Armin Sorooshian

Interests

Research

Armin's research focuses on the effect of aerosol particles on the environment, clouds and rainfall, climate, and public health/welfare. A suite of synergistic methods are used for this research, including laboratory experiments, ground and airborne field measurements, modeling, and remote sensing observations. Since 2004, he has participated in 15 airborne field projects, including six as a mission PI with the CIRPAS Twin Otter (sponsored by ONR). Currently, Armin is involved with a multi-year NASA project called CAMP2EX (Cloud and Aerosol Monsoonal Processes-Philippines Experiment; https://espo.nasa.gov/camp2ex/content/CAMP2Ex) and is serving as the PI of a NASA Earth Venture Suborbital-3 (EVS-3) mission called ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment; https://activate.larc.nasa.gov/).

Courses

Scholarly Contributions

Journals/Publications

• Brune, W. H., Ren, X., Zhang, L. i., Mao, J., Miller, D. O., Anderson, B. E., Blake, D. R., Cohen, R. C., Diskin, G. S., Hall, S. R., Hanisco, T. F., Huey, L. G., Nault, B. A., Peisch, J., Pollack, I., Ryerson, T. B., Shingler, T., Sorooshian, A., Ullmann, K., , Wisthaler, A., et al. (2018). Atmospheric oxidation in the presence of clouds during the Deep Convective Clouds and Chemistry (DC3) study. ATMOSPHERIC CHEMISTRY AND PHYSICS, 18(19), 14493-14510.
• Dadashazar, H., Braun, R. A., Crosbie, E., Chuang, P. Y., Woods, R. K., Jonsson, H. H., & Sorooshian, A. (2018). Aerosol characteristics in the entrainment interface layer in relation to the marine boundary layer and free troposphere. ATMOSPHERIC CHEMISTRY AND PHYSICS, 18(3), 1495-1506.
• Dehghani, M., Fazlzadeh, M., Sorooshian, A., Tabatabaee, H. R., Miri, M., Baghani, A. N., Delikhoon, M., Mahvi, A. H., & Rashidi, M. (2018). Characteristics and health effects of BTEX in a hot spot for urban pollution. ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY, 155, 133-143.
• Dehghani, M., Sorooshian, A., Ghorbani, M., Fazlzadeh, M., Miri, M., Badiee, P., Parvizi, A., Ansari, M., Baghani, A. N., & Delikhoon, M. (2018). Seasonal Variation in Culturable Bioaerosols in a Wastewater Treatment Plant. AEROSOL AND AIR QUALITY RESEARCH, 18(11), 2826-2839.
• Delikhoon, M., Fazlzadeh, M., Sorooshian, A., Baghani, A. N., Golaki, M., Ashournejad, Q., & Barkhordari, A. (2018). Characteristics and health effects of formaldehyde and acetaldehyde in an urban area in Iran. ENVIRONMENTAL POLLUTION, 242, 938-951.
• Ervens, B., Sorooshian, A., Aldhaif, A. M., Shingler, T., Crosbie, E., Ziemba, L., Campuzano-Jost, P., Jimenez, J. L., & Wisthaler, A. (2018). Is there an aerosol signature of chemical cloud processing?. ATMOSPHERIC CHEMISTRY AND PHYSICS, 18(21), 16099-16119.
• Farsani, M. H., Shirmardi, M., Alavi, N., Maleki, H., Sorooshian, A., Babaei, A., Asgharnia, H., Marzouni, M. B., & Goudarzi, G. (2018). Evaluation of the relationship between PM10 concentrations and heavy metals during normal and dusty days in Ahvaz, Iran. AEOLIAN RESEARCH, 33, 12-22.
• Iftikhar, M., Alam, K., Sorooshian, A., Syed, W. A., Bibi, S., & Bibi, H. (2018). Contrasting aerosol optical and radiative properties between dust and urban haze episodes in megacities of Pakistan. ATMOSPHERIC ENVIRONMENT, 173, 157-172.
• Keshavarzi, B., Abbasi, S., Moore, F., Mehravar, S., Sorooshian, A., Soltani, N., & Najmeddin, A. (2018). Contamination Level, Source Identification and Risk Assessment of Potentially Toxic Elements (PTEs) and Polycyclic Aromatic Hydrocarbons (PAHs) in Street Dust of an Important Commercial Center in Iran. ENVIRONMENTAL MANAGEMENT, 62(4), 803-818.
• Keshavarzi, B., Hassanaghaei, M., Moore, F., Mehr, M. R., Soltanian, S., Lahijanzadeh, A. R., & Sorooshian, A. (2018). Heavy metal contamination and health risk assessment in three commercial fish species in the Persian Gulf. MARINE POLLUTION BULLETIN, 129(1), 245-252.
• MacDonald, A. B., Dadashazar, H., Chuang, P. Y., Crosbie, E., Wang, H., Wang, Z., Jonsson, H. H., Flagan, R. C., Seinfeld, J. H., & Sorooshian, A. (2018). Characteristic Vertical Profiles of Cloud Water Composition in Marine Stratocumulus Clouds and Relationships With Precipitation. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 123(7), 3704-3723.
• Mardi, A. H., Khaghani, A., MacDonald, A. B., Nguyen, P., Karimi, N., Heidary, P., Karimi, N., Saemian, P., Sehatkashani, S., Tajrishy, M., & Sorooshian, A. (2018). The Lake Urmia environmental disaster in Iran: A look at aerosol pollution. SCIENCE OF THE TOTAL ENVIRONMENT, 633, 42-49.
• Naimabadi, A., Shirmardi, M., Maleki, H., Teymouri, P., Goudarzi, G., Shahsavani, A., Sorooshian, A., Babaei, A. A., Mehrabi, N., Baneshi, M. M., Zarei, M. R., Lababpou, A., & Ghozikali, M. G. (2018). On the chemical nature of precipitation in a populated Middle Eastern Region (Ahvaz, Iran) with diverse sources. ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY, 163, 558-566.
• Namdari, S., Karimi, N., Sorooshian, A., Mohammadi, G., & Sehatkashani, S. (2018). Impacts of climate and synoptic fluctuations on dust storm activity over the Middle East. ATMOSPHERIC ENVIRONMENT, 173, 265-276.
• Soltani, N., Keshavarzi, B., Sorooshian, A., Moore, F., Dunster, C., Dominguez, A. O., Kelly, F. J., Dhakal, P., Ahmadi, M. R., & Asadi, S. (2018). Oxidative potential (OP) and mineralogy of iron ore particulate matter at the Gol-E-Gohar Mining and Industrial Facility (Iran). ENVIRONMENTAL GEOCHEMISTRY AND HEALTH, 40(5), 1785-1802.
• Sorooshian, A., MacDonald, A. B., Dadashazar, H., Bates, K. H., Coggon, M. M., Craven, J. S., Crosbie, E., Hersey, S. P., Hodas, N., Lin, J. J., Marty, A. N., Maudlin, L. C., Metcalf, A. R., Murphy, S. M., Padro, L. T., Prabhakar, G., Rissman, T. A., Shingler, T., Varutbangkul, V., , Wang, Z., et al. (2018). A multi-year data set on aerosol-cloud-precipitation-meteorology interactions for marine stratocumulus clouds. SCIENTIFIC DATA, 5.
• Weiss-Penzias, P., Sorooshian, A., Coale, K., Heim, W., Crosbie, E., Dadashazar, H., MacDonald, A. B., Wang, Z., & Jonsson, H. (2018). Aircraft Measurements of Total Mercury and Monomethyl Mercury in Summertime Marine Stratus Cloudwater from Coastal California, USA. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 52(5), 2527-2537.
• Abbasi, S., Keshavarzi, B., Moore, F., Delshab, H., Soltani, N., & Sorooshian, A. (2017). Investigation of microrubbers, microplastics and heavy metals in street dust: a study in Bushehr city, Iran. ENVIRONMENTAL EARTH SCIENCES, 76(23).
• Braun, R. A., Dadashazar, H., MacDonald, A. B., Aldhaif, A. M., Maudlin, L. C., Crosbie, E., Aghdam, M. A., Mardi, A. H., & Sorooshian, A. (2017). Impact of Wildfire Emissions on Chloride and Bromide Depletion in Marine Aerosol Particles. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 51(16), 9013-9021.
• Dadashazar, H., Wang, Z., Crosbie, E., Brunke, M., Zeng, X., Jonsson, H., Woods, R. K., Flagan, R. C., Seinfeld, J. H., & Sorooshian, A. (2017). Relationships between giant sea salt particles and clouds inferred from aircraft physicochemical data. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(6), 3421-3434.
• Mora, M., Braun, R. A., Shingler, T., & Sorooshian, A. (2017). Analysis of remotely sensed and surface data of aerosols and meteorology for the Mexico Megalopolis Area between 2003 and 2015. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(16), 8705-8723.
• Perring, A. E., Schwarz, J. P., Markovic, M. Z., Fahey, D. W., Jimenez, J. L., Campuzano-Jost, P., Palm, B. D., Wisthaler, A., Mikoviny, T., Diskin, G., Sachse, G., Ziemba, L., Anderson, B., Shingler, T., Crosbie, E., Sorooshian, A., Yokelson, R., & Gao, R. (2017). In situ measurements of water uptake by black carbon-containing aerosol in wildfire plumes. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(2), 1086-1097.
• Schlosser, J. S., Braun, R. A., Bradley, T., Dadashazar, H., MacDonald, A. B., Aldhaif, A. A., Aghdam, M. A., Mardi, A. H., Xian, P., & Sorooshian, A. (2017). Analysis of aerosol composition data for western United States wildfires between 2005 and 2015: Dust emissions, chloride depletion, and most enhanced aerosol constituents. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(16), 8951-8966.
• Soltani, N., Keshavarzi, B., Moore, F., Sorooshian, A., & Ahmadi, M. R. (2017). Distribution of potentially toxic elements (PTEs) in tailings, soils, and plants around Gol-E-Gohar iron mine, a case study in Iran. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH, 24(23), 18798-18816.
• Sorooshian, A., Shingler, T., Crosbie, E., Barth, M. C., Homeyer, C. R., Campuzano-Jost, P., Day, D. A., Jimenez, J. L., Thornhill, K. L., Ziemba, L. D., Blake, D. R., & Fried, A. (2017). Contrasting aerosol refractive index and hygroscopicity in the inflow and outflow of deep convective storms: Analysis of airborne data from DC3. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(8), 4565-4577.
• Crosbie, E., Wang, Z., Sorooshian, A., Chuang, P. Y., Craven, J. S., Coggon, M. M., Brunke, M., Zeng, X., Jonsson, H., Woods, R. K., Flagan, R. C., & Seinfeld, J. H. (2016). Stratocumulus Cloud Clearings and Notable Thermodynamic and Aerosol Contrasts across the Clear-Cloudy Interface. JOURNAL OF THE ATMOSPHERIC SCIENCES, 73(3), 1083-1099.
• Jung, E., Albrecht, B. A., Sorooshian, A., Zuidema, P., & Jonsson, H. H. (2016). Precipitation susceptibility in marine stratocumulus and shallow cumulus from airborne measurements. ATMOSPHERIC CHEMISTRY AND PHYSICS, 16(17), 11395-11413.
• Lopez, D. H., Rabbani, M. R., Crosbie, E., Raman, A., Arellano Jr., A. F., & Sorooshian, A. (2016). Frequency and Character of Extreme Aerosol Events in the Southwestern United States: A Case Study Analysis in Arizona. ATMOSPHERE, 7(1).
• Maleki, H., Sorooshian, A., Goudarzi, G., Nikfal, A., & Baneshi, M. M. (2016). Temporal profile of PM10 and associated health effects in one of the most polluted cities of the world (Ahvaz, Iran) between 2009 and 2014. AEOLIAN RESEARCH, 22, 135-140.
• Raman, A., Arellano Jr., A. F., & Sorooshian, A. (2016). Decreasing Aerosol Loading in the North American Monsoon Region. ATMOSPHERE, 7(2).
• Sanchez, K. J., Russell, L. M., Modini, R. L., Frossard, A. A., Ahlm, L., Corrigan, C. E., Roberts, G. C., Hawkins, L. N., Schroder, J. C., Bertram, A. K., Zhao, R., Lee, A., Lin, J. J., Nenes, A., Wang, Z., Wonaschuetz, A., Sorooshian, A., Noone, K. J., Jonsson, H., , Toom, D., et al. (2016). Meteorological and aerosol effects on marine cloud microphysical properties. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 121(8), 4142-4161.
• Shingler, T., Crosbie, E., Ortega, A., Shiraiwa, M., Zuend, A., Beyersdorf, A., Ziemba, L., Anderson, B., Thornhill, L., Perring, A. E., Schwarz, J. P., Campazano-Jost, P., Day, D. A., Jimenez, J. L., Hair, J. W., Mikoviny, T., Wisthaler, A., & Sorooshian, A. (2016). Airborne characterization of subsaturated aerosol hygroscopicity and dry refractive index from the surface to 6.5km during the SEAC(4)RS campaign. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 121(8), 4188-4210.
• Shingler, T., Sorooshian, A., Ortega, A., Crosbie, E., Wonaschuetz, A., Perring, A. E., Beyersdorf, A., Ziemba, L., Jimenez, J. L., Campuzano-Jost, P., Mikoviny, T., Wisthaler, A., & Russell, L. M. (2016). Ambient observations of hygroscopic growth factor and f(RH) below 1: Case studies from surface and airborne measurements. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 121(22), 13661-13677.
• Soleimani, Z., Goudarzi, G., Sorooshian, A., Marzouni, M. B., & Maleki, H. (2016). Impact of Middle Eastern dust storms on indoor and outdoor composition of bioaerosol. ATMOSPHERIC ENVIRONMENT, 138, 135-143.
• Wang, Z., Ramirez, M. M., Dadashazar, H., MacDonald, A. B., Crosbie, E., Bates, K. H., Coggon, M. M., Craven, J. S., Lynch, P., Campbell, J. R., Aghdam, M. A., Woods, R. K., Jonsson, H., Flagan, R. C., Seinfeld, J. H., & Sorooshian, A. (2016). Contrasting cloud composition between coupled and decoupled marine boundary layer clouds. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 121(19), 11679-11691.
• Youn, J., Csavina, J., Rine, K. P., Shingler, T., Taylor, M. P., Saez, A. E., Betterton, E. A., & Sorooshian, A. (2016). Hygroscopic Properties and Respiratory System Deposition Behavior of Particulate Matter Emitted By Mining and Smelting Operations. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 50(21), 11706-11713.
• Zhang, X., Dalleska, N. F., Huang, D. D., Bates, K. H., Sorooshian, A., Flagan, R. C., & Seinfeld, J. H. (2016). Time-resolved molecular characterization of organic aerosols by PILS plus UPLC/ESI-Q-TOFMS. ATMOSPHERIC ENVIRONMENT, 130, 180-189.
• Asa-Awuku, A., Sorooshian, A., Flagan, R. C., Seinfeld, J. H., & Nenes, A. (2015). CCN Properties of Organic Aerosol Collected Below and within Marine Stratocumulus Clouds near Monterey, California. ATMOSPHERE, 6(11), 1590-1607.
• Crosbie, E., Youn, J., Balch, B., Wonaschuetz, A., Shingler, T., Wang, Z., Conant, W. C., Betterton, E. A., & Sorooshian, A. (2015). On the competition among aerosol number, size and composition in predicting CCN variability: a multi-annual field study in an urbanized desert. ATMOSPHERIC CHEMISTRY AND PHYSICS, 15(12), 6943-6958.
• Hersey, S. P., Garland, R. M., Crosbie, E., Shingler, T., Sorooshian, A., Piketh, S., & Burger, R. (2015). An overview of regional and local characteristics of aerosols in South Africa using satellite, ground, and modeling data. ATMOSPHERIC CHEMISTRY AND PHYSICS, 15(8), 4259-4278.
• Jung, E., Albrecht, B. A., Jonsson, H. H., Chen, Y. -., Seinfeld, J. H., Sorooshian, A., Metcalf, A. R., Song, S., Fang, M., & Russell, L. M. (2015). Precipitation effects of giant cloud condensation nuclei artificially introduced into stratocumulus clouds. ATMOSPHERIC CHEMISTRY AND PHYSICS, 15(10), 5645-5658.
• Maudlin, L. C., Wang, Z., Jonsson, H. H., & Sorooshian, A. (2015). Impact of wildfires on size-resolved aerosol composition at a coastal California site. ATMOSPHERIC ENVIRONMENT, 119, 59-68.
• Modini, R. L., Frossard, A. A., Ahlm, L., Russell, L. M., Corrigan, C. E., Roberts, G. C., Hawkins, L. N., Schroder, J. C., Bertram, A. K., Zhao, R., Lee, A. K., Abbatt, J. P., Lin, J., Nenes, A., Wang, Z., Wonaschuetz, A., Sorooshian, A., Noone, K. J., Jonsson, H., , Seinfeld, J. H., et al. (2015). Primary marine aerosol-cloud interactions off the coast of California. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 120(9), 4282-4303.
• Sorooshian, A., Crosbie, E., Maudlin, L. C., Youn, J., Wang, Z., Shingler, T., Ortega, A. M., Hersey, S., & Woods, R. K. (2015). Surface and airborne measurements of organosulfur and methanesulfonate over the western United States and coastal areas. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 120(16), 8535-8548.
• Sorooshian, A., Prabhakar, G., Jonsson, H., Woods, R. K., Flagan, R. C., & Seinfeld, J. H. (2015). On the presence of giant particles downwind of ships in the marine boundary layer. GEOPHYSICAL RESEARCH LETTERS, 42(6), 2024-2030.
• Youn, J. -., Crosbie, E., Maudlin, L. C., Wang, Z., & Sorooshian, A. (2015). Dimethylamine as a major alkyl amine species in particles and cloud water: Observations in semi-arid and coastal regions. ATMOSPHERIC ENVIRONMENT, 122, 250-258.
• Fountoukis, C., Nenes, A., Meskhidze, N., Bahreini, R., Conant, W. C., Jonsson, H., Murphy, S., Sorooshian, A., Varutbangkul, V., Brechtel, F., Flagan, R. C., & Seinfeld, J. H. (2014). Aerosol-cloud drop concentration closure for clouds sampled during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D10).
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  [ 1] This study analyzes 27 cumuliform and stratiform clouds sampled aboard the CIRPAS Twin Otter during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) experiment. The data set was used to assess cloud droplet closure using ( 1) a detailed adiabatic cloud parcel model and ( 2) a state-of-the-art cloud droplet activation parameterization. A unique feature of the data set is the sampling of highly polluted clouds within the vicinity of power plant plumes. Remarkable closure was achieved ( much less than the 20% measurement uncertainty) for both parcel model and parameterization. The highly variable aerosol did not complicate the cloud droplet closure, since the clouds had low maximum supersaturation and were not sensitive to aerosol variations ( which took place at small particle sizes). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06, but can range between 0.03 and 1.0. The sensitivity of cloud droplet prediction error to changes in the uptake coefficient, organic solubility and surface tension depression suggest that organics exhibit limited solubility. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America; future in situ studies will determine the robustness of our findings.
• Jiang, H., Feingold, G., & Sorooshian, A. (2014). Effect of Aerosol on the Susceptibility and Efficiency of Precipitation in Warm Trade Cumulus Clouds. JOURNAL OF THE ATMOSPHERIC SCIENCES, 67(11), 3525-3540.
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  Large eddy simulations of warm trade wind cumulus clouds are conducted for a range of aerosol conditions with a focus on precipitating clouds Individual clouds are tracked over the course of their lifetimes Precipitation rate decreases progressively as aerosol Increases For larger precipitating clouds the polluted clouds have longer lifetimes because of precipitation suppression For clean aerosol conditions there is good agreement between the average model precipitation rate and that calculated based on observed radar reflectivity Z and precipitation rate R relationships Precipitation rate can be expressed as a power law function of liquid water path (LWP) and N(d) to reasonable accuracy The respective powers for LWP and N(d) are of similar magnitude compared to those based on observational studies of stratocumulus clouds The time integrated precipitation rate represented by a power law function of LWP N(d) and cloud lifetime is much more reliably predicted than is R expressed in terms of LWP and N(d) alone The precipitation susceptibility (So = dInR/dInN(d)) that quantifies the sensitivity of precipitation to changes in N(d) depends strongly on LWP and exhibits nonmonotonic behavior with a maximum at intermediate LWP values The relationship between So and precipitation efficiency is explored and the Importance of including dependence on N(d) in the latter is highlighted The results provide trade cumulus cloud population statistics as well as relationships between (memicrophysical/macrophysical properties and precipitation that are amenable for use in larger scale models
• Lance, S., Nenes, A., Mazzoleni, C., Dubey, M. K., Gates, H., Varutbangkul, V., Rissman, T. A., Murphy, S. M., Sorooshian, A., Flagan, R. C., Seinfeld, J. H., Feingold, G., & Jonsson, H. H. (2014). Cloud condensation nuclei activity, closure, and droplet growth kinetics of Houston aerosol during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 114.
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  In situ cloud condensation nuclei (CCN) measurements were obtained in the boundary layer over Houston, Texas, during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) campaign onboard the CIRPAS Twin Otter. Polluted air masses in and out of cloudy regions were sampled for a total of 22 flights, with CCN measurements obtained for 17 of these flights. In this paper, we focus on CCN closure during two flights, within and downwind of the Houston regional plume and over the Houston Ship Channel. During both flights, air was sampled with particle concentrations exceeding 25,000 cm(-3) and CCN concentrations exceeding 10,000 cm(-3). CCN closure is evaluated by comparing measured concentrations with those predicted on the basis of measured aerosol size distributions and aerosol mass spectrometer particle composition. Different assumptions concerning the internally mixed chemical composition result in average CCN overprediction ranging from 3% to 36% (based on a linear fit). It is hypothesized that the externally mixed fraction of the aerosol contributes much of the CCN closure scatter, while the internally mixed fraction largely controls the overprediction bias. On the basis of the droplet sizes of activated CCN, organics do not seem to impact, on average, the CCN activation kinetics.
• Prabhakar, G., Ervens, B., Wang, Z., Maudlin, L. C., Coggon, M. M., Jonsson, H. H., Seinfeld, J. H., & Sorooshian, A. (2014). Sources of nitrate in stratocumulus cloud water: Airborne measurements during the 2011 E-PEACE and 2013 NiCE studies. ATMOSPHERIC ENVIRONMENT, 97, 166-173.
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  This study examines the sources of NO3- in stratocumulus clouds over the eastern Pacific Ocean off the California coast using airborne and surface measurement data from the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE; 2011) and Nucleation in California Experiment (NiCE; 2013). Average NO3- air-equivalent concentrations in cloud water samples categorized as having been influenced by ship exhaust (2.5 mu g m(-3)), strong marine emissions (2.5 mu g m(-3)) and fires (2.0 mu g m(-3)) were more than twice that in the background cloud water (0.9 mu g m(-3)). During periods when biomass burning plumes resided above cloud top, 16 of 29 cloud water samples were impacted due to instability in the entrainment interface layer with NO3- levels reaching as high as 9.0 mu g m(-3). Nucleation scavenging of chloride depleted sea-salt is a source of cloud water NO3-, with the lowest Cl-:Na+ ratio (1.5) observed in ship-influenced samples. Surface aerosol measurements show that NOT concentrations peak in the particle diameter range of 1.0-5.6 mu m, similar to Na, Cl- and Si, suggesting that drop activation of crustal particles and sea salt could be an important source of NO3- in cloud water. The contrasting behavior of NOT and SO42- is emphasized by the NO3-:SO42- mass concentration ratio which is highest in cloud water (by more than a factor of two) followed by above cloud aerosol, droplet residual particles, and below cloud aerosol. Trends of a decreasing NO3-:SO42- ratio with altitude in clouds are confirmed by parcel model studies due to the higher rate of in-cloud sulfate formation as compared to HNO3 uptake by droplets. (C) 2014 Elsevier Ltd. All rights reserved.
• Sorooshian, A., Wonaschuetz, A., Jarjour, E. G., Hashimoto, B. I., Schichtel, B. A., & Betterton, E. A. (2014). An aerosol climatology for a rapidly growing arid region (southern Arizona): Major aerosol species and remotely sensed aerosol properties. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 116.
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  This study reports a comprehensive characterization of atmospheric aerosol particle properties in relation to meteorological and back trajectory data in the southern Arizona region, which includes two of the fastest growing metropolitan areas in the United States (Phoenix and Tucson). Multiple data sets (MODIS, AERONET, OMI/TOMS, MISR, GOCART, ground-based aerosol measurements) are used to examine monthly trends in aerosol composition, aerosol optical depth (AOD), and aerosol size. Fine soil, sulfate, and organics dominate PM2.5 mass in the region. Dust strongly influences the region between March and July owing to the dry and hot meteorological conditions and back trajectory patterns. Because monsoon precipitation begins typically in July, dust levels decrease, while AOD, sulfate, and organic aerosol reach their maximum levels because of summertime photochemistry and monsoon moisture. Evidence points to biogenic volatile organic compounds being a significant source of secondary organic aerosol in this region. Biomass burning also is shown to be a major contributor to the carbonaceous aerosol budget in the region, leading to enhanced organic and elemental carbon levels aloft at a sky-island site north of Tucson (Mt. Lemmon). Phoenix exhibits different monthly trends for aerosol components in comparison with the other sites owing to the strong influence of fossil carbon and anthropogenic dust. Trend analyses between 1988 and 2009 indicate that the strongest statistically significant trends are reductions in sulfate, elemental carbon, and organic carbon, and increases in fine soil during the spring (March-May) at select sites. These results can be explained by population growth, land-use changes, and improved source controls.
• Craven, J. S., Metcalf, A. R., Bahreini, R., Middlebrook, A., Hayes, P. L., Duong, H. T., Sorooshian, A., Jimenez, J. L., Flagan, R. C., & Seinfeld, J. H. (2013). Los Angeles Basin airborne organic aerosol characterization during CalNex. Journal of Geophysical Research D: Atmospheres, 118(19), 11453-11467.
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  Abstract: We report airborne organic aerosol (OA) measurements over Los Angeles carried out in May 2010 as part of the CalNex field campaign. The principal platform for the airborne data reported here was the CIRPAS Twin Otter (TO); airborne data from NOAA WP-3D aircraft and Pasadena CalNex ground-site data acquired during simultaneous TO flybys are also presented. Aerodyne aerosol mass spectrometer measurements constitute the main source of data analyzed. The increase in organic aerosol oxidation from west to east in the basin was sensitive to OA mass loading, with a greater spatial trend in O:C associated with lower mass concentration. Three positive matrix factorization (PMF) components (hydrocarbon-like organic aerosol (HOA), semi-volatile oxidized organic aerosol (SVOOA), and low volatility oxidized organic aerosol (LVOOA)) were resolved for the one flight that exhibited the largest variability in estimated O:C ratio. Comparison of the PMF factors with two optical modes of refractory black carbon (rBC)-containing aerosol revealed that the coating of thinly coated rBC-containing aerosol, dominant in the downtown region, is likely composed of HOA, whereas more thickly coated rBC-containing aerosol, dominant in the Banning pass outflow, is composed of SVOOA and LVOOA. The correlation of water-soluble organic mass to oxidized organic aerosol (OOA) is higher in the outflows than in the basin due to the higher mass fraction of OOA/OA in the outflows. By comparison, the average OA concentration over Mexico City MILAGRO (Megacity Initiative: Local and Global Research Observations) campaign was ∼7 times higher than the airborne average during CalNex. Key Points Airborne organic aerosol measurements over LA are reported Spatial pattern of O:C depends on amount of organic aerosol present PMF factors are correlated with two modes of rBC containing aerosol ©2013. American Geophysical Union. All Rights Reserved.
• Duong, H. T., Sorooshian, A., Craven, J. S., Hersey, S. P., Metcalf, A. R., Zhang, X., Weber, R. J., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2013). Water-soluble organic aerosol in the Los Angeles Basin and outflow regions: Airborne and ground measurements during the 2010 CalNex field campaign. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 116.
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  A particle-into-liquid sampler coupled to a total organic carbon analyzer (PILS-TOC) quantified particulate water-soluble organic carbon (WSOC) mass concentrations during the May 2010 deployment of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter in the CalNex field study. WSOC data collected during 16 flights provide the first spatiotemporal maps of WSOC in the San Joaquin Valley, Los Angeles Basin, and outflow regions of the Basin. WSOC was consistently higher in concentration within the Los Angeles Basin, where sea breeze transport and Basin topography strongly influence the spatial distribution of WSOC. The highest WSOC levels were associated with fire plumes, highlighting the importance of both primary and secondary sources for WSOC in the region. Residual pollution layers enriched with WSOC are observed aloft up to an altitude of 3.2 km and the highest WSOC levels for each flight were typically observed above 500 m. Simultaneous ground WSOC measurements during aircraft overpasses in Pasadena and Riverside typically exhibit lower levels, especially when relative humidity (RH) was higher aloft suggestive of the influence of aerosol-phase water. This points to the underestimation of the radiative effects of WSOC when using only surface measurements. Reduced aerosol-phase water in the eastern desert outflow region likely promotes the re-partitioning of WSOC to the gas phase and suppression of processes to produce these species (partitioning, multiphase chemistry, photolytic production); as a result, WSOC is reduced relative to sulfate (but not as much as nitrate) as aerosol is advected from the Basin to the outflows.
• Feingold, G., McComiskey, A., Rosenfeld, D., & Sorooshian, A. (2013). On the relationship between cloud contact time and precipitation susceptibility to aerosol. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 118(18), 10544-10554.
• Feingold, G., McComiskey, A., Rosenfeld, D., & Sorooshian, A. (2013). On the relationship between cloud contact time and precipitation susceptibility to aerosol. Journal of Geophysical Research D: Atmospheres, 118(18), 10544-10554.
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  Abstract: The extent to which the rain rate from shallow, liquid-phase clouds is microphysically influenced by aerosol, and therefore drop concentration N d perturbations, is addressed through analysis of the precipitation susceptibility, So. Previously published work, based on both models and observations, disagrees on the qualitative behavior of So with respect to variables such as liquid water path L or the ratio between accretion and autoconversion rates. Two primary responses have emerged: (i) So decreases monotonically with increasing L and (ii) So increases with L, reaches a maximum, and decreases thereafter. Here we use a variety of modeling frameworks ranging from box models of (size-resolved) collision-coalescence, to trajectory ensembles based on large eddy simulation to explore the role of time available for collision-coalescence tc in determining the So response. The analysis shows that an increase in tc shifts the balance of rain production from autoconversion (a Nd-dependent process) to accretion (roughly independent of N d), all else (e.g., L) equal. Thus, with increasing cloud contact time, warm rain production becomes progressively less sensitive to aerosol, all else equal. When the time available for collision-coalescence is a limiting factor, So increases with increasing L whereas when there is ample time available, So decreases with increasing L. The analysis therefore explains the differences between extant studies in terms of an important precipitation-controlling parameter, namely the integrated liquid water history over the course of an air parcel's contact with a cloud. Key PointsTime-integrated liquid water determines precipitation susceptibility to aerosolRain susceptibility to aerosol is a non-monotonic function of liquid water ©2013. American Geophysical Union. All Rights Reserved.
• Gilardoni, S., Russell, L. M., Sorooshian, A., Flagan, R. C., Seinfeld, J. H., Bates, T. S., Quinn, P. K., Allan, J. D., Williams, B., Goldstein, A. H., Onasch, T. B., & Worsnop, D. R. (2013). Regional variation of organic functional groups in aerosol particles on four US east coast platforms during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D10).
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  [1] Submicron atmospheric aerosol samples were collected during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 campaign on four platforms: Chebogue Point ( Nova Scotia, Canada), Appledore Island ( Maine), the CIRPAS Twin Otter over Ohio, and the NOAA R/V Ronald H. Brown in the Gulf of Maine. Saturated aliphatic C-C-H, unsaturated aliphatic C=C - H, aromatic C=C - H, organosulfur C-O-S, carbonyl C=O, and organic hydroxyl C-OH functional groups were measured by calibrated Fourier Transform Infrared ( FTIR) spectroscopy at all four sampling platforms. The ratio of molar concentrations of carbonyl C=O to saturated aliphatic C-C-H groups was nearly constant at each sampling platform, with the Twin Otter samples having the lowest ratio at 0.1 and the three more coastal platforms having ratios of 0.4 and 0.5. Organic mass ( OM) to organic carbon (OC) ratios follow similar trends for the four platforms, with the Twin Otter having the lowest ratio of 1.4 and the coastal platforms having slightly higher values typically between 1.5 and 1.6. Organosulfur compounds were occasionally observed. Collocated organic aerosol sampling with two Aerodyne aerosol mass spectrometers for OM, a Sunset Laboratory thermo-optical analysis instrument for OC, and an ion chromatography-particle into liquid sampler (IC-PILS) for speciated carboxylic acids provided comparable results for most of the project, tracking the time series of FTIR OM, OC, and carbonyl groups, respectively, and showing simultaneous peaks of similar magnitude during most of the project. The FTIR/IC-PILS comparison suggests that about 9% of the carbonyl groups found in submicron organic particles on the Twin Otter are typically associated with low molecular weight carboxylic acids.
• Hersey, S. P., Craven, J. S., Metcalf, A. R., Lin, J., Lathem, T., Suski, K. J., Cahill, J. F., Duong, H. T., Sorooshian, A., Jonsson, H. H., Shiraiwa, M., Zuend, A., Nenes, A., Prather, K. A., Flagan, R. C., & Seinfeld, J. H. (2013). Composition and hygroscopicity of the Los Angeles Aerosol: CalNex. Journal of Geophysical Research D: Atmospheres, 118(7), 3016-3036.
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  Abstract: Aircraft-based measurements of aerosol composition, either bulk or single-particle, and both subsaturated and supersaturated hygroscopicity were made in the Los Angeles Basin and its outflows during May 2010 during the CalNex field study. Aerosol composition evolves from source-rich areas in the western Basin to downwind sites in the eastern Basin, evidenced by transition from an external to internal mixture, as well as enhancements in organic O: C ratio, the amount of organics and nitrate internally mixed on almost all particle types, and coating thickness on refractory black carbon (rBC). Transport into hot, dilute outflow regions leads to significant volatilization of semivolatile material, resulting in a unimodal aerosol comprising primarily oxygenated, low-volatility, water-soluble organics and sulfate. The fraction of particles with rBC or soot cores is between 27 and 51% based on data from a Single Particle Soot Photometer (SP2) and Aerosol Time of Flight Mass Spectrometer (ATOFMS). Secondary organics appear to inhibit subsaturated water uptake in aged particles, while CCN activity is enhanced with photochemical age. A biomass-burning event resulted in suppression of subsaturated hygroscopicity but enhancement in CCN activity, suggesting that BB particles may be nonhygroscopic at subsaturated RH but are important sources of CCN. Aerosol aging and biomass burning can lead to discrepancies between subsaturated and supersaturated hygroscopicity that may be related to mixing state. In the cases of biomass burning aerosol and aged particles coated with secondary material, more than a single parameter representation of subsaturated hygroscopicity and CCN activity is needed. © 2013. American Geophysical Union. All Rights Reserved.
• Moore, R. H., Ingall, E. D., Sorooshian, A., & Nenes, A. (2013). Molar mass, surface tension, and droplet growth kinetics of marine organics from measurements of CCN activity. GEOPHYSICAL RESEARCH LETTERS, 35(7).
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  The CCN-relevant properties and droplet growth kinetics are determined for marine organic matter isolated from seawater collected near the Georgia coast. The organic matter is substantially less CCN active than (NH(4))(2)SO(4), but droplet growth kinetics are similar. Kohler Theory Analysis (KTA) is used to determine the average organic molar masses of two samples, which are 4370 +/- 24% and 4340 +/- 18% kg kmol(-1). KTA is used to infer surface tension depression, which is in excellent agreement with direct measurements. For the first time it is shown that direct measurements of surface tension are relevant for CCN activation, and this study highlights the power of KTA.
• Murphy, S. M., Sorooshian, A., Kroll, J. H., Ng, N. L., Chhabra, P., Tong, C., Surratt, J. D., Knipping, E., Flagan, R. C., & Seinfeld, J. H. (2013). Secondary aerosol formation from atmospheric reactions of aliphatic amines. ATMOSPHERIC CHEMISTRY AND PHYSICS, 7(9), 2313-2337.
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  Although aliphatic amines have been detected in both urban and rural atmospheric aerosols, little is known about the chemistry leading to particle formation or the potential aerosol yields from reactions of gas-phase amines. We present here the first systematic study of aerosol formation from the atmospheric reactions of amines. Based on laboratory chamber experiments and theoretical calculations, we evaluate aerosol formation from reaction of OH, ozone, and nitric acid with trimethylamine, methylamine, triethylamine, diethylamine, ethylamine, and ethanolamine. Entropies of formation for alkylammonium nitrate salts are estimated by molecular dynamics calculations enabling us to estimate equilibrium constants for the reactions of amines with nitric acid. Though subject to significant uncertainty, the calculated dissociation equilibrium constant for diethylammonium nitrate is found to be sufficiently small to allow for its atmospheric formation, even in the presence of ammonia which competes for available nitric acid. Experimental chamber studies indicate that the dissociation equilibrium constant for triethylammonium nitrate is of the same order of magnitude as that for ammonium nitrate. All amines studied form aerosol when photooxidized in the presence of NOx with the majority of the aerosol mass present at the peak of aerosol growth consisting of aminium (R3NH+) nitrate salts, which repartition back to the gas phase as the parent amine is consumed. Only the two tertiary amines studied, trimethylamine and triethylamine, are found to form significant non-salt organic aerosol when oxidized by OH or ozone; calculated organic mass yields for the experiments conducted are similar for ozonolysis (15% and 5% respectively) and photooxidation (23% and 8% respectively). The non-salt organic aerosol formed appears to be more stable than the nitrate salts and does not quickly repartition back to the gas phase.
• Ng, N. L., Kwan, A. J., Surratt, J. D., Chan, A. W., Chhabra, P. S., Sorooshian, A., Pye, H. O., Crounse, J. D., Wennberg, P. O., Flagan, R. C., & Seinfeld, J. H. (2013). Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO3). ATMOSPHERIC CHEMISTRY AND PHYSICS, 8(14), 4117-4140.
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  Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO3) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH < 10%) using N2O5 as a source of NO3 radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield ( defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO2) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO2+RO2 reaction ( self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% ( corresponding to M-o congruent to 10 mu g m(-3)), it is estimated that similar to 2 to 3 Tg yr(-1) of SOA results from isoprene+ NO3. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals in the nighttime troposphere.
• Partridge, D. G., Vrugt, J. A., Tunved, P., Ekman, A. M., Gorea, D., & Sorooshian, A. (2013). Inverse modeling of cloud-aerosol interactions - Part 1: Detailed response surface analysis. ATMOSPHERIC CHEMISTRY AND PHYSICS, 11(14), 7269-7287.
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  New methodologies are required to probe the sensitivity of parameters describing cloud droplet activation. This paper presents an inverse modeling-based method for exploring cloud-aerosol interactions via response surfaces. The objective function, containing the difference between the measured and model predicted cloud droplet size distribution is studied in a two-dimensional framework, and presented for pseudo-adiabatic cloud parcel model parameters that are pair-wise selected. From this response surface analysis it is shown that the susceptibility of cloud droplet size distribution to variations in different aerosol physiochemical parameters is highly dependent on the aerosol environment and meteorological conditions. In general the cloud droplet size distribution is most susceptible to changes in the up-draft velocity. A shift towards an increase in the importance of chemistry for the cloud nucleating ability of particles is shown to exist somewhere between marine average and rural continental aerosol regimes.
• Russell, L. M., Sorooshian, A., Seinfeld, J. H., Albrecht, B. A., Nenes, A., Ahlm, L., Chen, Y., Coggon, M., Craven, J. S., Flagan, R. C., Frossard, A. A., Jonsson, H., Jung, E., Lin, J. J., Metcalf, A. R., Modini, R., Mülmenstädt, J., Roberts, G. C., Shingler, T., , Song, S., et al. (2013). Eastern pacific emitted aerosol cloud experiment. Bulletin of the American Meteorological Society, 94(5), 709-729.
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  Abstract: A coordinated field experiment, the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) campaign, in which the effects of well-defined aerosol perturbations on marine stratocumulus clouds were probed via in situ aircraft and satellite observations. E-PEACE combined a targeted aircraft campaign off the coast of Monterey in July and August 2011 with embedded ship and satellite observations and modeling studies. The research vessel (R/V) Point Sur was applied to measure the aerosol below cloud and as a platform for well-characterized smoke emissions to produce a uniquely identifiable cloud signature. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft was used with a full payload of instruments to measure particle and cloud droplet number, mass, and composition. The novel aspect of using E-PEACE observations for ACP studies is that the starting point can be constrained with the particle number, size, and composition of emitted particles, and predict their activation in cloud.
• Russell, L. M., Sorooshian, A., Seinfeld, J. H., Albrecht, B. A., Nenes, A., Leaitch, W. R., Macdonald, A. M., Ahlm, L., Chen, Y., Coggon, M., Corrigan, A., Craven, J. S., Flagan, R. C., Frossard, A. A., Hawkins, L. N., Jonsson, H., Jung, E., Lin, J. J., Metcalf, A. R., , Modini, R., et al. (2013). Observed aerosol effects on marine cloud nucleation and supersaturation. AIP Conference Proceedings, 1527, 696-701.
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  Abstract: Aerosol particles in the marine boundary layer include primary organic and salt particles from sea spray and combustion-derived particles from ships and coastal cities. These particle types serve as nuclei for marine cloud droplet activation, although the particles that activate depend on the particle size and composition as well as the supersaturation that results from cloud updraft velocities. The Eastern Pacific Emitted Aerosol Cloud Experiment (EPEACE) 2011 was a targeted aircraft campaign to assess how different particle types nucleate cloud droplets. As part of E-PEACE 2011, we studied the role of marine particles as cloud droplet nuclei and used emitted particle sources to separate particle-induced feedbacks from dynamical variability. The emitted particle sources included shipboard smoke-generated particles with 0.05-1 μm diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke) and combustion particles from container ships with 0.05-0.2 μm diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components) [1]. Three central aspects of the collaborative E-PEACE results are: (1) the size and chemical composition of the emitted smoke particles compared to ship-track-forming cargo ship emissions as well as background marine particles, with particular attention to the role of organic particles, (2) the characteristics of cloud track formation for smoke and cargo ships, as well as the role of multi-layered low clouds, and (3) the implications of these findings for quantifying aerosol indirect effects. For comparison with the E-PEACE results, the preliminary results of the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) 2012 provided evidence of the cloud-nucleating roles of both marine organic particles and coastal urban pollution, with simultaneous measurements of the effective supersaturations of the clouds in the California coastal region. © 2013 AIP Publishing LLC.
• Ryerson, T. B., Andrews, A. E., Angevine, W. M., Bates, T. S., Brock, C. A., Cairns, B., Cohen, R. C., Cooper, O. R., Gouw, J. D., Fehsenfeld, F. C., Ferrare, R. A., Fischer, M. L., Flagan, R. C., Goldstein, A. H., Hair, J. W., Hardesty, R. M., Hostetler, C. A., Jimenez, J. L., Langford, A. O., , McCauley, E., et al. (2013). The 2010 California Research at the Nexus of Air Quality and Climate Change (CalNex) field study. Journal of Geophysical Research D: Atmospheres, 118(11), 5830-5866.
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  Abstract: The California Research at the Nexus of Air Quality and Climate Change (CalNex) field study was conducted throughout California in May, June, and July of 2010. The study was organized to address issues simultaneously relevant to atmospheric pollution and climate change, including (1) emission inventory assessment, (2) atmospheric transport and dispersion, (3) atmospheric chemical processing, and (4) cloud-aerosol interactions and aerosol radiative effects. Measurements from networks of ground sites, a research ship, tall towers, balloon-borne ozonesondes, multiple aircraft, and satellites provided in situ and remotely sensed data on trace pollutant and greenhouse gas concentrations, aerosol chemical composition and microphysical properties, cloud microphysics, and meteorological parameters. This overview report provides operational information for the variety of sites, platforms, and measurements, their joint deployment strategy, and summarizes findings that have resulted from the collaborative analyses of the CalNex field study. Climate-relevant findings from CalNex include that leakage from natural gas infrastructure may account for the excess of observed methane over emission estimates in Los Angeles. Air-quality relevant findings include the following: mobile fleet VOC significantly declines, and NOx emissions continue to have an impact on ozone in the Los Angeles basin; the relative contributions of diesel and gasoline emission to secondary organic aerosol are not fully understood; and nighttime NO3 chemistry contributes significantly to secondary organic aerosol mass in the San Joaquin Valley. Findings simultaneously relevant to climate and air quality include the following: marine vessel emissions changes due to fuel sulfur and speed controls result in a net warming effect but have substantial positive impacts on local air quality. © 2013. American Geophysical Union. All Rights Reserved.
• Sorooshian, A., Murphy, S. M., Hersey, S., Bahreini, R., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2013). Constraining the contribution of organic acids and AMS m/z 44 to the organic aerosol budget: On the importance of meteorology, aerosol hygroscopicity, and region. GEOPHYSICAL RESEARCH LETTERS, 37.
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  Airborne measurements in regions of varying meteorology and pollution are used to quantify the contribution of organic acids and a mass spectral marker for oxygenated aerosols, m/z 44, to the total organic aerosol budget. Organic acids and m/z 44 separately are shown to exhibit their highest organic mass fractions in the vicinity of clouds. The contribution of such oxygenated species is shown to increase as a function of relative humidity, aerosol hygroscopicity (and decreasing organic mass fraction), and is typically greater off the California coast versus the continental atmospheres studied. Reasons include more efficient chemistry and partitioning of organic acid precursors with increasing water in the reaction medium, and high aqueous-phase processing times in boundary layers with higher cloud volume fractions. These results highlight the importance of secondary organic aerosol formation in both wet aerosols and cloud droplets. Citation: Sorooshian, A., S. M. Murphy, S. Hersey, R. Bahreini, H. Jonsson, R. C. Flagan, and J. H. Seinfeld (2010), Constraining the contribution of organic acids and AMS m/z 44 to the organic aerosol budget: On the importance of meteorology, aerosol hygroscopicity, and region, Geophys. Res. Lett., 37, L21807, doi: 10.1029/2010GL044951.
• Sorooshian, A., Shingler, T., Harpold, A., Feagles, C. W., Meixner, T., & Brooks, P. D. (2013). Aerosol and precipitation chemistry in the southwestern United States: Spatiotemporal trends and interrelationships. Atmospheric Chemistry and Physics, 13(15), 7361-7379.
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  Abstract: This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December-February) and during the monsoon season (July-September). Rain and snow pH levels are usually between 5-6, with crustal-derived species playing a major role in acid neutralization. These species (Ca2+, Mg2+, K+, Na+) exhibit their highest concentrations between March and June in both PM2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO42-, NO3-, and Cl-, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO42- show a statistically significant correlation with rain SO42- unlike snow SO42-, which may be related to some combination of the vertical distribution of SO 42- (and precursors) and the varying degree to which SO42-enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO3-: SO42- ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate NO3-: SO42- ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the NO3-: SO42- ratio in rain increased at the majority of sites due mostly to air pollution regulations of SO42- precursors. © 2013 Author(s).
• Sorooshian, A., Wang, Z., Coggon, M. M., Jonsson, H. H., & Ervens, B. (2013). Observations of Sharp Oxalate Reductions in Stratocumulus Clouds at Variable Altitudes: Organic Acid and Metal Measurements During the 2011 E-PEACE Campaign. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 47(14), 7747-7756.
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  This work examines organic acid and metal concentrations in northeastern Pacific Ocean stratocumulus cloudwater samples collected by the CIRPAS Twin Otter between July and August 2011. Correlations between a suite of various monocarboxylic and dicarboxylic acid concentrations are consistent with documented aqueous-phase mechanistic relationships leading up to oxalate production. Monocarboxylic and dicarboxylic acids exhibited contrasting spatial profiles reflecting their different sources; the former were higher in concentration near the continent due to fresh organic emissions. Concentrations of sea salt crustal tracer species, oxalate, and malonate were positively correlated with low-level wind speed suggesting that an important route for oxalate and malonate entry in cloudwater is via some combination of association with coarse particles and gaseous precursors emitted from the ocean surface. Three case flights show that oxalate (and no other organic acid) concentrations drop by nearly an order of magnitude relative to samples in the same vicinity. A consistent feature in these cases was an inverse relationship between oxalate and several metals (Fe, Mn, K, Na, Mg, Ca), especially Fe. By means of box model studies we show that the loss of oxalate due to the photolysis of iron oxalato complexes is likely a significant oxalate sink in the study region due to the ubiquity of oxalate precursors, clouds, and metal emissions from ships, the ocean, and continental sources.
• Sorooshian, A., Wang, Z., Coggon, M. M., Jonsson, H. H., & Ervens, B. (2013). Observations of sharp oxalate reductions in stratocumulus clouds at variable altitudes: Organic acid and metal measurements during the 2011 E-PEACE campaign. Environmental Science and Technology, 47(14), 7747-7756.
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  PMID: 23786214;Abstract: This work examines organic acid and metal concentrations in northeastern Pacific Ocean stratocumulus cloudwater samples collected by the CIRPAS Twin Otter between July and August 2011. Correlations between a suite of various monocarboxylic and dicarboxylic acid concentrations are consistent with documented aqueous-phase mechanistic relationships leading up to oxalate production. Monocarboxylic and dicarboxylic acids exhibited contrasting spatial profiles reflecting their different sources; the former were higher in concentration near the continent due to fresh organic emissions. Concentrations of sea salt crustal tracer species, oxalate, and malonate were positively correlated with low-level wind speed suggesting that an important route for oxalate and malonate entry in cloudwater is via some combination of association with coarse particles and gaseous precursors emitted from the ocean surface. Three case flights show that oxalate (and no other organic acid) concentrations drop by nearly an order of magnitude relative to samples in the same vicinity. A consistent feature in these cases was an inverse relationship between oxalate and several metals (Fe, Mn, K, Na, Mg, Ca), especially Fe. By means of box model studies we show that the loss of oxalate due to the photolysis of iron oxalato complexes is likely a significant oxalate sink in the study region due to the ubiquity of oxalate precursors, clouds, and metal emissions from ships, the ocean, and continental sources. © 2013 American Chemical Society.
• Sorooshian, A., Wang, Z., Feingold, G., & L'Ecuyer, T. S. (2013). A satellite perspective on cloud water to rain water conversion rates and relationships with environmental conditions. Journal of Geophysical Research D: Atmospheres, 118(12), 6643-6650.
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  Abstract: A two-year satellite remote sensing data set from the NASA A-Train is used to examine conversion rates of cloud water to rain water for warm maritime clouds with different ranges of mean cloud-layer radar reflectivity and rain rate. Recent work has demonstrated the utility of a novel procedure that relies on the differing sensitivities of passive MODIS measurements and active CloudSat radar measurements to estimate warm cloud conversion rates and associated time scales. That work is extended here to examine regional differences in conversion rates, including sensitivity to environmental parameters such as atmospheric stability and the presence of different aerosol types defined based on values of aerosol optical depth, fine mode fraction, and Ångstrom Exponent. Among eight subregions examined, the tropical Pacific Ocean is characterized by the highest average conversion rate while subtropical stratocumulus cloud regions (far northeastern Pacific Ocean, far southeastern Pacific Ocean, Western Africa coastal region) exhibit the lowest rates. Conversion rates are generally higher at reduced values of lower tropospheric static stability (LTSS). When examining data in two selected ranges for LTSS, higher conversion rates are coincident with higher LWP and factors covarying or rooted in the presence of aerosol types exhibiting lower aerosol index values. Key Points Cloud-to-rain water conversion parameters quantified using A-Train data Conversion faster in unstable environments with lower aerosol concentrations Technique is shown to be useful to examine relative trends in a global sense © 2013. American Geophysical Union. All Rights Reserved.
• Sorooshian, A., Youn, J., Wang, Z., Wonaschütz, A., Arellano, A., Betterton, E. A., & Sorooshian, A. -. (2013). Evidence of aqueous secondary organic aerosol formation from biogenic emissions in the North American Sonoran Desert. Geophysical research letters, 40(13).
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  This study examines the role of aqueous secondary organic aerosol formation in the North American Sonoran Desert as a result of intense solar radiation, enhanced moisture, and biogenic volatile organic compounds (BVOCs). The ratio of water-soluble organic carbon (WSOC) to organic carbon (OC) nearly doubles during the monsoon season relative to other seasons of the year. When normalized by mixing height, the WSOC enhancement during monsoon months relative to preceding dry months (May-June) exceeds that of sulfate by nearly a factor of 10. WSOC:OC and WSOC are most strongly correlated with moisture parameters, temperature, and concentrations of O3 and BVOCs. No positive relationship was identified between WSOC or WSOC:OC and anthropogenic tracers such as CO over a full year. This study points at the need for further work to understand the effect of BVOCs and moisture in altering aerosol properties in understudied desert regions.
• Surratt, J. D., Murphy, S. M., Kroll, J. H., Ng, a. L., Hildebrandt, L., Sorooshian, A., Szmigielski, R., Vermeylen, R., Maenhaut, W., Claeys, M., Flagan, R. C., & Seinfeld, J. H. (2013). Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene. JOURNAL OF PHYSICAL CHEMISTRY A, 110(31), 9665-9690.
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  Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C5H8) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NOx conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NOx level, with acidic products formed under high-NOx conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NOx conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise similar to 22-34% of the high-NOx SOA mass. Under low-NOx conditions, organic peroxides contribute significantly to the low-NOx SOA mass (similar to 61% when SOA forms by nucleation and similar to 25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C-5 alkene triols and 2-methyltetrols under low-NOx conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NOx chamber experiments.
• Wonaschütz, A., Coggon, M., Sorooshian, A., Modini, R., Frossard, A. A., Ahlm, L., Mülmenstädt, J., Roberts, G. C., Russell, L. M., Dey, S., Brechtel, F. J., & Seinfeld, J. H. (2013). Hygroscopic properties of smoke-generated organic aerosol particles emitted in the marine atmosphere. Atmospheric Chemistry and Physics, 13(19), 9819-9835.
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  Abstract: During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plumewere studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92 %, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm.-3), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at supersaturations between 0.07 and 0.88 %. Ratios of oxygen to carbon (O : C) andwater-soluble organic mass (WSOM) increased with plume age: From. © 2013 CC Attribution 3.0 License.
• Youn, J., Wang, Z., Wonaschütz, A., Arellano, A., Betterton, E. A., & Sorooshian, A. (2013). Evidence of aqueous secondary organic aerosol formation from biogenic emissions in the North American Sonoran Desert. Geophysical Research Letters, 40(13), 3468-3472.
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  Abstract: This study examines the role of aqueous secondary organic aerosol formation in the North American Sonoran Desert as a result of intense solar radiation, enhanced moisture, and biogenic volatile organic compounds (BVOCs). The ratio of water-soluble organic carbon (WSOC) to organic carbon (OC) nearly doubles during the monsoon season relative to other seasons of the year. When normalized by mixing height, the WSOC enhancement during monsoon months relative to preceding dry months (May-June) exceeds that of sulfate by nearly a factor of 10. WSOC:OC and WSOC are most strongly correlated with moisture parameters, temperature, and concentrations of O3 and BVOCs. No positive relationship was identified between WSOC or WSOC:OC and anthropogenic tracers such as CO over a full year. This study points at the need for further work to understand the effect of BVOCs and moisture in altering aerosol properties in understudied desert regions. Key Points Evidence of aqueous SOA formation in the Sonoran Desert Biogenic emissions assist in WSOC formation in this desert region WSOC production greatly exceeds that of sulfate during the monsoon season ©2013. American Geophysical Union. All Rights Reserved.
• Chen, Y. -., Christensen, M. W., Xue, L., Sorooshian, A., Stephens, G. L., Rasmussen, R. M., & Seinfeld, J. H. (2012). Occurrence of lower cloud albedo in ship tracks. Atmospheric Chemistry and Physics, 12(17), 8223-8235.
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  Abstract: The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming. © Author(s) 2012.
• Coggon, M. M., Sorooshian, A., Wang, Z., Metcalf, A. R., Frossard, A. A., Lin, J. J., Craven, J. S., Nenes, A., Jonsson, H. H., Russell, L. M., Flagan, R. C., & Seinfeld, J. H. (2012). Ship impacts on the marine atmosphere: Insights into the contribution of shipping emissions to the properties of marine aerosol and clouds. Atmospheric Chemistry and Physics, 12(18), 8439-8458.
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  Abstract: We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 μg m -3 and 510 cm -3, respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f/42) and 99 (f/99) in ship-impacted clouds. These ions are well correlated to each other (R 2>0.64) both in and out of cloud and constitute 14% (f/44) and 3% (f/99) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f/42 > 0.15; f/99 > 0.04) would imply heavy influence from shipping emissions, (0.05 < f/42 < 0.15; 0.01 < f/99 < 0.04) would imply moderate, but persistent, influences from ships, and (f/42 < 0.05; f/99 < 0.01) would imply clean, non-ship-influenced air. © 2012 Author(s).
• Duong, H. T., Sorooshian, A., & Feingold, G. (2012). Investigating potential biases in observed and modeled metrics of aerosol-cloud-precipitation interactions. ATMOSPHERIC CHEMISTRY AND PHYSICS, 11(9), 4027-4037.
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  This study utilizes large eddy simulation, aircraft measurements, and satellite observations to identify factors that bias the absolute magnitude of metrics of aerosol-cloud-precipitation interactions for warm clouds. The metrics considered are precipitation susceptibility So, which examines rain rate sensitivity to changes in drop number, and a cloud-precipitation metric, chi, which relates changes in rain rate to those in drop size. While wide ranges in rain rate exist at fixed cloud drop concentration for different cloud liquid water amounts, chi and S-o are shown to be relatively insensitive to the growth phase of the cloud for large datasets that include data representing the full spectrum of cloud lifetime. Spatial resolution of measurements is shown to influence the liquid water path-dependent behavior of S-o and chi. Other factors of importance are the choice of the minimum rain rate threshold, and how to quantify rain rate, drop size, and the cloud condensation nucleus proxy. Finally, low biases in retrieved aerosol amounts owing to wet scavenging and high biases associated with above-cloud aerosol layers should be accounted for. The paper explores the impact of these effects for model, satellite, and aircraft data.
• Lu, M., Sorooshian, A., Jonsson, H. H., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2012). Marine stratocumulus aerosol-cloud relationships in the MASE-II experiment: Precipitation susceptibility in eastern Pacific marine stratocumulus. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 114.
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  Observational data on aerosol-cloud-drizzle relationships in marine stratocumulus are presented from the second Marine Stratus/Stratocumulus Experiment (MASE-II) carried out in July 2007 over the eastern Pacific near Monterey, California. Observations, carried out in regions of essentially uniform meteorology with localized aerosol enhancements due to ship exhaust ("ship tracks''), demonstrate, in accord with those from numerous other field campaigns, that increased cloud drop number concentration N-c and decreased cloud top effective radius r(e) are associated with increased subcloud aerosol concentration. Modulation of drizzle by variations in aerosol levels is clearly evident. Variations of cloud base drizzle rate R-cb are found to be consistent with the proportionality, R-cb proportional to H-3/N-c, where H is cloud depth. Simultaneous aircraft and A-Train satellite observations are used to quantify the precipitation susceptibility of clouds to aerosol perturbations in the eastern Pacific region.
• Metcalf, A. R., Craven, J. S., Ensberg, J. J., Brioude, J., Angevine, W., Sorooshian, A., Duong, H. T., Jonsson, H. H., Flagan, R. C., & Seinfeld, J. H. (2012). Black carbon aerosol over the Los Angeles Basin during CalNex. Journal of Geophysical Research D: Atmospheres, 117(8).
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  Abstract: Refractory black carbon (rBC) mass and number concentrations were quantified by a Single Particle Soot Photometer (SP2) in the CalNex 2010 field study on board the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter in the Los Angeles (LA) Basin in May, 2010. The mass concentrations of rBC in the LA Basin ranged from 0.002-0.530 g m -3, with an average of 0.172 g m -3. Lower concentrations were measured in the Basin outflow regions and above the inversion layer. The SP2 afforded a quantification of the mixing state of rBC aerosols through modeling the scattering cross-section with a core-and-shell Mie model to determine coating thickness. The rBC particles above the inversion layer were more thickly coated by a light-scattering substance than those below, indicating a more aged aerosol in the free troposphere. Near the surface, as the LA plume is advected from west to east with the sea breeze, a coating of scattering material grows on rBC particles, coincident with a clear growth of ammonium nitrate within the LA Basin and the persistence of water-soluble organic compounds as the plume travels through the outflow regions. Detailed analysis of the rBC mixing state reveals two modes of coated rBC particles; a mode with smaller rBC core diameters (∼90 nm) but thick (>200 nm) coating diameters and a mode with larger rBC cores (∼145 nm) with a thin (
• Murphy, S. M., Agrawal, H., Sorooshian, A., Padro, L. T., Gates, H., Hersey, S., Welch, W. A., Jung, H., Miller, J. W., Cocker, D. R., Nenes, A., Jonsson, H. H., Flagan, R. C., & Seinfeld, J. H. (2012). Comprehensive Simultaneous Shipboard and Airborne Characterization of Exhaust from a Modern Container Ship at Sea. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 43(13), 4626-4640.
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  We report the first joint shipboard and airborne study focused on the chemical composition and water-uptake behavior of particulate ship emissions, The study focuses on emissions from the main propulsion engine of a Post-Panamax class container ship cruising off the central coast of California and burning heavy fuel oil. Shipboard sampling included micro-orifice uniform deposit impactors (MOUDI) with subsequent offline analysis, whereas airborne measurements involved a number of real-time analyzers to characterize the plume aerosol, aged from a few seconds to over an hour. The mass ratio of particulate organic carbon to sulfate at the base of the ship stack was 0.23 +/- 0.03, and increased to 0.30 +/- 0.01 in the airborne exhaust plume, with the additional organic mass in the airborne plume being concentrated largely in particles below 100 nm in diameter, The organic to sulfate mass ratio in the exhaust aerosol remained constant during the first hour of plume dilution into the marine boundary layer. The mass spectrum of the organic fraction of the exhaust aerosol strongly resembles that of emissions from other diesel sources and appears to be predominantly hydrocarbon-like organic (HOA) material, Background aerosol which, based on air mass back trajectories, probably consisted of aged ship emissions and marine aerosol, contained a lower organic mass fraction than the fresh plume and had a much more oxidized organic component A volume-weighted mixing rule is able to accurately predict hygroscopic growth factors in the background aerosol but measured and calculated growth factors do not agree for aerosols in the ship exhaust plume. Calculated CCN concentrations, at supersaturations ranging from 0.1 to 0.33%, agree well with measurements in the ship-exhaust plume. Using size-resolved chemical composition instead of bulk submicrometer composition has little effect on the predicted CCN concentrations because the cutoff diameter for CCN activation is larger than the diameter where the mass fraction of organic aerosol begins to increase significantly. The particle number emission factor estimated from this study is 1.3 x 10(16) (kg fuel)(-1), with less than 1/10 of the particles having diameters above 100 nm; 24% of particles (> 10 nm in diameter) activate into cloud droplets at 0.3% supersaturation.
• Ng, N. L., Chhabra, P. S., Chan, A. W., Surratt, J. D., Kroll, J. H., Kwan, A. J., McCabe, D. C., Wennberg, P. O., Sorooshian, A., Murphy, S. M., Dalleska, N. F., Flagan, R. C., & Seinfeld, J. H. (2012). Effect of NOx level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes. ATMOSPHERIC CHEMISTRY AND PHYSICS, 7(19), 5159-5174.
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  Secondary organic aerosol (SOA) formation from the photooxidation of one monoterpene (alpha- pinene) and two sesquiterpenes (longifolene and aromadendrene) is investigated in the Caltech environmental chambers. The effect of NOx on SOA formation for these biogenic hydrocarbons is evaluated by performing photooxidation experiments under varying NOx conditions. The NOx dependence of alpha-pinene SOA formation follows the same trend as that observed previously for a number of SOA precursors, including isoprene, in which SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) decreases as NOx level increases. The NOx dependence of SOA yield for the sesquiterpenes, longifolene and aromadendrene, however, differs from that determined for isoprene and alpha-pinene; the aerosol yield under high-NOx conditions substantially exceeds that under low-NOx conditions. The reversal of the NOx dependence of SOA formation for the sesquiterpenes is consistent with formation of relatively low-volatility organic nitrates, and/or the isomerization of large alkoxy radicals leading to less volatile products. Analysis of the aerosol chemical composition for longifolene confirms the presence of organic nitrates under high-NOx conditions. Consequently the formation of SOA from certain biogenic hydrocarbons such as sesquiterpenes (and possibly large anthropogenic hydrocarbons as well) may be more efficient in polluted air.
• Partridge, D. G., Vrugt, J. A., Tunved, P., Ekman, A. M., Struthers, H., & Sorooshian, A. (2012). Inverse modelling of cloud-aerosol interactions - Part 2: Sensitivity tests on liquid phase clouds using a Markov chain Monte Carlo based simulation approach. Atmospheric Chemistry and Physics, 12(6), 2823-2847.
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  Abstract: This paper presents a novel approach to investigate cloud-aerosol interactions by coupling a Markov chain Monte Carlo (MCMC) algorithm to an adiabatic cloud parcel model. Despite the number of numerical cloud-aerosol sensitivity studies previously conducted few have used statistical analysis tools to investigate the global sensitivity of a cloud model to input aerosol physiochemical parameters. Using numerically generated cloud droplet number concentration (CDNC) distributions (i.e. synthetic data) as cloud observations, this inverse modelling framework is shown to successfully estimate the correct calibration parameters, and their underlying posterior probability distribution. The employed analysis method provides a new, integrative framework to evaluate the global sensitivity of the derived CDNC distribution to the input parameters describing the lognormal properties of the accumulation mode aerosol and the particle chemistry. To a large extent, results from prior studies are confirmed, but the present study also provides some additional insights. There is a transition in relative sensitivity from very clean marine Arctic conditions where the lognormal aerosol parameters representing the accumulation mode aerosol number concentration and mean radius and are found to be most important for determining the CDNC distribution to very polluted continental environments (aerosol concentration in the accumulation mode >1000 cm-3) where particle chemistry is more important than both number concentration and size of the accumulation mode. The competition and compensation between the cloud model input parameters illustrates that if the soluble mass fraction is reduced, the aerosol number concentration, geometric standard deviation and mean radius of the accumulation mode must increase in order to achieve the same CDNC distribution. This study demonstrates that inverse modelling provides a flexible, transparent and integrative method for efficiently exploring cloud-aerosol interactions with respect to parameter sensitivity and correlation. © Author(s) 2012.
• Russell, L. M., Sorooshian, A., Seinfeld, J. H., Albrecht, B. A., Nenes, A., Ahlm, L., Chen, Y., Coggon, M., Craven, J. S., Flagan, R. C., Frossard, A. A., Jonsson, H., Jung, E., Lin, J. J., Metcalf, A. R., Modini, R., Muelmenstaedt, J., Roberts, G. C., Shingler, T., , Song, S., et al. (2012). EASTERN PACIFIC EMITTED AEROSOL CLOUD EXPERIMENT. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 94(5), 709-+.
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  Aerosol-cloud-radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models in addressing these important questions but quantifying them correctly is nontrivial, thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft and the research vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. E-PEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely 1) shipboard smoke-generated particles with 0.05-1-mu m diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), 2) combustion particles from container ships with 0.05-0.2-mu m diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and 3) aircraft-based milled salt particles with 3-5-mu m diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large-eddy simulations of deeper clouds in ship tracks and aerosol cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm-cloud microphysics.
• Shingler, T., Dey, S., Sorooshian, A., Brechtel, F. J., Wang, Z., Metcalf, A., Coggon, M., Mülmenstädt, J., Russell, L. M., Jonsson, H. H., & Seinfeld, J. H. (2012). Characterisation and airborne deployment of a new counterflow virtual impactor inlet. Atmospheric Measurement Techniques, 5(6), 1259-1269.
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  Abstract: A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min-1) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7-13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter. © 2012 Author(s).
• Sorooshian, A., Csavina, J., Shingler, T., Dey, S., Brechtel, F. J., Sáez, A. E., & Betterton, E. A. (2012). Hygroscopic and chemical properties of aerosols collected near a copper smelter: Implications for public and environmental health. Environmental Science and Technology, 46(17), 9473-9480.
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  PMID: 22852879;PMCID: PMC3435440;Abstract: Particulate matter emissions near active copper smelters and mine tailings in the southwestern United States pose a potential threat to nearby environments owing to toxic species that can be inhaled and deposited in various regions of the body depending on the composition and size of the particles, which are linked by particle hygroscopic properties. This study reports the first simultaneous measurements of size-resolved chemical and hygroscopic properties of particles next to an active copper smelter and mine tailings by the towns of Hayden and Winkelman in southern Arizona. Size-resolved particulate matter samples were examined with inductively coupled plasma mass spectrometry, ion chromatography, and a humidified tandem differential mobility analyzer. Aerosol particles collected at the measurement site are enriched in metals and metalloids (e.g., arsenic, lead, and cadmium) and water-uptake measurements of aqueous extracts of collected samples indicate that the particle diameter range of particles most enriched with these species (0.18-0.55 μm) overlaps with the most hygroscopic mode at a relative humidity of 90% (0.10-0.32 μm). These measurements have implications for public health, microphysical effects of aerosols, and regional impacts owing to the transport and deposition of contaminated aerosol particles. © 2012 American Chemical Society.
• Sorooshian, A., Feingold, G., Lebsock, M. D., Jiang, H., & Stephens, G. L. (2012). Deconstructing the precipitation susceptibility construct: Improving methodology for aerosol-cloud precipitation studies. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 115.
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  It is generally thought that an increase in aerosol particles suppresses precipitation in warm clouds. The nature and magnitude of this effect are highly uncertain owing to numerous microphysical and macrophysical processes that influence clouds over a wide range of spatial and temporal scales. This work addresses the need to improve the evidence for and quantification of aerosol effects on precipitation by using observational data. Previous work introduced the concept of precipitation susceptibility as a metric for changes in precipitation that result from aerosol perturbations. Motivated by the difficulty in obtaining statistically significant aerosol measurements in the vicinity of clouds, this study explores breaking up the precipitation susceptibility construct into separate components: an aerosol-cloud interaction component and a cloud-precipitation component. These are used to quantify precipitation susceptibility, while also accounting for meteorological factors that could obfuscate the response of clouds to aerosol perturbations. The utility of this technique is demonstrated using a diverse set of tools, including data from NASA's A-Train constellation of satellites, aircraft measurements, and models of various complexities. Employing this method results in increased confidence in causal relationships between aerosol perturbations and precipitation.
• Sorooshian, A., Ng, N. L., Chan, A. W., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2012). Particulate organic acids and overall water-soluble aerosol composition measurements from the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D13).
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  [1] The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter participated in the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) mission during August-September 2006. A particle-into-liquid sampler (PILS) coupled to ion chromatography was used to characterize the water-soluble ion composition of aerosol and cloud droplet residual particles (976 5-min PM1.0 samples in total). Sulfate and ammonium dominated the water-soluble mass (NH4+ + SO42- = 84 +/- 14%), while organic acids contributed 3.4 +/- 3.7%. The average NH4+:SO42- molar ratio was 1.77 +/- 0.85. Particulate concentrations of organic acids increased with decreasing carbon number from C-9 to C-2. Organic acids were most abundant above cloud, presumably as a result of aqueous phase chemistry in cloud droplets, followed by subsequent droplet evaporation above cloud tops; the main product of this chemistry was oxalic acid. The evolution of organic acids with increasing altitude in cloud provides evidence for the multistep nature of oxalic acid production; predictions from a cloud parcel model are consistent with the observed oxalate: glyoxylate ratio as a function of altitude in GoMACCS cumuli. Suppressed organic acid formation was observed in clouds with relatively acidic droplets, as determined by high particulate nitrate concentrations (presumably high HNO3 levels too) and lower liquid water content, as compared to other cloud fields probed. In the Houston Ship Channel region, an area with significant volatile organic compound emissions, oxalate, acetate, formate, benzoate, and pyruvate, in decreasing order, were the most abundant organic acids. Photo-oxidation of m-xylene in laboratory chamber experiments leads to a particulate organic acid product distribution consistent with the Ship Channel area observations.
• Wonaschuetz, A., Sorooshian, A., Ervens, B., Chuang, P. Y., Feingold, G., Murphy, S. M., Gouw, J. D., Warneke, C., & Jonsson, H. H. (2012). Aerosol and gas re-distribution by shallow cumulus clouds: An investigation using airborne measurements. Journal of Geophysical Research D: Atmospheres, 117(17).
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  Abstract: Aircraft measurements during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) are used to examine the influence of shallow cumulus clouds on vertical profiles of aerosol chemical composition, size distributions, and secondary aerosol precursor gases. The data show signatures of convective transport of particles, gases and moisture from near the surface to higher altitudes, and of aqueous-phase production of aerosol mass (sulfate and organics) in cloud droplets and aerosol water. In cloudy conditions, the average aerosol volume concentration at an altitude of 2850 m, above typical cloud top levels, was found to be 34% of that at 450 m; for clear conditions, the same ratio was 13%. Both organic and sulfate mass fractions were on average constant with altitude (around 50%); however, the ratio of oxalate to organic mass increased with altitude (from 1% at 450 m to almost 9% at 3450 m), indicative of the influence of in-cloud production on the vertical abundance and characteristics of secondary organic aerosol (SOA) mass. A new metric termed "residual cloud fraction" is introduced as a way of quantifying the "cloud processing history" of an air parcel. Results of a parcel model simulating aqueous phase production of sulfate and organics reproduce observed trends and point at a potentially important role of SOA production, especially oligomers, in deliquesced aerosols. The observations emphasize the importance of shallow cumulus clouds in altering the vertical distribution of aerosol properties that influence both their direct and indirect effect on climate. © 2012. American Geophysical Union. All Rights Reserved.
• Duong, H. T., Sorooshian, A., & Feingold, G. (2011). Investigating potential biases in observed and modeled metrics of aerosol-cloud-precipitation interactions. Atmospheric Chemistry and Physics, 11(9), 4027-4037.
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  Abstract: This study utilizes large eddy simulation, aircraft measurements, and satellite observations to identify factors that bias the absolute magnitude of metrics of aerosol-cloud-precipitation interactions for warm clouds. The metrics considered are precipitation susceptibility So, which examines rain rate sensitivity to changes in drop number, and a cloud-precipitation metric, X, which relates changes in rain rate to those in drop size. While wide ranges in rain rate exist at fixed cloud drop concentration for different cloud liquid water amounts, X and So are shown to be relatively insensitive to the growth phase of the cloud for large datasets that include data representing the full spectrum of cloud lifetime. Spatial resolution of measurements is shown to influence the liquid water path-dependent behavior of So and X. Other factors of importance are the choice of the minimum rain rate threshold, and how to quantify rain rate, drop size, and the cloud condensation nucleus proxy. Finally, low biases in retrieved aerosol amounts owing to wet scavenging and high biases associated with above-cloud aerosol layers should be accounted for. The paper explores the impact of these effects for model, satellite, and aircraft data. © 2011 Author(s).
• Duong, H. T., Sorooshian, A., Craven, J. S., Hersey, S. P., Metcalf, A. R., Zhang, X., Weber, R. J., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2011). Water-soluble organic aerosol in the Los Angeles Basin and outflow regions: Airborne and ground measurements during the 2010 CalNex field campaign. Journal of Geophysical Research D: Atmospheres, 116(22).
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  Abstract: A particle-into-liquid sampler coupled to a total organic carbon analyzer (PILS-TOC) quantified particulate water-soluble organic carbon (WSOC) mass concentrations during the May 2010 deployment of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter in the CalNex field study. WSOC data collected during 16 flights provide the first spatiotemporal maps of WSOC in the San Joaquin Valley, Los Angeles Basin, and outflow regions of the Basin. WSOC was consistently higher in concentration within the Los Angeles Basin, where sea breeze transport and Basin topography strongly influence the spatial distribution of WSOC. The highest WSOC levels were associated with fire plumes, highlighting the importance of both primary and secondary sources for WSOC in the region. Residual pollution layers enriched with WSOC are observed aloft up to an altitude of 3.2 km and the highest WSOC levels for each flight were typically observed above 500 m. Simultaneous ground WSOC measurements during aircraft overpasses in Pasadena and Riverside typically exhibit lower levels, especially when relative humidity (RH) was higher aloft suggestive of the influence of aerosol-phase water. This points to the underestimation of the radiative effects of WSOC when using only surface measurements. Reduced aerosol-phase water in the eastern desert outflow region likely promotes the re-partitioning of WSOC to the gas phase and suppression of processes to produce these species (partitioning, multiphase chemistry, photolytic production); as a result, WSOC is reduced relative to sulfate (but not as much as nitrate) as aerosol is advected from the Basin to the outflows. Copyright 2011 by the American Geophysical Union.
• Hersey, S. P., Craven, J. S., Schilling, K. A., Metcalf, A. R., Sorooshian, A., Chan, M. N., Flagan, R. C., & Seinfeld, J. H. (2011). The Pasadena Aerosol Characterization Observatory (PACO): Chemical and physical analysis of the Western Los Angeles basin aerosol. Atmospheric Chemistry and Physics, 11(15), 7417-7443.
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  Abstract: The Pasadena Aerosol Characterization Observatory (PACO) represents the first major aerosol characterization experiment centered in the Western/Central Los Angeles Basin. The sampling site, located on the campus of the California Institute of Technology in Pasadena, was positioned to sample a continuous afternoon influx of transported urban aerosol with a photochemical age of 1-2 h and generally free from major local contributions. Sampling spanned 5 months during the summer of 2009, which were broken into 3 regimes on the basis of distinct meteorological conditions. Regime I was characterized by a series of low pressure systems, resulting in high humidity and rainy periods with clean conditions. Regime II typified early summer meteorology, with significant morning marine layers and warm, sunny afternoons. Regime III was characterized by hot, dry conditions with little marine layer influence. Regardless of regime, organic aerosol (OA) is the most significant constituent of nonrefractory submicron Los Angeles aerosol (42, 43, and 55 % of total submicron mass in regimes I, II, and III, respectively). The overall oxidation state remains relatively constant on timescales of days to weeks (O:C Combining double low line 0.44 ± 0.08, 0.55 ± 0.05, and 0.48 ± 0.08 during regimes I, II, and III, respectively), with no difference in O:C between morning and afternoon periods. Periods characterized by significant morning marine layer influence followed by photochemically favorable afternoons displayed significantly higher aerosol mass and O:C ratio, suggesting that aqueous processes may be important in the generation of secondary aerosol and oxidized organic aerosol (OOA) in Los Angeles. Online analysis of water soluble organic carbon (WSOC) indicates that water soluble organic mass (WSOM) reaches maxima near 14:00-15:00 local time (LT), but the percentage of AMS organic mass contributed by WSOM remains relatively constant throughout the day. Sulfate and nitrate reside predominantly in accumulation mode aerosol, while afternoon SOA production coincides with the appearance of a distinct fine mode dominated by organics. Particulate NH4NO3 and (NH4)2SO4 appear to be NH3-limited in regimes I and II, but a significant excess of particulate NH4+ in the hot, dry regime III suggests less SO42- and the presence of either organic amines or NH4+-associated organic acids. C-ToF-AMS data were analyzed by Positive Matrix Factorization (PMF), which resolved three factors, corresponding to a hydrocarbon-like OA (HOA), semivolatile OOA (SV-OOA), and low-volatility OOA (LV-OOA). HOA appears to be a periodic plume source, while SV-OOA exhibits a strong diurnal pattern correlating with ozone. Peaks in SV-OOA concentration correspond to peaks in DMA number concentration and the appearance of a fine organic mode. LV-OOA appears to be an aged accumulation mode constituent that may be associated with aqueous-phase processing, correlating strongly with sulfate and representing the dominant background organic component. Periods characterized by high SV-OOA and LV-OOA were analyzed by filter analysis, revealing a complex mixture of species during periods dominated by SV-OOA and LV-OOA, with LV-OOA periods characterized by shorter-chain dicarboxylic acids (higher O:C ratio), as well as appreciable amounts of nitrate-and sulfate-substituted organics. Phthalic acid was ubiquitous in filter samples, suggesting that PAH photochemistry may be an important SOA pathway in Los Angeles. Aerosol composition was related to water uptake characteristics, and it is concluded that hygroscopicity is largely controlled by organic mass fraction (OMF). The hygroscopicity parameter averaged 0.31 ± 0.08, approaching 0.5 at low OMF and 0.1 at high OMF, with increasing OMF suppressing hygroscopic growth and increasing critical dry diameter for CCN activation (Dd). An experiment-averaged of 0.14 was calculated, indicating that the highly-oxidized organic fraction of aerosol in Los Angeles is appreciably more hygroscopic than previously reported in urban areas. Finally, PACO will provide context for results forthcoming from the CalNex field campaign, which involved ground sampling in Pasadena during the spring and summer of 2010. © 2011 Author(s).
• Hersey, S. P., Sorooshian, A., Murphy, S. M., Flagan, R. C., & Seinfeld, J. H. (2011). Aerosol hygroscopicity in the marine atmosphere: a closure study using high-time-resolution, multiple-RH DASH-SP and size-resolved C-ToF-AMS data. ATMOSPHERIC CHEMISTRY AND PHYSICS, 9(7), 2543-2554.
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  We have conducted the first airborne hygroscopic growth closure study to utilize data from an Aerodyne compact Time-of-Flight Aerosol Mass Spectrometer (C-ToF-AMS) coupled with size-resolved, multiple-RH, high-time-resolution hygroscopic growth factor (GF) measurements from the differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP). These data were collected off the coast of Central California during seven of the 16 flights carried out during the MASE-II field campaign in July 2007. Two of the seven flights were conducted in airmasses characterized by continental origin. These flights exhibited elevated organic volume fractions (VF(organic) = 0.56 +/- 0.19, as opposed to 0.39 +/- 0.20 for all other flights), corresponding to significantly suppressed GFs at high RH (1.61 +/- 0.14 at 92% RH, as compared with 1.91 +/- 0.07 for all other flights), more moderate GF suppression at intermediate RH (1.53 +/- 0.10 at 85%, compared with 1.58 +/- 0.08 for all other flights), and no measurable GF suppression at low RH (1.31 +/- 0.06 at 74%, compared with 1.31 +/- 0.07 for all other flights). Organic loadings were slightly elevated in above-cloud aerosols, as compared with below-cloud aerosols, and corresponded to a similar trend of significantly suppressed GF at high RH, but more moderate impacts at lower values of RH. A hygroscopic closure based on a volume-weighted mixing rule provided good agreement with DASH-SP measurements (R(2) = 0.78). Minimization of root mean square error between observations and predictions indicated mission-averaged organic GFs of 1.22, 1.45, and 1.48 at 74, 85, and 92% RH, respectively. These values agree with previously reported values for water-soluble organics such as dicarboxylic and multifunctional acids, and correspond to a highly oxidized, presumably water-soluble, organic fraction (mission-averaged O:C=0.92 +/- 0.33). Finally, a backward stepwise linear regression revealed that, other than RH, the most important predictor for GF is VF(organic), indicating that a simple emperical model relating GF, RH, and the relative abundance of organic material can provide accurate predictions (R(2) = 0.77) of hygroscopic growth for the California coast.
• Partridge, D. G., Vrugt, J. A., Tunved, P., Ekman, A. M., Gorea, D., & Sorooshian, A. (2011). Inverse modeling of cloud-aerosol interactions-Part 1: Detailed response surface analysis. Atmospheric Chemistry and Physics, 11(14), 7269-7287.
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  Abstract: New methodologies are required to probe the sensitivity of parameters describing cloud droplet activation. This paper presents an inverse modeling-based method for exploring cloud-aerosol interactions via response surfaces. The objective function, containing the difference between the measured and model predicted cloud droplet size distribution is studied in a two-dimensional framework, and presented for pseudo-adiabatic cloud parcel model parameters that are pair-wise selected. From this response surface analysis it is shown that the susceptibility of cloud droplet size distribution to variations in different aerosol physiochemical parameters is highly dependent on the aerosol environment and meteorological conditions. In general the cloud droplet size distribution is most susceptible to changes in the updraft velocity. A shift towards an increase in the importance of chemistry for the cloud nucleating ability of particles is shown to exist somewhere between marine average and rural continental aerosol regimes. We also use these response surfaces to explore the feasibility of inverse modeling to determine cloud-aerosol interactions. It is shown that the "cloud- aerosol" inverse problem is particularly difficult to solve due to significant parameter interaction, presence of multiple regions of attraction, numerous local optima, and considerable parameter insensitivity. The identifiability of the model parameters will be dependent on the choice of the objective function. Sensitivity analysis is performed to investigate the location of the information content within the calibration data to confirm that our choice of objective function maximizes information retrieval from the cloud droplet size distribution. Cloud parcel models that employ a moving-centre based calculation of the cloud droplet size distribution pose additional difficulties when applying automatic search algorithms for studying cloud-aerosol interactions. To aid future studies, an increased resolution of the region of the size spectrum associated with droplet activation within cloud parcel models, or further development of fixed-sectional cloud models would be beneficial. Despite these improvements, it is demonstrated that powerful search algorithms remain necessary to efficiently explore the parameter space and successfully solve the cloud-aerosol inverse problem. © 2011 Author(s).
• Partridge, D. G., Vrugt, J. A., Tunved, P., Ekman, A. M., Struthers, H., & Sorooshian, A. (2011). Inverse modelling of cloud-aerosol interactions - Part 2: Sensitivity tests on liquid phase clouds using a Markov chain Monte Carlo based simulation approach. ATMOSPHERIC CHEMISTRY AND PHYSICS, 12(6), 2823-2847.
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  This paper presents a novel approach to investigate cloud-aerosol interactions by coupling a Markov chain Monte Carlo (MCMC) algorithm to an adiabatic cloud parcel model. Despite the number of numerical cloud-aerosol sensitivity studies previously conducted few have used statistical analysis tools to investigate the global sensitivity of a cloud model to input aerosol physiochemical parameters. Using numerically generated cloud droplet number concentration (CDNC) distributions (i.e. synthetic data) as cloud observations, this inverse modelling framework is shown to successfully estimate the correct calibration parameters, and their underlying posterior probability distribution.
• Sorooshian, A., Lu, M., Brechtel, F. J., Jonsson, H., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2011). On the source of organic acid aerosol layers above clouds. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 41(13), 4647-4654.
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  During the July 2005 Marine Stratus/Stratocumulus Experiment (MASE) and the August-September 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS),the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed aerosols and cumulus clouds in the eastern Pacific Ocean off the coast of northern California and in southeastern Texas, respectively. An on-board particle-into-liquid sampler PILS) quantified inorganic and organic acid species with
• Sorooshian, A., Wonaschtz, A., Jarjour, E. G., Hashimoto, B. I., Schichtel, B. A., & Betterton, E. A. (2011). An aerosol climatology for a rapidly growing arid region (southern Arizona): Major aerosol species and remotely sensed aerosol properties. Journal of Geophysical Research D: Atmospheres, 116(19).
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  Abstract: This study reports a comprehensive characterization of atmospheric aerosol particle properties in relation to meteorological and back trajectory data in the southern Arizona region, which includes two of the fastest growing metropolitan areas in the United States (Phoenix and Tucson). Multiple data sets (MODIS, AERONET, OMI/TOMS, MISR, GOCART, ground-based aerosol measurements) are used to examine monthly trends in aerosol composition, aerosol optical depth (AOD), and aerosol size. Fine soil, sulfate, and organics dominate PM 2.5 mass in the region. Dust strongly influences the region between March and July owing to the dry and hot meteorological conditions and back trajectory patterns. Because monsoon precipitation begins typically in July, dust levels decrease, while AOD, sulfate, and organic aerosol reach their maximum levels because of summertime photochemistry and monsoon moisture. Evidence points to biogenic volatile organic compounds being a significant source of secondary organic aerosol in this region. Biomass burning also is shown to be a major contributor to the carbonaceous aerosol budget in the region, leading to enhanced organic and elemental carbon levels aloft at a sky-island site north of Tucson (Mt. Lemmon). Phoenix exhibits different monthly trends for aerosol components in comparison with the other sites owing to the strong influence of fossil carbon and anthropogenic dust. Trend analyses between 1988 and 2009 indicate that the strongest statistically significant trends are reductions in sulfate, elemental carbon, and organic carbon, and increases in fine soil during the spring (March-May) at select sites. These results can be explained by population growth, land-use changes, and improved source controls. Copyright © 2011 by the American Geophysical Union.
• Szmigielski, R., Surratt, J. D., Vermeylen, R., Szmigielska, K., Kroll, J. H., Ng, N. L., Murphy, S. M., Sorooshian, A., Seinfeld, J. H., & Claeys, M. (2011). Characterization of 2-methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry. JOURNAL OF MASS SPECTROMETRY, 42(1), 101-116.
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  In the present work, we have characterized in detail the chemical structures of secondary organic aerosol (SOA) components that were generated in a smog chamber and result from the photooxidation of isoprene under high-NOx conditions typical for a polluted atmosphere. Isoprene high-NOx SOA contains 2-methylglyceric acid (2-MG) and oligoester derivatives thereof. Trimethylsilylation, in combination with capillary gas chromatography (GQ/ion trap mass spectrometry (MS) and detailed interpretation of the MS data, allowed structural characterization the polar oxygenated compounds present in isoprene SOA up to 2-MG trimers. GC separation was achieved between 2-MG linear and branched dimers or trimers, as well as between the 2-MG linear dimer and isomeric mono-acetate derivatives thereof. The electron ionization (EI) spectra of the trimethylsilyl derivatives contain a wealth of structural information, including information about the molecular weight (MW), oligoester linkages, terminal carboxylic and hydroxymethyl groups, and esterification sites. Only part of this information can be achieved with a soft ionization technique such as electrospray (ESI) in combination with collision-induced dissociation (CID). The methane chemical ionization (CI) data were used to obtain supporting MW information. Interesting El spectral differences were observed between the trimethylsilyl derivatives of 2-MG linear and branched dimers or trimers and between 2-MG linear dimer mono-acetate isomers. Copyright (c) 2006 John Wiley & Sons, Ltd.
• Wonaschuetz, A., Hersey, S. P., Sorooshian, A., Craven, J. S., Metcalf, A. R., Flagan, R. C., & Seinfeld, J. H. (2011). Impact of a large wildfire on water-soluble organic aerosol in a major urban area: the 2009 Station Fire in Los Angeles County. ATMOSPHERIC CHEMISTRY AND PHYSICS, 11(16), 8257-8270.
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  This study examines the nature of water-soluble organic aerosol measured in Pasadena, CA, under typical conditions and under the influence of a large wildfire (the 2009 Station Fire). During non-fire periods, water-soluble organic carbon (WSOC) variability was driven by photochemical production processes and sea breeze transport, resulting in an average diurnal cycle with a maximum at 15: 00 local time (up to 4.9 mu g Cm-3). During the Station Fire, primary production was a key formation mechanism for WSOC. High concentrations of WSOC (up to 41 mu g Cm-3) in smoke plumes advected to the site in the morning hours were tightly correlated with nitrate and chloride, numerous aerosol mass spectrometer (AMS) organic mass spectral markers, and total non-refractory organic mass. Processed residual smoke was transported to the measurement site by the sea breeze later in the day, leading to higher afternoon WSOC levels than on non-fire days. Parameters representing higher degrees of oxidation of organics, including the ratios of the organic metrics m/z 44: m/z 57 and m/z 44: m/z 43, were elevated in those air masses. Intercomparisons of relative amounts of WSOC, organics, m/z 44, and m/z 43 show that the fraction of WSOC comprising acid-oxygenates increased as a function of photochemical aging owing to the conversion of aliphatic and non-acid oxygenated organics to more acid-like organics. The contribution of water-soluble organic species to the organic mass budget (10th-90th percentile values) ranged between 27%-72% and 27%-68% during fire and non-fire periods, respectively. The seasonal incidence of wildfires in the Los Angeles Basin greatly enhances the importance of water-soluble organics, which has implications for the radiative and hygroscopic properties of the regional aerosol.
• Wonaschütz, A., Hersey, S. P., Sorooshian, A., Craven, J. S., Metcalf, A. R., Flagan, R. C., & Seinfeld, J. H. (2011). Impact of a large wildfire on water-soluble organic aerosol in a major urban area: The 2009 station fire in Los Angeles County. Atmospheric Chemistry and Physics, 11(16), 8257-8270.
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  Abstract: This study examines the nature of water-soluble organic aerosol measured in Pasadena, CA, under typical conditions and under the influence of a large wildfire (the 2009 Station Fire). During non-fire periods, water-soluble organic carbon (WSOC) variability was driven by photochemical production processes and sea breeze transport, resulting in an average diurnal cycle with a maximum at 15:00 local time (up to 4.9 μg Cm-3). During the Station Fire, primary production was a key formation mechanism for WSOC. High concentrations of WSOC (up to 41 μg Cm-3) in smoke plumes advected to the site in the morning hours were tightly correlated with nitrate and chloride, numerous aerosol mass spectrometer (AMS) organic mass spectral markers, and total non-refractory organic mass. Processed residual smoke was transported to the measurement site by the sea breeze later in the day, leading to higher afternoon WSOC levels than on non-fire days. Parameters representing higher degrees of oxidation of organics, including the ratios of the organic metrics m/z 44:;m/z 57 and ;m/z 44:;m/z 43, were elevated in those air masses. Intercomparisons of relative amounts of WSOC, organics, ;m/z 44, and m/z 43 show that the fraction of WSOC comprising acid-oxygenates increased as a function of photochemical aging owing to the conversion of aliphatic and non-acid oxygenated organics to more acid-like organics. The contribution of water-soluble organic species to the organic mass budget (10th-90th percentile values) ranged between 27 %-72 % and 27 %-68 % during fire and non-fire periods, respectively. The seasonal incidence of wildfires in the Los Angeles Basin greatly enhances the importance of water-soluble organics, which has implications for the radiative and hygroscopic properties of the regional aerosol. © 2011 Author(s).
• Jiang, H., Feingold, G., & Sorooshian, A. (2010). Effect of aerosol on the susceptibility and efficiency of precipitation in warm trade cumulus clouds. Journal of the Atmospheric Sciences, 67(11), 3525-3540.
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  Abstract: Large-eddy simulations of warm, trade wind cumulus clouds are conducted for a range of aerosol conditions with a focus on precipitating clouds. Individual clouds are tracked over the course of their lifetimes. Precipitation rate decreases progressively as aerosol increases. For larger, precipitating clouds, the polluted clouds have longer lifetimes because of precipitation suppression. For clean aerosol conditions, there is good agreement between the average model precipitation rate and that calculated based on observed radar reflectivity Z and precipitation rate R relationships. Precipitation rate can be expressed as a power-law function of liquid water path (LWP) and Nd, to reasonable accuracy. The respective powers for LWP and Nd are of similar magnitude compared to those based on observational studies of stratocumulus clouds. The timeintegrated precipitation rate represented by a power-law function of LWP, Nd, and cloud lifetime is much more reliably predicted than is R expressed in terms of LWP and Nd alone. The precipitation susceptibility (So = -dlnR/dlnNd) that quantifies the sensitivity of precipitation to changes in Nd depends strongly on LWP and exhibits nonmonotonic behavior with a maximumat intermediate LWP values. The relationship between So and precipitation efficiency is explored and the importance of including dependence on Nd in the latter is highlighted. The results provide trade cumulus cloud population statistics, as well as relationships between microphysical/macrophysical properties and precipitation, that are amenable for use in larger-scale models. © 2010 American Meteorological Society.
• Sorooshian, A., Feingold, G., Lebsock, M. D., Jiang, H., & Stephens, G. L. (2010). Deconstructing the precipitation susceptibility construct: Improving methodology for aerosol-cloud precipitation studies. Journal of Geophysical Research D: Atmospheres, 115(17).
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  Abstract: It is generally thought that an increase in aerosol particles suppresses precipitation in warm clouds. The nature and magnitude of this effect are highly uncertain owing to numerous microphysical and macrophysical processes that influence clouds over a wide range of spatial and temporal scales. This work addresses the need to improve the evidence for and quantification of aerosol effects on precipitation by using observational data. Previous work introduced the concept of precipitation susceptibility as a metric for changes in precipitation that result from aerosol perturbations. Motivated by the difficulty in obtaining statistically significant aerosol measurements in the vicinity of clouds, this study explores breaking up the precipitation susceptibility construct into separate components: an aerosol-cloud interaction component and a cloud-precipitation component. These are used to quantify precipitation susceptibility, while also accounting for meteorological factors that could obfuscate the response of clouds to aerosol perturbations. The utility of this technique is demonstrated using a diverse set of tools, including data from NASA's A-Train constellation of satellites, aircraft measurements, and models of various complexities. Employing this method results in increased confidence in causal relationships between aerosol perturbations and precipitation. © Copyright 2010 by the American Geophysical Union.
• Sorooshian, A., Murphy, S. M., Hersey, S., Bahreini, R., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2010). Constraining the contribution of organic acids and AMS m/z 44 to the organic aerosol budget: On the importance of meteorology, aerosol hygroscopicity, and region. Geophysical Research Letters, 37(21).
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  Abstract: Airborne measurements in regions of varying meteorology and pollution are used to quantify the contribution of organic acids and a mass spectral marker for oxygenated aerosols, m/z 44, to the total organic aerosol budget. Organic acids and m/z 44 separately are shown to exhibit their highest organic mass fractions in the vicinity of clouds. The contribution of such oxygenated species is shown to increase as a function of relative humidity, aerosol hygroscopicity (and decreasing organic mass fraction), and is typically greater off the California coast versus the continental atmospheres studied. Reasons include more efficient chemistry and partitioning of organic acid precursors with increasing water in the reaction medium, and high aqueous-phase processing times in boundary layers with higher cloud volume fractions. These results highlight the importance of secondary organic aerosol formation in both wet aerosols and cloud droplets. Copyright 2010 by the American Geophysical Union.
• Surratt, J. D., Kroll, J. H., Kleindienst, T. E., Edney, E. O., Claeys, M., Sorooshian, A., Ng, N. L., Offenberg, J. H., Lewandowski, M., Jaoui, M., Flagan, R. C., & Seinfeld, J. H. (2010). Evidence for organosulfates in secondary organic aerosol. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 41(2), 517-527.
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  Recent work has shown that particle-phase reactions contribute to the formation of secondary organic aerosol (SOA), with enhancements of SOA yields in the presence of acidic seed aerosol. In this study, the chemical composition of SOA from the photooxidations of alpha-pinene and isoprene, in the presence or absence of sulfate seed aerosol, is investigated through a series of controlled chamber experiments in two separate laboratories. By using electrospray ionization-mass spectrometry, sulfate esters in SOA produced in laboratory photooxidation experiments are identified for the first time. Sulfate esters are found to account for a larger fraction of the SOA mass when the acidity of seed aerosol is increased, a result consistent with aerosol acidity increasing SOA formation. Many of the isoprene and alpha-pinene sulfate esters identified in these chamber experiments are also found in ambient aerosol collected at several locations in the southeastern U.S. It is likely that this pathway is important for other biogenic terpenes, and may be important in the formation of humic-like substances (HULIS) in ambient aerosol.
• Wonaschuetz, A., Coggon, M., Sorooshian, A., Modini, R., Frossard, A. A., Ahlm, L., Muelmenstaedt, J., Roberts, G. C., Russell, L. M., Dey, S., Brechtel, F. J., & Seinfeld, J. H. (2010). Hygroscopic properties of smoke-generated organic aerosol particles emitted in the marine atmosphere. ATMOSPHERIC CHEMISTRY AND PHYSICS, 13(19), 9819-9835.
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  During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plume were studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92 %, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm(-3)), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at super-saturations between 0.07 and 0.88 %. Ratios of oxygen to carbon (O : C) and water-soluble organic mass (WSOM) increased with plume age: from
• Wonaschuetz, A., Sorooshian, A., Ervens, B., Chuang, P. Y., Feingold, G., Murphy, S. M., de Gouw, J., Warneke, C., & Jonsson, H. H. (2010). Aerosol and gas re-distribution by shallow cumulus clouds: An investigation using airborne measurements. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 117.
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  Aircraft measurements during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) are used to examine the influence of shallow cumulus clouds on vertical profiles of aerosol chemical composition, size distributions, and secondary aerosol precursor gases. The data show signatures of convective transport of particles, gases and moisture from near the surface to higher altitudes, and of aqueous-phase production of aerosol mass (sulfate and organics) in cloud droplets and aerosol water. In cloudy conditions, the average aerosol volume concentration at an altitude of 2850 m, above typical cloud top levels, was found to be 34% of that at 450 m; for clear conditions, the same ratio was 13%. Both organic and sulfate mass fractions were on average constant with altitude (around 50%); however, the ratio of oxalate to organic mass increased with altitude (from 1% at 450 m to almost 9% at 3450 m), indicative of the influence of in-cloud production on the vertical abundance and characteristics of secondary organic aerosol (SOA) mass. A new metric termed "residual cloud fraction" is introduced as a way of quantifying the "cloud processing history" of an air parcel. Results of a parcel model simulating aqueous phase production of sulfate and organics reproduce observed trends and point at a potentially important role of SOA production, especially oligomers, in deliquesced aerosols. The observations emphasize the importance of shallow cumulus clouds in altering the vertical distribution of aerosol properties that influence both their direct and indirect effect on climate.
• Coggon, M. M., Sorooshian, A., Wang, Z., Metcalf, A. R., Frossard, A. A., Lin, J. J., Craven, J. S., Nenes, A., Jonsson, H. H., Russell, L. M., Flagan, R. C., & Seinfeld, J. H. (2009). Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds. ATMOSPHERIC CHEMISTRY AND PHYSICS, 12(18), 8439-8458.
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  We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 mu g m(-3) and 510 cm(-3), respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f(42)) and 99 (f(99)) in ship-impacted clouds. These ions are well correlated to each other (R-2 > 0.64) both in and out of cloud and constitute 14% (f(44)) and 3% (f(99)) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f(42) > 0.15; f(99) > 0.04) would imply heavy influence from shipping emissions, (0.05 < f(42) < 0.15; 0.01 < f(99) < 0.04) would imply moderate, but persistent, influences from ships, and (f(42) < 0.05; f(99) < 0.01) would imply clean, non-ship-influenced air.
• Crosbie, E., Sorooshian, A., Monfared, N. A., Shingler, T., & Esmaili, O. (2009). A Multi-Year Aerosol Characterization for the Greater Tehran Area Using Satellite, Surface, and Modeling Data. ATMOSPHERE, 5(2), 178-197.
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  This study reports a multi-year (2000-2009) aerosol characterization for metropolitan Tehran and surrounding areas using multiple datasets (Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging Spectroradiometer (MISR), Total Ozone Mapping Spectrometer (TOMS), Goddard Ozone Chemistry Aerosol Radiation and Transport (GOCART), and surface and upper air data from local stations). Monthly trends in aerosol characteristics are examined in the context of the local meteorology, regional and local emission sources, and air mass back-trajectory data. Dust strongly affects the region during the late spring and summer months (May-August) when aerosol optical depth (AOD) is at its peak and precipitation accumulation is at a minimum. In addition, the peak AOD that occurs in July is further enhanced by a substantial number of seasonal wildfires in upwind regions. Conversely, AOD is at a minimum during winter; however, reduced mixing heights and a stagnant lower atmosphere trap local aerosol emissions near the surface and lead to significant reductions in visibility within Tehran. The unique meteorology and topographic setting makes wintertime visibility and surface aerosol concentrations particularly sensitive to local anthropogenic sources and is evident in the noteworthy improvement in visibility observed on weekends. Scavenging of aerosol due to precipitation is evident during the winter when a consistent increase in surface visibility and concurrent decrease in AOD is observed in the days after rain compared with the days immediately before rain.
• Hersey, S. P., Craven, J. S., Schilling, K. A., Metcalf, A. R., Sorooshian, A., Chan, M. N., Flagan, R. C., & Seinfeld, J. H. (2009). The Pasadena Aerosol Characterization Observatory (PACO): chemical and physical analysis of the Western Los Angeles basin aerosol. ATMOSPHERIC CHEMISTRY AND PHYSICS, 11(15), 7417-7443.
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  The Pasadena Aerosol Characterization Observatory (PACO) represents the first major aerosol characterization experiment centered in the Western/Central Los Angeles Basin. The sampling site, located on the campus of the California Institute of Technology in Pasadena, was positioned to sample a continuous afternoon influx of transported urban aerosol with a photochemical age of 1-2 h and generally free from major local contributions. Sampling spanned 5 months during the summer of 2009, which were broken into 3 regimes on the basis of distinct meteorological conditions. Regime I was characterized by a series of low pressure systems, resulting in high humidity and rainy periods with clean conditions. Regime II typified early summer meteorology, with significant morning marine layers and warm, sunny afternoons. Regime III was characterized by hot, dry conditions with little marine layer influence. Regardless of regime, organic aerosol (OA) is the most significant constituent of nonrefractory submicron Los Angeles aerosol (42, 43, and 55% of total submicron mass in regimes I, II, and III, respectively). The overall oxidation state remains relatively constant on timescales of days to weeks (O:C = 0.44 +/- 0.08, 0.55 +/- 0.05, and 0.48 +/- 0.08 during regimes I, II, and III, respectively), with no difference in O:C between morning and afternoon periods. Periods characterized by significant morning marine layer influence followed by photochemically favorable afternoons displayed significantly higher aerosol mass and O:C ratio, suggesting that aqueous processes may be important in the generation of secondary aerosol and oxidized organic aerosol (OOA) in Los Angeles. Online analysis of water soluble organic carbon (WSOC) indicates that water soluble organic mass (WSOM) reaches maxima near 14:00-15:00 local time (LT), but the percentage of AMS organic mass contributed by WSOM remains relatively constant throughout the day. Sulfate and nitrate reside predominantly in accumulation mode aerosol, while afternoon SOA production coincides with the appearance of a distinct fine mode dominated by organics. Particulate NH4NO3 and (NH4)(2)SO4 appear to be NH3-limited in regimes I and II, but a significant excess of particulate NH4+ in the hot, dry regime III suggests less SO42- and the presence of either organic amines or NH4+-associated organic acids. C-ToF-AMS data were analyzed by Positive Matrix Factorization (PMF), which resolved three factors, corresponding to a hydrocarbon-like OA (HOA), semivolatile OOA (SV-OOA), and low-volatility OOA (LV-OOA). HOA appears to be a periodic plume source, while SV-OOA exhibits a strong diurnal pattern correlating with ozone. Peaks in SV-OOA concentration correspond to peaks in DMA number concentration and the appearance of a fine organic mode. LV-OOA appears to be an aged accumulation mode constituent that may be associated with aqueous-phase processing, correlating strongly with sulfate and representing the dominant background organic component. Periods characterized by high SV-OOA and LV-OOA were analyzed by filter analysis, revealing a complex mixture of species during periods dominated by SV-OOA and LV-OOA, with LV-OOA periods characterized by shorter-chain dicarboxylic acids (higher O:C ratio), as well as appreciable amounts of nitrate-and sulfate-substituted organics.
• Hersey, S. P., Sorooshian, A., Murphy, S. M., Flagan, R. C., & Seinfeld, J. H. (2009). Aerosol hygroscopicity in the marine atmosphere: A closure study using high-time-resolution, multiple-RH DASH-SP and size-resolved C-ToF-AMS data. Atmospheric Chemistry and Physics, 9(7), 2543-2554.
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  Abstract: We have conducted the first airborne hygroscopic growth closure study to utilize data from an Aerodyne compact Time-of-Flight Aerosol Mass Spectrometer (C-ToF-AMS) coupled with size-resolved, multiple-RH, high-time-resolution hygroscopic growth factor (GF) measurements from the differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP). These data were collected off the coast of Central California during seven of the 16 flights carried out during the MASE-II field campaign in July 2007. Two of the seven flights were conducted in airmasses characterized by continental origin. These flights exhibited elevated organic volume fractions (VForganic=0.56±0. 19, as opposed to 0.39±0.20 for all other flights), corresponding to significantly suppressed GFs at high RH (1.61±0.14 at 92% RH, as compared with 1.91±0.07 for all other flights), more moderate GF suppression at intermediate RH (1.53±0.10 at 85%, compared with 1.58±0.08 for all other flights), and no measurable GF suppression at low RH (1.31±0.06 at 74%, compared with 1.31±0.07 for all other flights). Organic loadings were slightly elevated in above-cloud aerosols, as compared with below-cloud aerosols, and corresponded to a similar trend of significantly suppressed GF at high RH, but more moderate impacts at lower values of RH. A hygroscopic closure based on a volume-weighted mixing rule provided good agreement with DASH-SP measurements (R2=0.78). Minimization of root mean square error between observations and predictions indicated mission-averaged organic GFs of 1.22, 1.45, and 1.48 at 74, 85, and 92% RH, respectively. These values agree with previously reported values for water-soluble organics such as dicarboxylic and multifunctional acids, and correspond to a highly oxidized, presumably water-soluble, organic fraction (mission-averaged O:C=0.92±0.33). Finally, a backward stepwise linear regression revealed that, other than RH, the most important predictor for GF is Vívanlo indicating that a simple emperical model relating GF, RH, and the relative abundance of organic material can provide accurate predictions (R2=0.77) of hygroscopic growth for the California coast.
• Lu, M., Sorooshian, A., Jonsson, H. H., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2009). Marine stratocumulus aerosol-cloud relationships in the MASE-II experiment: Precipitation susceptibility in eastern Pacific marine stratocumulus. Journal of Geophysical Research D: Atmospheres, 114(24).
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  Abstract: [1] Observational data on aerosol-cloud-drizzle relationships in marine stratocumulus are presented from the second Marine Stratus/Stratocumulus Experiment (MASE-II) carried out in July 2007 over the eastern Pacific near Monterey, California. Observations, carried out in regions of essentially uniform meteorology with localized aerosol enhancements due to ship exhaust ("ship tracks"), demonstrate, in accord with those from numerous other field campaigns, that increased cloud drop number concentration Nc and decreased cloud top effective radius re are associated with increased subcloud aerosol concentration. Modulation of drizzle by variations in aerosol levels is clearly evident. Variations of cloud base drizzle rate R cb are found to be consistent with the proportionality, R cb ∝ H3/Nc, where H is cloud depth. Simultaneous aircraft and A-Train satellite observations are used to quantify the precipitation susceptibility of clouds to aerosol perturbations in the eastern Pacific region. Copyright 2009 by the American Geophysical Union.
• Murphy, S., Agrawal, H., Sorooshian, A., Padró, L. T., Gates, H., Hersey, S., Welch, W. A., Jung, H., Miller, J. W., R., D., Nenes, A., Jonsson, H. H., Flagan, R. C., & Seinfeld, J. H. (2009). Comprehensive simultaneous shipboard and airborne characterization of exhaust from a modern container ship at sea. Environmental Science and Technology, 43(13), 4626-4640.
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  PMID: 19673244;Abstract: We report the first joint shipboard and airborne study focused on the chemical composition and water-uptake behavior of particulate ship emissions. The study focuses on emissions from the main propulsion engine of a Post-Panamax class container ship cruising off the central coast of California and burning heavy fuel oil. Shipboard sampling included micro-orifice uniform deposit impactors (MOUDI) with subsequent offline analysis, whereas airborne measurements involved a number of real-time analyzers to characterize the plume aerosol, aged from a few seconds to over an hour. The mass ratio of particulate organic carbon to sulfate at the base of the ship stack was 0.23 ± 0.03, and increased to 0.30 ± 0.01 in the airborne exhaust plume, with the additional organic mass in the airborne plume being concentrated largely in particles below 100 nm in diameter. The organic to sulfate mass ratio in the exhaust aerosol remained constant during the first hour of plume dilution into the marine boundary layer. The mass spectrum of the organic fraction of the exhaust aerosol strongly resembles that of emissions from other diesel sources and appears to be predominantly hydrocarbon-like organic (HOA) material. Background aerosol which, based on air mass back trajectories, probably consisted of aged ship emissions and marine aerosol, contained a lower organic mass fraction than the fresh plume and had a much more oxidized organic component. A volume-weighted mixing rule is able to accurately predict hygroscopic growth factors in the background aerosol but measured and calculated growth factors do not agree for aerosols in the ship exhaust plume. Calculated CCN concentrations, at supersaturations ranging from 0.1 to 0.33%, agree well with measurements in the ship-exhaust plume. Using size-resolved chemical composition instead of bulk submicrometer composition has little effect on the predicted CCN concentrations because the cutoff diameter for CCN activation is larger than the diameter where the mass fraction of organic aerosol begins to increase significantly. The particle number emission factor estimated from this study is 1.3 × 1016 (kg fuel)-1, with less than 1/10 of the particles having diameters above 100 nm; 24% of particles (>10 nm in diameter) activate into cloud droplets at 0.3% supersaturation. © 2009 American Chemical Society.
• Sorooshian, A., Feingold, G., Lebsock, M. D., Jiang, H., & Stephens, G. L. (2009). On the precipitation susceptibility of clouds to aerosol perturbations. Geophysical Research Letters, 36(13).
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  Abstract: Atmospheric aerosol particles act as cloud condensation nuclei, affording them the ability to influence cloud microphysics, planetary albedo, and precipitation. Models of varying complexity and satellite observations from NASA's A-Train constellation of satellites are used to determine what controls the precipitation susceptibility of warm clouds to aerosol perturbations. Three susceptibility regimes are identified: (i) clouds with low liquid water path (LWP) generate very little rain and are least susceptible to aerosol; (ii) clouds with intermediate LWP where aerosol most effectively suppress precipitation; and (iii) clouds with high LWP, where the susceptibility begins to decrease because the precipitation process is efficient owing to abundant liquid water. Remarkable qualitative agreement between remote sensing observations and model predictions provides the first suggestions that certain regions of the Earth might be more vulnerable to pollution aerosol. Targeted pollution control strategies in such regions would most benefit water availability via precipitation. Copyright 2009 by the American Geophysical Union.
• Sorooshian, A., Padro, L. T., Nenes, A., Feingold, G., McComiskey, A., Hersey, S. P., Gates, H., Jonsson, H. H., Miller, S. D., Stephens, G. L., Flagan, R. C., & Seinfeld, J. H. (2009). On the link between ocean biota emissions, aerosol, and maritime clouds: Airborne, ground, and satellite measurements off the coast of California. Global Biogeochemical Cycles, 23(4).
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  Abstract: Surface, airborne, and satellite measurements over the eastern Pacific Ocean off the coast of California during the period between 2005 and 2007 are used to explore the relationship between ocean chlorophyll a, aerosol, and marine clouds. Periods of enhanced chlorophyll a and wind speed are coincident with increases in particulate diethylamine and methanesulfonate concentrations. The measurements indicate that amines are a source of secondary organic aerosol in the marine atmosphere. Subsaturated aerosol hygroscopic growth measurements indicate that the organic component during periods of high chlorophyll a and wind speed exhibit considerable water uptake ability. Increased average cloud condensation nucleus (CCN) activity during periods of increased chlorophyll a levels likely results from both size distribution and aerosol composition changes. The available data over the period of measurements indicate that the cloud microphysical response, as represented by either cloud droplet number concentration or cloud droplet effective radius, is likely influenced by a combination of atmospheric dynamics and aerosol perturbations during periods of high chlorophyll a concentrations. Copyright 2009 by the American Geophysical Union.
• Sorooshian, A., Sorooshian, A., Feingold, G., Feingold, G., Lebsock, M. D., Lebsock, M. D., Jiang, H., Jiang, H., Stephens, G. L., & Stephens, G. L. (2009). On the precipitation susceptibility of clouds to aerosol perturbations. GEOPHYSICAL RESEARCH LETTERS, 36.
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  Atmospheric aerosol particles act as cloud condensation nuclei, affording them the ability to influence cloud microphysics, planetary albedo, and precipitation. Models of varying complexity and satellite observations from NASA's A-Train constellation of satellites are used to determine what controls the precipitation susceptibility of warm clouds to aerosol perturbations. Three susceptibility regimes are identified: (i) clouds with low liquid water path (LWP) generate very little rain and are least susceptible to aerosol; (ii) clouds with intermediate LWP where aerosol most effectively suppress precipitation; and (iii) clouds with high LWP, where the susceptibility begins to decrease because the precipitation process is efficient owing to abundant liquid water. Remarkable qualitative agreement between remote sensing observations and model predictions provides the first suggestions that certain regions of the Earth might be more vulnerable to pollution aerosol. Targeted pollution control strategies in such regions would most benefit water availability via precipitation. Citation: Sorooshian, A., G. Feingold, M. D. Lebsock, H. Jiang, and G. L. Stephens (2009), On the precipitation susceptibility of clouds to aerosol perturbations, Geophys. Res. Lett., 36, L13803, doi: 10.1029/2009GL038993.
• Sorooshian, A., Varutbangkul, V., Brechtel, F. J., Ervens, B., Feingold, G., Bahreini, R., Murphy, S. M., Holloway, J. S., Atlas, E. L., Buzorius, G., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2009). Oxalic acid in clear and cloudy atmospheres: Analysis of data from International Consortium for Atmospheric Research on Transport and Transformation 2004. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 111(D23).
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  Oxalic acid is often the leading contributor to the total dicarboxylic acid mass in ambient organic aerosol particles. During the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign, nine inorganic ions (including SO42-) and five organic acid ions (including oxalate) were measured on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter research aircraft by a particle-into-liquid sampler (PILS) during flights over Ohio and surrounding areas. Five local atmospheric conditions were studied: (1) cloud-free air, (2) power plant plume in cloud-free air with precipitation from scattered clouds overhead, (3) power plant plume in cloud-free air, (4) power plant plume in cloud, and (5) clouds uninfluenced by local pollution sources. The aircraft sampled from two inlets: a counterflow virtual impactor (CVI) to isolate droplet residuals in clouds and a second inlet for sampling total aerosol. A strong correlation was observed between oxalate and SO42- when sampling through both inlets in clouds. Predictions from a chemical cloud parcel model considering the aqueous-phase production of dicarboxylic acids and SO42- show good agreement for the relative magnitude of SO42- and oxalate growth for two scenarios: power plant plume in clouds and clouds uninfluenced by local pollution sources. The relative contributions of the two aqueous-phase routes responsible for oxalic acid formation were examined; the oxidation of glyoxylic acid was predicted to dominate over the decay of longer-chain dicarboxylic acids. Clear evidence is presented for aqueous-phase oxalic acid production as the primary mechanism for oxalic acid formation in ambient aerosols.
• Sorooshian, A., Wang, Z., Feingold, G., & L'Ecuyer, T. S. (2009). A satellite perspective on cloud water to rain water conversion rates and relationships with environmental conditions. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 118(12), 6643-6650.
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  A two-year satellite remote sensing data set from the NASA A-Train is used to examine conversion rates of cloud water to rain water for warm maritime clouds with different ranges of mean cloud-layer radar reflectivity and rain rate. Recent work has demonstrated the utility of a novel procedure that relies on the differing sensitivities of passive MODIS measurements and active CloudSat radar measurements to estimate warm cloud conversion rates and associated time scales. That work is extended here to examine regional differences in conversion rates, including sensitivity to environmental parameters such as atmospheric stability and the presence of different aerosol types defined based on values of aerosol optical depth, fine mode fraction, and angstrom ngstrom Exponent. Among eight subregions examined, the tropical Pacific Ocean is characterized by the highest average conversion rate while subtropical stratocumulus cloud regions (far northeastern Pacific Ocean, far southeastern Pacific Ocean, Western Africa coastal region) exhibit the lowest rates. Conversion rates are generally higher at reduced values of lower tropospheric static stability (LTSS). When examining data in two selected ranges for LTSS, higher conversion rates are coincident with higher LWP and factors covarying or rooted in the presence of aerosol types exhibiting lower aerosol index values.
• Youn, J., Wang, Z., Wonaschuetz, A., Arellano, A., Betterton, E. A., & Sorooshian, A. (2009). Evidence of aqueous secondary organic aerosol formation from biogenic emissions in the North American Sonoran Desert. GEOPHYSICAL RESEARCH LETTERS, 40(13), 3468-3472.
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  This study examines the role of aqueous secondary organic aerosol formation in the North American Sonoran Desert as a result of intense solar radiation, enhanced moisture, and biogenic volatile organic compounds (BVOCs). The ratio of water-soluble organic carbon (WSOC) to organic carbon (OC) nearly doubles during the monsoon season relative to other seasons of the year. When normalized by mixing height, the WSOC enhancement during monsoon months relative to preceding dry months (May-June) exceeds that of sulfate by nearly a factor of 10. WSOC: OC and WSOC are most strongly correlated with moisture parameters, temperature, and concentrations of O-3 and BVOCs. No positive relationship was identified between WSOC or WSOC: OC and anthropogenic tracers such as CO over a full year. This study points at the need for further work to understand the effect of BVOCs and moisture in altering aerosol properties in understudied desert regions.
• Chen, Y. -., Christensen, M. W., Xue, L., Sorooshian, A., Stephens, G. L., Rasmussen, R. M., & Seinfeld, J. H. (2008). Occurrence of lower cloud albedo in ship tracks. ATMOSPHERIC CHEMISTRY AND PHYSICS, 12(17), 8223-8235.
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  The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming.
• Ervens, B., Sorooshian, A., Lim, Y. B., & Turpin, B. J. (2008). Key parameters controlling OH-initiated formation of secondary organic aerosol in the aqueous phase (aqSOA). JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(7), 3997-4016.
• Moore, R. H., Ingall, E. D., Sorooshian, A., & Nenes, A. (2008). Molar mass, surface tension, and droplet growth kinetics of marine organics from measurements of CCN activity. Geophysical Research Letters, 35(7).
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  Abstract: The CCN-relevant properties and droplet growth kinetics are determined for marine organic matter isolated from seawater collected near the Georgia coast. The organic matter is substantially less CCN active than (NH4)2SO4, but droplet growth kinetics are similar. Köhler Theory Analysis (KTA) is used to determine the average organic molar masses of two samples, which are 4370 ± 24% and 4340 ± 18% kg kmol-1. KTA is used to infer surface tension depression, which is in excellent agreement with direct measurements. For the first time it is shown that direct measurements of surface tension are relevant for CCN activation, and this study highlights the power of KTA. Copyright 2008 by the American Geophysical Union.
• Ng, N. L., Kwan, A. J., Surratt, J. D., Chan, A. W., Chhabra, P. S., Sorooshian, A., Pye, H. O., Crounse, J. D., Wennberg, P. O., Flagan, R. C., & Seinfeld, J. H. (2008). Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO₃). Atmospheric Chemistry and Physics, 8(14), 4117-4140.
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  Abstract: Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO3) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH<10%) using N2O5 as a source of NO3 radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO2) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO 2+RO2 reaction (self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% (corresponding to Mo≅10 μg m-3), it is estimated that ∼2 to 3 Tg yr−1 of SOA results from isoprene+NO3. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals in the nighttime troposphere.
• Padro, L. T., Gates, H., Murphy, S. M., Sorooshian, A., Jonsson, H., Flagan, R. C., Seinfeld, J. H., & Nenes, A. (2008). Airborne size-resolved ccn activity and droplet growth kinetic measurements in pristine and polluted airmasses. AIChE Annual Meeting, Conference Proceedings.
• Shingler, T., Dey, S., Sorooshian, A., Brechtel, F. J., Wang, Z., Metcalf, A., Coggon, M., Muelmenstaedt, J., Russell, L. M., Jonsson, H. H., & Seinfeld, J. H. (2008). Characterisation and airborne deployment of a new counterflow virtual impactor inlet. ATMOSPHERIC MEASUREMENT TECHNIQUES, 5(6), 1259-1269.
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  A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (similar to 15 l min(-1)) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7-13.1 mu m. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.
• Sorooshian, A., Csavina, J., Shingler, T., Dey, S., Brechtel, F. J., Saez, A. E., & Betterton, E. A. (2008). Hygroscopic and Chemical Properties of Aerosols Collected near a Copper Smelter: Implications for Public and Environmental Health. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 46(17), 9473-9480.
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  Particulate matter emissions near active copper smelters and mine tailings in the southwestern United States pose a potential threat to nearby environments owing to toxic species that can be inhaled and deposited in various regions of the body depending on the composition and size of the particles, which are linked by particle hygroscopic properties. This study reports the first simultaneous measurements of size-resolved chemical and hygroscopic properties of particles next to an active copper smelter and mine tailings by the towns of Hayden and Winkelman in southern Arizona. Size-resolved particulate matter samples were examined with inductively coupled plasma mass spectrometry, ion chromatography, and a humidified tandem differential mobility analyzer. Aerosol particles collected at the measurement site are enriched in metals and metalloids (e.g., arsenic, lead, and cadmium) and water-uptake measurements of aqueous extracts of collected samples indicate that the particle diameter range of particles most enriched with these species (0.18-0.55 mu m) overlaps with the most hygroscopic mode at a relative humidity of 90% (0.10-0.32 mu m). These measurements have implications for public health, microphysical effects of aerosols, and regional impacts owing to the transport and deposition of contaminated aerosol particles.
• Sorooshian, A., Hersey, S., Brechtel, F. J., Corless, A., Flagan, R. C., & Seinfeld, J. H. (2008). Rapid, size-resolved aerosol hygroscopic growth measurements: Differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP). Aerosol Science and Technology, 42(6), 445-464.
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  Abstract: We report on a new instrument developed to perform rapid, size-resolved aerosol hygroscopicity measurements. The differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP) employs differential mobility analysis in-concert with multiple humidification and optical sizing steps to determine dry optical size and hygroscopic growth factors for size-selected aerosols simultaneously at three elevated relative humidities. The DASH-SP has been designed especially for aircraft-based measurements, with time resolution as short as a few seconds. The minimum particle diameter detected with 50% efficiency in the optical particle counters (OPCs) is 135 ± 8 nm, while the maximum detectable particle diameter is in excess of 1 μm. An iterative data processing algorithm quantifies growth factors and "effective" refractive indices for humidified particles using an empirically derived three-dimensional surface (OPC pulse height-refractive index-particle size), based on a calculated value of the "effective" dry particle refractive index. Excellent agreement is obtained between DASH-SP laboratory data and thermodynamic model predictions for growth factor dependence on relative humidity for various inorganic salts. Growth factor data are also presented for several organic acids. Oxalic, malonic, glutaric, and glyoxylic acids grow gradually with increasing relative humidity up to 94%, while succinic and adipic acids show no growth. Airborne measurements of hygroscopic growth factors of ship exhaust aerosol during the 2007 Marine Stratus/Stratocumulus Experiment (MASE II) field campaign off the central coast of California are presented as the first report of the aircraft integration of the DASH-SP. Copyright © American Association for Aerosol Research.
• Sorooshian, A., Murphy, S. M., Hersey, S., Gates, H., Padro, L. T., Nenes, A., Brechtel, F. J., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2008). Comprehensive airborne characterization of aerosol from a major bovine source. Atmospheric Chemistry and Physics, 8(17), 5489-5520.
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  Abstract: We report an extensive airborne characterization of aerosol downwind of a massive bovine source in the San Joaquin Valley (California) on two flights during July 2007. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed chemical composition, particle size distribution, mixing state, sub- and supersaturated water uptake behavior, light scattering properties, and the interrelationship between these parameters and meteorology. Total PM1.0 levels and concentrations of organics, nitrate, and ammonium were enhanced in the plume from the source as compared to the background aerosol. Organics dominated the plume aerosol mass (∼56ĝ€"64%), followed either by sulfate or nitrate, and then ammonium. Particulate amines were detected in the plume aerosol by a particle-into-liquid sampler (PILS) and via mass spectral markers in the Aerodyne C-ToF-AMS. Amines were found to be a significant atmospheric base even in the presence of ammonia; particulate amine concentrations are estimated as at least 14ĝ€"23% of that of ammonium in the plume. Enhanced sub- and supersaturated water uptake and reduced refractive indices were coincident with lower organic mass fractions, higher nitrate mass fractions, and the detection of amines. The likelihood of suppressed droplet growth owing to kinetic limitations from hydrophobic organic material is explored. After removing effects associated with size distribution and mixing state, the normalized activated fraction of cloud condensation nuclei (CCN) increased as a function of the subsaturated hygroscopic growth factor, with the highest activated fractions being consistent with relatively lower organic mass fractions and higher nitrate mass fractions. Subsaturated hygroscopic growth factors for the organic fraction of the aerosol are estimated based on employing the Zdanovskii-Stokes Robinson (ZSR) mixing rule. Representative values for a parameterization treating particle water uptake in both the sub- and supersaturated regimes are reported for incorporation into atmospheric models.
• Coggon, M. M., Sorooshian, A., Wang, Z., Craven, J. S., Metcalf, A. R., Lin, J. J., Nenes, A., Jonsson, H. H., Flagan, R. C., & Seinfeld, J. H. (2007). Observations of continental biogenic impacts on marine aerosol and clouds off the coast of California. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(11), 6724-6748.
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  During the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and 2013 Nucleation in California Experiment (NiCE) field campaigns, a predominantly organic aerosol (> 85% by mass) was observed in the free troposphere over marine stratocumulus off the coast of California. These particles originated from a densely forested region in the Northwestern United States. The organic mass spectrum resolved by positive matrix factorization is consistent with the mass spectra of previously measured biogenic organic aerosol. Particulate organic mass exhibits a latitudinal gradient that corresponds to the geographical distribution of vegetation density and composition, with the highest concentration over regions impacted by densely populated monoterpene sources. Due to meteorological conditions during summer months, cloud-clearing events transport aerosol from the Northwestern United States into the free troposphere above marine stratocumulus. Based on the variation of meteorological variables with altitude, dry air containing enhanced biogenic organic aerosol is shown to entrain into the marine boundary layer. Fresh impacts on cloud water composition are observed north of San Francisco, CA which is consistent with fresh continental impacts on the marine atmosphere at higher latitudes. Continental aerosol size distributions are bimodal. Particles in the 100nm mode are impacted by biogenic sources, while particles in the approximate to 30nm mode may originate from fresh biogenic emissions. Continental aerosol in the 100nm mode is cloud condensation nuclei active and may play a role in modulating marine stratocumulus microphysics.
• Craven, J. S., Metcalf, A. R., Bahreini, R., Middlebrook, A., Hayes, P. L., Duong, H. T., Sorooshian, A., Jimenez, J. L., Flagan, R. C., & Seinfeld, J. H. (2007). Los Angeles Basin airborne organic aerosol characterization during CalNex. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 118(19), 11453-11467.
• Fountoukis, C., Nenes, A., Meskhidze, N., Bahreini, R., Conant, W. C., Jonsson, H., Murphy, S., Sorooshian, A., Varutbangkul, V., Brechtel, F., Flagan, R. C., & Seinfeld, J. H. (2007). Aerosol-cloud drop concentration closure for clouds sampled during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign. Journal of Geophysical Research D: Atmospheres, 112(10).
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  Abstract: This study analyzes 27 cumuliform and stratiform clouds sampled aboard the CIRPAS Twin Otter during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) experiment. The data set was used to assess cloud droplet closure using (1) a detailed adiabatic cloud parcel model and (2) a state-of-the-art cloud droplet activation parameterization. A unique feature of the data set is the sampling of highly polluted clouds within the vicinity of power plant plumes. Remarkable closure was achieved (much less than the 20% measurement uncertainty) for both parcel model and parameterization. The highly variable aerosol did not complicate the cloud droplet closure, since the clouds had low maximum supersaturation and were not sensitive to aerosol variations (which took place at small particle sizes). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06, but can range between 0.03 and 1.0. The sensitivity of cloud droplet prediction error to changes in the uptake coefficient, organic solubility and surface tension depression suggest that organics exhibit limited solubility. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America; future in situ studies will determine the robustness of our findings. Copyright 2007 by the American Geophysical Union.
• Gilardoni, S., Russell, L. M., Sorooshian, A., Flagan, R. C., Seinfeld, J. H., Bates, T. S., Quinn, P. K., Allan, J. D., Williams, B., Goldstein, A. H., Onasch, T. B., & Worsnop, D. R. (2007). Regional variation of organic functional groups in aerosol particles on four U.S. east coast platforms during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign. Journal of Geophysical Research D: Atmospheres, 112(10).
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  Abstract: Submicron atmospheric aerosol samples were collected during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 campaign on four platforms: Chebogue Point (Nova Scotia, Canada), Appledore Island (Maine), the CIRPAS Twin Otter over Ohio, and the NOAA R/V Ronald H. Brown in the Gulf of Maine. Saturated aliphatic C-C-H, unsaturated aliphatic C=C-H, aromatic C=C-H, organosulfur C-O-S, carbonyl C=O, and organic hydroxyl C-OH functional groups were measured by calibrated Fourier Transform Infrared (FTIR) spectroscopy at all four sampling platforms. The ratio of molar concentrations of carbonyl C=O to saturated aliphatic C-C-H groups was nearly constant at each sampling platform, with the Twin Otter samples having the lowest ratio at 0.1 and the three more coastal platforms having ratios of 0.4 and 0.5. Organic mass (OM) to organic carbon (OC) ratios follow similar trends for the four platforms, with the Twin Otter having the lowest ratio of 1.4 and the coastal platforms having slightly higher values typically between 1.5 and 1.6. Organosulfur compounds were occasionally observed. Collocated organic aerosol sampling with two Aerodyne aerosol mass spectrometers for OM, a Sunset Laboratory thermo-optical analysis instrument for OC, and an ion chromatography-particle into liquid sampler (IC-PILS) for speciated carboxylic acids provided comparable results for most of the project, tracking the time series of FTIR OM, OC, and carbonyl groups, respectively, and showing simultaneous peaks of similar magnitude during most of the project. The FTIR/IC-PILS comparison suggests that about 9% of the carbonyl groups found in submicron organic particles on the Twin Otter are typically associated with low molecular weight carboxylic acids. Copyright 2007 by the American Geophysical Union.
• Metcalf, A. R., Craven, J. S., Ensberg, J. J., Brioude, J., Angevine, W., Sorooshian, A., Duong, H. T., Jonsson, H. H., Flagan, R. C., & Seinfeld, J. H. (2007). Black carbon aerosol over the Los Angeles Basin during CalNex. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 117.
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  Refractory black carbon (rBC) mass and number concentrations were quantified by a Single Particle Soot Photometer (SP2) in the CalNex 2010 field study on board the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter in the Los Angeles (LA) Basin in May, 2010. The mass concentrations of rBC in the LA Basin ranged from 0.002-0.530 mu g m(-3), with an average of 0.172 mu g m(-3). Lower concentrations were measured in the Basin outflow regions and above the inversion layer. The SP2 afforded a quantification of the mixing state of rBC aerosols through modeling the scattering cross-section with a core-and-shell Mie model to determine coating thickness. The rBC particles above the inversion layer were more thickly coated by a light-scattering substance than those below, indicating a more aged aerosol in the free troposphere. Near the surface, as the LA plume is advected from west to east with the sea breeze, a coating of scattering material grows on rBC particles, coincident with a clear growth of ammonium nitrate within the LA Basin and the persistence of water-soluble organic compounds as the plume travels through the outflow regions. Detailed analysis of the rBC mixing state reveals two modes of coated rBC particles; a mode with smaller rBC core diameters (similar to 90 nm) but thick (>200 nm) coating diameters and a mode with larger rBC cores (similar to 145 nm) with a thin (
• Murphy, S. M., Sorooshian, A., Kroll, J. H., Ng, N. L., Chhabra, P., Tong, C., Surratt, J. D., Knipping, E., Flagan, R. C., & Seinfeld, J. H. (2007). Secondary aerosol formation from atmospheric reactions of aliphatic amines. Atmospheric Chemistry and Physics, 7(9), 2313-2337.
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  Abstract: Although aliphatic amines have been detected in both urban and rural atmospheric aerosols, little is known about the chemistry leading to particle formation or the potential aerosol yields from reactions of gas-phase amines. We present here the first systematic study of aerosol formation from the atmospheric reactions of amines. Based on laboratory chamber experiments and theoretical calculations, we evaluate aerosol formation from reaction of OH, ozone, and nitric acid with trimethylamine, methylamine, triethylamine, diethylamine, ethylamine, and ethanolamine. Entropies of formation for alkylammonium nitrate salts are estimated by molecular dynamics calculations enabling us to estimate equilibrium constants for the reactions of amines with nitric acid. Though subject to significant uncertainty, the calculated dissociation equilibrium constant for diethylammonium nitrate is found to be sufficiently small to allow for its atmospheric formation, even in the presence of ammonia which competes for available nitric acid. Experimental chamber studies indicate that the dissociation equilibrium constant for triethylammonium nitrate is of the same order of magnitude as that for ammonium nitrate. All amines studied form aerosol when photooxidized in the presence of NOx with the majority of the aerosol mass present at the peak of aerosol growth consisting of aminium (R3NH+) nitrate salts, which repartition back to the gas phase as the parent amine is consumed. Only the two tertiary amines studied, trimethylamine and triethylamine, are found to form significant nonsalt organic aerosol when oxidized by OH or ozone; calculated organic mass yields for the experiments conducted are similar for ozonolysis (15% and 5% respectively) and photooxidation (23% and 8% respectively). The non-salt organic aerosol formed appears to be more stable than the nitrate salts and does not quickly repartition back to the gas phase.
• Ng, N. L., Chhabra, P. S., Chan, A. W., Surratt, J. D., Kroll, J. H., Kwan, A. J., McCabe, D. C., Wennberg, P. O., Sorooshian, A., Murphy, S. M., Dalleska, N. F., Flagan, R. C., & Seinfeld, J. H. (2007). Effect of NO_x level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes. Atmospheric Chemistry and Physics, 7(19), 5159-5174.
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  Abstract: Secondary organic aerosol (SOA) formation from the photooxidation of one monoterpene (a-pinene) and two sesquiterpenes (longifolene and aromadendrene) is investigated in the Caltech environmental chambers. The effect of NO x on SOA formation for these biogenic hydrocarbons is evaluated by performing photooxidation experiments under varying NOx conditions. The NOx dependence of a-pinene SOA formation follows the same trend as that observed previously for a number of SOA precursors, including isoprene, in which SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) decreases as NOx level increases. The NOx dependence of SOA yield for the sesquiterpenes, longifolene and aromadendrene, however, differs from that determined for isoprene and a-pinene; the aerosol yield under high-NOx conditions substantially exceeds that under low-NOx conditions. The reversal of the NOx dependence of SOA formation for the sesquiterpenes is consistent with formation of relatively low-volatility organic nitrates, and/or the isomerization of large alkoxy radicals leading to less volatile products. Analysis of the aerosol chemical composition for longifolene confirms the presence of organic nitrates under highNOx conditions. Consequently the formation of SOA from certain biogenic hydrocarbons such as sesquiterpenes (and possibly large anthropogenic hydrocarbons as well) may be more efficient in polluted air.
• Prabhakar, G., Sorooshian, A., Toffol, E., Arellano, A. F., & Betterton, E. A. (2007). Spatiotemporal distribution of airborne particulate metals and metalloids in a populated arid region. ATMOSPHERIC ENVIRONMENT, 92, 339-347.
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  A statistical analysis of data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network of aerosol samplers has been used to study the spatial and temporal concentration trends in airborne particulate metals and metalloids for southern Arizona. The study region is a rapidly growing area in southwestern North America characterized by high fine soil concentrations (among the highest in the United States), anthropogenic emissions from an area within the fastest growing region in the United States, and a high density of active and abandoned mining sites. Crustal tracers in the region are most abundant in the summer (April-June) followed by fall (October-November) as a result of dry meteorological conditions which favor dust emissions from natural and anthropogenic activity. A distinct day-of-week cycle is evident for crustal tracer mass concentrations, with the greatest amplitude evident in urban areas. There have been significant reductions since 1988 in the concentrations of toxic species that are typically associated with smelting and mining. Periods with high fine soil concentrations coincide with higher concentrations of metals and metalloids in the atmosphere, with the enhancement being higher at urban sites. (C) 2014 Elsevier Ltd. All rights reserved.
• Sorooshian, A., & Duong, H. T. (2007). Ocean Emission Effects on Aerosol-Cloud Interactions: Insights from Two Case Studies. ADVANCES IN METEOROLOGY.
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  Two case studies are discussed that evaluate the effect of ocean emissions on aerosol-cloud interactions. A review of the first case study from the eastern Pacific Ocean shows that simultaneous aircraft and space-borne observations are valuable in detecting links between ocean biota emissions and marine aerosols, but that the effect of the former on cloud microphysics is less clear owing to interference from background anthropogenic pollution and the difficulty with field experiments in obtaining a wide range of aerosol conditions to robustly quantify ocean effects on aerosol-cloud interactions. To address these limitations, a second case was investigated using remote sensing data over the less polluted Southern Ocean region. The results indicate that cloud drop size is reduced more for a fixed increase in aerosol particles during periods of higher ocean chlorophyll A. Potential biases in the results owing to statistical issues in the data analysis are discussed.
• Sorooshian, A., Lu, M., Brechtel, F. J., Jonsson, H., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2007). On the source of organic acid aerosol layers above clouds. Environmental Science and Technology, 41(13), 4647-4654.
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  PMID: 17695910;Abstract: During the July 2005 Marine Stratus/Stratocumulus Experiment (MASE) and the August-September 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed aerosols and cumulus clouds in the eastern Pacific Ocean off the coast of northern California and in southeastern Texas, respectively. An on-board particle-into-liquid sampler (PILS) quantified inorganic and organic acid species with ≤5-min time resolution. Ubiquitous organic aerosol layers above cloud with enhanced organic acid levels were observed in both locations. The data suggest that aqueous-phase reactions to produce organic acids, mainly oxalic acid, followed by droplet evaporation is a source of elevated organic acid aerosol levels above cloud. Oxalic acid is observed to be produced more efficiently relative to sulfate as the cloud liquid water content increases, corresponding to larger and less acidic droplets. As derived from large eddy simulations of stratocumulus under the conditions of MASE, both Lagrangian trajectory analysis and diurnal cloudtop evolution provide evidence that a significant fraction of the aerosol mass concentration above cloud can be accounted for by evaporated droplet residual particles. Methanesulfonate data suggest that entrainment of free tropospheric aerosol can also be a source of organic acids above boundary layer clouds. © 2007 American Chemical Society.
• Sorooshian, A., Ng, N. L., Chan, A. W., Feingold, G., Flagan, R. C., & Seinfeld, J. H. (2007). Particulate organic acids and overall water-soluble aerosol composition measurements from the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). Journal of Geophysical Research D: Atmospheres, 112(13).
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  Abstract: The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter participated in the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) mission during August-September 2006. A particle-into-liquid sampler (PILS) coupled to ion chromatography was used to characterize the water-soluble ion composition of aerosol and cloud droplet residual particles (976 5-min PM1.0 samples in total). Sulfate and ammonium dominated the water-soluble mass (NH4+ + SO42 = 84 ± 14%), while organic acids contributed 3.4 ± 3.7%. The average NH4+: SO42- molar ratio was 1.77 ± 0.85. Particulate concentrations of organic acids increased with decreasing carbon number from C9 to C2. Organic acids were most abundant above cloud, presumably as a result of aqueous phase chemistry in cloud droplets, followed by subsequent droplet evaporation above cloud tops; the main product of this chemistry was oxalic acid. The evolution of organic acids with increasing altitude in cloud provides evidence for the multistep nature of oxalic acid production; predictions from a cloud parcel model are consistent with the observed oxalate:glyoxylate ratio as a function of altitude in GoMACCS cumuli. Suppressed organic acid formation was observed in clouds with relatively acidic droplets, as determined by high particulate nitrate concentrations (presumably high HNO3 levels too) and lower liquid water content, as compared to other cloud fields probed. In the Houston Ship Channel region, an area with significant volatile organic compound emissions, oxalate, acetate, formate, benzoate, and pyruvate, in decreasing order, were the most abundant organic acids. Photo-oxidation of m-xylene in laboratory chamber experiments leads to a particulate organic acid product distribution consistent with the Ship Channel area observations. Copyright 2007 by the American Geophysical Union.
• Surratt, J. D., Kroll, J. H., Kleindienst, T. E., Edney, E. O., Claeys, M., Sorooshian, A., Ng, N. L., Offenberg, J. H., Lewandowski, M., Jaoui, M., Flagan, R. C., & Seinfeld, J. H. (2007). Evidence for organosulfates in secondary organic aerosol. Environmental Science and Technology, 41(2), 517-527.
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  PMID: 17310716;Abstract: Recent work has shown that particle-phase reactions contribute to the formation of secondary organic aerosol (SOA), with enhancements of SOA yields in the presence of acidic seed aerosol. In this study, the chemical composition of SOA from the photooxidations of α-pinene and isoprene, in the presence or absence of sulfate seed aerosol, is investigated through a series of controlled chamber experiments in two separate laboratories. By using electrospray ionization-mass spectrometry, sulfate esters in SOA produced in laboratory photooxidation experiments are identified for the first time. Sulfate esters are found to account for a larger fraction of the SOA mass when the acidity of seed aerosol is increased, a result consistent with aerosol acidity increasing SOA formation. Many of the isoprene and α-pinene sulfate esters identified in these chamber experiments are also found in ambient aerosol collected at several locations in the southeastern U.S. It is likely that this pathway is important for other biogenic terpenes, and may be important in the formation of humic-like substances (HULIS) in ambient aerosol. © 2007 American Chemical Society.
• Szmigielski, R., Surratt, J. D., Vermeylen, R., Szmigielska, K., Kroll, J. H., Ng, N. L., Murphy, S. M., Sorooshian, A., Seinfeld, J. H., & Claeys, M. (2007). Characterization of 2-methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry. Journal of Mass Spectrometry, 42(1), 101-116.
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  PMID: 17154243;Abstract: In the present work, we have characterized in detail the chemical structures of secondary organic aerosol (SOA) components that were generated in a smog chamber and result from the photooxidation of isoprene under high-NO x conditions typical for a polluted atmosphere. Isoprene high-NO x SOA contains 2-methylglyceric acid (2-MG) and oligoester derivatives thereof. Trimethylsilylation, in combination with capillary gas chromatography (GC)/ion trap mass spectrometry (MS) and detailed interpretation of the MS data, allowed structural characterization the polar oxygenated compounds present in isoprene SOA up to 2-MG trimers. GC separation was achieved between 2-MG linear and branched dimers or trimers, as well as between the 2-MG linear dimer and isomeric mono-acetate derivatives thereof. The electron ionization (EI) spectra of the trimethylsilyl derivatives contain a wealth of structural information, including information about the molecular weight (MW), oligoester linkages, terminal carboxylic and hydroxymethyl groups, and esterification sites. Only part of this information can be achieved with a soft ionization technique such as electrospray (ESI) in combination with collision-induced dissociation (CID). The methane chemical ionization (CI) data were used to obtain supporting MW information. Interesting EI spectral differences were observed between the trimethylsilyl derivatives of 2-MG linear and branched dimers or trimers and between 2-MG linear dimer mono-acetate isomers. Copyright © 2006 John Wiley & Sons, Ltd.
• Sorooshian, A., Brechtel, F., Ma, Y., Weber, R., Corless, A., Flagan, R., & Seinfeld, J. (2006). Modeling and characterization of a particle-into-liquid sampler (PILS). AEROSOL SCIENCE AND TECHNOLOGY, 40(6), 396-409.
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  A modified particle-into-liquid sampler (PILS), based on the original design of Weber et al. (2001), is presented. The principal modification in this design is that collected liquid sample is delivered to vials held on a rotating carousel as opposed to an online analytical detector. A model is developed to predict aerosol mass concentrations measured by a PILS based on operating parameters and characteristics of the sampled aerosol. A backward model predicts the concentrations of the sampled aerosol based on operating parameters and concentrations measured by the PILS. Both models, which consider plumbing transmission efficiencies, droplet growth, mixing effects, and volatilization losses, predict mass concentrations that are consistent with laboratory tests for step changes in concentration. The average collection efficiency for species (Na+, K+, SO42-, Cl-, NO3-) from a variety of aerosols compared to simultaneous measurements with a differential mobility analyzer (DMA) exceeded 96% except for NH4+ (88%); NH4+ is theoretically shown to be the most vulnerable to volatilization, followed by Cl- and then NO3-, with greater losses caused by increasing droplet pH and temperature. The characterization tests highlight the importance of reducing NH4+ volatilization by keeping a stable tip temperature of 100 degrees C at the point where steam and ambient air mix in the condensation chamber. Maintaining a stable tip temperature also avoids fluctuations in supersaturations that lead to increased deposition losses of larger droplets. Sample data from the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign are presented.
• Sorooshian, A., Hersey, S., Brechtel, F. J., Corless, A., Flagan, R. C., & Seinfeld, J. H. (2006). Rapid, size-resolved aerosol hygroscopic growth measurements: Differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP). AEROSOL SCIENCE AND TECHNOLOGY, 42(6), 445-464.
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  We report on a new instrument developed to perform rapid, size-resolved aerosol hygroscopicity measurements. The differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP) employs differential mobility analysis in-concert with multiple humidification and optical sizing steps to determine dry optical size and hygroscopic growth factors for size-selected aerosols simultaneously at three elevated relative humidities. The DASH-SP has been designed especially for aircraft-based measurements, with time resolution as short as a few seconds. The minimum particle diameter detected with 50% efficiency in the optical particle counters (OPCs) is 135 +/- 8 nm, while the maximum detectable particle diameter is in excess of 1 m. An iterative data processing algorithm quantifies growth factors and effective refractive indices for humidified particles using an empirically derived three-dimensional surface (OPC pulse height-refractive index-particle size), based on a calculated value of the effective dry particle refractive index. Excellent agreement is obtained between DASH-SP laboratory data and thermodynamic model predictions for growth factor dependence on relative humidity for various inorganic salts. Growth factor data are also presented for several organic acids. Oxalic, malonic, glutaric, and glyoxylic acids grow gradually with increasing relative humidity up to 94%, while succinic and adipic acids show no growth. Airborne measurements of hygroscopic growth factors of ship exhaust aerosol during the 2007 Marine Stratus/Stratocumulus Experiment (MASE II) field campaign off the central coast of California are presented as the first report of the aircraft integration of the DASH-SP.
• Sorooshian, A., Varutbangkul, V., Brechtel, F. J., Ervens, B., Feingold, G., Bahreini, R., Murphy, S. M., Holloway, J. S., Atlas, E. L., Buzorius, G., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2006). Oxalic acid in clear and cloudy atmospheres: Analysis of data from International Consortium for Atmospheric Research on Transport and Transformation 2004. Journal of Geophysical Research D: Atmospheres, 111(23).
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  Abstract: Oxalic acid is often the leading contributor to the total dicarboxylic acid mass in ambient organic aerosol particles. During the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign, nine inorganic ions (including SO42-) and five organic acid ions (including oxalate) were measured on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter research aircraft by a particle-into-liquid sampler (PILS) during flights over Ohio and surrounding areas. Five local atmospheric conditions were studied: (1) cloud-free air, (2) power plant plume in cloud-free air with precipitation from scattered clouds overhead, (3) power plant plume in cloud-free air, (4) power plant plume in cloud, and (5) clouds uninfluenced by local pollution sources. The aircraft sampled from two inlets: a counterflow virtual impactor (CVI) to isolate droplet residuals in clouds and a second inlet for sampling total aerosol. A strong correlation was observed between oxalate and SO42- when sampling through both inlets in clouds. Predictions from a chemical cloud parcel model considering the aqueous-phase production of dicarboxylic acids and SO42- show good agreement for the relative magnitude of SO42- and oxalate growth for two scenarios: power plant plume in clouds and clouds uninfluenced by local pollution sources. The relative contributions of the two aqueous-phase routes responsible for oxalic acid formation were examined; the oxidation of glyoxylic acid was predicted to dominate over the decay of longer-chain dicarboxylic acids. Clear evidence is presented for aqueous-phase oxalic acid production as the primary mechanism for oxalic acid formation in ambient aerosols. Copyright 2006 by the American Geophysical Union.
• Surratt, J. D., Murphy, S. M., Kroll, J. H., Ng, N. L., Hildebrandt, L., Sorooshian, A., Szmigielski, R., Vermeylen, R., Maenhaut, W., Claeys, M., Flagan, R. C., & Seinfeld, J. H. (2006). Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene. Journal of Physical Chemistry A, 110(31), 9665-9690.
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  PMID: 16884200;Abstract: Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C 5H 8) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO x conditions-is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO x level, with acidic products formed under high-NO x conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the-further oxidation of the isoprene oxidation product methacrolein under high-NO x conditions produces polyesters involving 2-methylgryceric acid as a key monomeric unit. These oligomers comprise ∼22-34% of the high-NO x SOA mass. Under low-No x conditions, organic peroxides contribute significantly to the low-NO x SOA mass (∼61% when SOA forms by nucleation and ∼25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C 5 alkene triols and 2-methyltetrols under low-NO x conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO x chamber experiments. © 2006 American Chemical Society.
• Wang, Z., Sorooshian, A., Prabhakar, G., Coggon, M. M., & Jonsson, H. H. (2014). Impact of emissions from shipping, land, and the ocean on stratocumulus cloud water elemental composition during the 2011 E-PEACE field campaign. ATMOSPHERIC ENVIRONMENT, 89, 570-580.
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  This study reports on cloud water chemical and pH measurements off the California coast during the July August 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). Eighty two cloud water samples were collected by a slotted-rod cloud water collector protruding above the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter in boundary layer stratocumulus clouds impacted to varying degrees by ocean-derived emissions, ship exhaust, and land emissions. Cloud water pH ranged between 2.92 and 7.58, with an average of 4.46. Peak pH values were observed north of San Francisco, simultaneous with the highest concentrations of Si, B, and Cs, and air masses originating over land. The lowest pH values were observed south of San Francisco due to ship emissions resulting in the highest concentrations of sulfate, nitrate, V, Fe, Al, P, Cd, Ti, Sb, P, and Mn. Many of these species act as important agents in aqueous-phase reactions in cloud drops and are critical ocean micronutrients after subsequent wet deposition in an ocean system that can be nutrient-limited. E-PEACE measurements suggest that conditions in the California coastal zone region can promote the conversion of micronutrients to more soluble forms, if they are not already, due to acidic cloud water conditions, the ubiquity of important organic agents such as oxalic acid, and the persistence of stratocumulus clouds to allow for continuous cloud processing. (C) 2014 Elsevier Ltd. All rights reserved.
• Sorooshian, A., Ashwani, R., Choi, H. K., Moinpour, M., Oehler, A., & Tregub, A. (2004). Effect of particle interaction on agglomeration of silica-based CMP slurries. Materials Research Society Symposium Proceedings, 816, 125-131.
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  Abstract: Chemical Mechanical Planarization has become a method of choice for planarization of metal and oxide layers in microelectronics industry. A CMP process includes up to 16 variables that need to be controlled to achieve a stable CMP process [1], One of the major variables in CMP is related to slurry compositions. In particularly, a uniform distribution of the sizes of the abrasive particle in slurry is crucial for a stable CMP performance. The agglomerates can be unstable, since their size depends on addition of chemical additives and shearing during the CMP process. In this work, the authors studied agglomeration of the fumed and colloidal silica-based slurries using dynamic rheometry, zeta potential tests, and an accusizer. Slurry viscosity, determined using a steady state rheometry, was correlated to the particle charge, characterized by zeta potential, and to the particle sizes obtained using the particle size analyzer. Additionally, rheometer was used for slurry shearing to study effect of shear on slurry characteristics. Particle agglomeration due to slurry shearing and storage was observed and corroborated using rheometry, zeta potential, and particle size measurements.
• Sorooshian, A., Shingler, T., Harpold, A., Feagles, C. W., Meixner, T., & Brooks, P. D. (2004). Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships. ATMOSPHERIC CHEMISTRY AND PHYSICS, 13(15), 7361-7379.
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  This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December-February) and during the monsoon season (July-September). Rain and snow pH levels are usually between 5-6, with crustal-derived species playing a major role in acid neutralization. These species (Ca2+, Mg2+, K+, Na+) exhibit their highest concentrations between March and June in both PM2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO42-, NO3-, and Cl-, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO42- show a statistically significant correlation with rain SO42- unlike snow SO42-, which may be related to some combination of the vertical distribution of SO42- (and precursors) and the varying degree to which SO42--enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO3-:SO42- ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate NO3- : SO42- ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the NO3-:SO42- ratio in rain increased at the majority of sites due mostly to air pollution regulations of SO42- precursors.

Proceedings Publications

• Sorooshian, A., Ashwani, R., Choi, H., Moinpour, M., Oehler, A., Tregub, A., Boning, D., Bartha, J., Philipossian, A., Shinn, G., & Vos, . (2014). Effect of particle interaction on agglomeration of silica-based CMP slurries.. In ADVANCES IN CHEMICAL-MECHANICAL POLISHING, 816, 125-131.
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  Chemical Mechanical Planarization has become a method of choice for planarization of metal and oxide layers in microelectronics industry. A CMP process includes up to 16 variables that need to be controlled to achieve a stable CMP process [1]. One of the major variables in CMP is related to slurry compositions. In particularly, a uniform distribution of the sizes of the abrasive particle in slurry is crucial for a stable CMP performance. The agglomerates can be unstable, since their size depends on addition of chemical additives and shearing during the CMP process.
• Russell, L. M., Sorooshian, A., Seinfeld, J. H., Albrecht, B. A., Nenes, A., Leaitch, W. R., Macdonald, A. M., Ahlm, L., Chen, Y., Coggon, M., Corrigan, A., Craven, J. S., Flagan, R. C., Frossard, A. A., Hawkins, L. N., Jonsson, H., Jung, E., Lin, J. J., Metcalf, A. R., , Modini, R., et al. (2010). Observed Aerosol Effects on Marine Cloud Nucleation and Supersaturation. In NUCLEATION AND ATMOSPHERIC AEROSOLS, 1527, 696-701.
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  Aerosol particles in the marine boundary layer include primary organic and salt particles from sea spray and combustion-derived particles from ships and coastal cities. These particle types serve as nuclei for marine cloud droplet activation, although the particles that activate depend on the particle size and composition as well as the supersaturation that results from cloud updraft velocities. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign to assess how different particle types nucleate cloud droplets. As part of E-PEACE 2011, we studied the role of marine particles as cloud droplet nuclei and used emitted particle sources to separate particle-induced feedbacks from dynamical variability. The emitted particle sources included shipboard smoke-generated particles with 0.05-1 mu m diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke) and combustion particles from container ships with 0.05-0.2 mu m diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components) [1]. Three central aspects of the collaborative E-PEACE results are: (1) the size and chemical composition of the emitted smoke particles compared to ship-track-forming cargo ship emissions as well as background marine particles, with particular attention to the role of organic particles, (2) the characteristics of cloud track formation for smoke and cargo ships, as well as the role of multi-layered low clouds, and (3) the implications of these findings for quantifying aerosol indirect effects. For comparison with the E-PEACE results, the preliminary results of the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) 2012 provided evidence of the cloud-nucleating roles of both marine organic particles and coastal urban pollution, with simultaneous measurements of the effective supersaturations of the clouds in the California coastal region.

Reviews

• Sorooshian, A., Padro, L. T., Nenes, A., Feingold, G., McComiskey, A., Hersey, S. P., Gates, H., Jonsson, H. H., Miller, S. D., Stephens, G. L., Flagan, R. C., & Seinfeld, J. H. (2011. On the link between ocean biota emissions, aerosol, and maritime clouds: Airborne, ground, and satellite measurements off the coast of California.
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  Surface, airborne, and satellite measurements over the eastern Pacific Ocean off the coast of California during the period between 2005 and 2007 are used to explore the relationship between ocean chlorophyll a, aerosol, and marine clouds. Periods of enhanced chlorophyll a and wind speed are coincident with increases in particulate diethylamine and methanesulfonate concentrations. The measurements indicate that amines are a source of secondary organic aerosol in the marine atmosphere. Subsaturated aerosol hygroscopic growth measurements indicate that the organic component during periods of high chlorophyll a and wind speed exhibit considerable water uptake ability. Increased average cloud condensation nucleus (CCN) activity during periods of increased chlorophyll a levels likely results from both size distribution and aerosol composition changes. The available data over the period of measurements indicate that the cloud microphysical response, as represented by either cloud droplet number concentration or cloud droplet effective radius, is likely influenced by a combination of atmospheric dynamics and aerosol perturbations during periods of high chlorophyll a concentrations.
• Hersey, S. P., Craven, J. S., Metcalf, A. R., Lin, J., Lathem, T., Suski, K. J., Cahill, J. F., Duong, H. T., Sorooshian, A., Jonsson, H. H., Shiraiwa, M., Zuend, A., Nenes, A., Prather, K. A., Flagan, R. C., & Seinfeld, J. H. (2007. Composition and hygroscopicity of the Los Angeles Aerosol: CalNex(pp 3016-3036).
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  Aircraft-based measurements of aerosol composition, either bulk or single-particle, and both subsaturated and supersaturated hygroscopicity were made in the Los Angeles Basin and its outflows during May 2010 during the CalNex field study. Aerosol composition evolves from source-rich areas in the western Basin to downwind sites in the eastern Basin, evidenced by transition from an external to internal mixture, as well as enhancements in organic O:C ratio, the amount of organics and nitrate internally mixed on almost all particle types, and coating thickness on refractory black carbon (rBC). Transport into hot, dilute outflow regions leads to significant volatilization of semivolatile material, resulting in a unimodal aerosol comprising primarily oxygenated, low-volatility, water-soluble organics and sulfate. The fraction of particles with rBC or soot cores is between 27 and 51% based on data from a Single Particle Soot Photometer (SP2) and Aerosol Time of Flight Mass Spectrometer (ATOFMS). Secondary organics appear to inhibit subsaturated water uptake in aged particles, while CCN activity is enhanced with photochemical age. A biomass-burning event resulted in suppression of subsaturated hygroscopicity but enhancement in CCN activity, suggesting that BB particles may be nonhygroscopic at subsaturated RH but are important sources of CCN. Aerosol aging and biomass burning can lead to discrepancies between subsaturated and supersaturated hygroscopicity that may be related to mixing state. In the cases of biomass burning aerosol and aged particles coated with secondary material, more than a single parameter representation of subsaturated hygroscopicity and CCN activity is needed.
• Ryerson, T. B., Andrews, A. E., Angevine, W. M., Bates, T. S., Brock, C. A., Cairns, B., Cohen, R. C., Cooper, O. R., de Gouw, J. A., Fehsenfeld, F. C., Ferrare, R. A., Fischer, M. L., Flagan, R. C., Goldstein, A. H., Hair, J. W., Hardesty, R. M., Hostetler, C. A., Jimenez, J. L., Langford, A. O., , McCauley, E., et al. (2007. The 2010 California Research at the Nexus of Air Quality and Climate Change (CalNex) field study(pp 5830-5866).
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  The California Research at the Nexus of Air Quality and Climate Change (CalNex) field study was conducted throughout California in May, June, and July of 2010. The study was organized to address issues simultaneously relevant to atmospheric pollution and climate change, including (1) emission inventory assessment, (2) atmospheric transport and dispersion, (3) atmospheric chemical processing, and (4) cloud-aerosol interactions and aerosol radiative effects. Measurements from networks of ground sites, a research ship, tall towers, balloon-borne ozonesondes, multiple aircraft, and satellites provided in situ and remotely sensed data on trace pollutant and greenhouse gas concentrations, aerosol chemical composition and microphysical properties, cloud microphysics, and meteorological parameters. This overview report provides operational information for the variety of sites, platforms, and measurements, their joint deployment strategy, and summarizes findings that have resulted from the collaborative analyses of the CalNex field study. Climate-relevant findings from CalNex include that leakage from natural gas infrastructure may account for the excess of observed methane over emission estimates in Los Angeles. Air-quality relevant findings include the following: mobile fleet VOC significantly declines, and NOx emissions continue to have an impact on ozone in the Los Angeles basin; the relative contributions of diesel and gasoline emission to secondary organic aerosol are not fully understood; and nighttime NO3 chemistry contributes significantly to secondary organic aerosol mass in the San Joaquin Valley. Findings simultaneously relevant to climate and air quality include the following: marine vessel emissions changes due to fuel sulfur and speed controls result in a net warming effect but have substantial positive impacts on local air quality.
• Sorooshian, A., Murphy, S. N., Hersey, S., Gates, H., Padro, L. T., Nenes, A., Brechtel, F. J., Jonsson, H., Flagan, R. C., & Seinfeld, J. H. (2007. Comprehensive airborne characterization of aerosol from a major bovine source(pp 5489-5520).
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  We report an extensive airborne characterization of aerosol downwind of a massive bovine source in the San Joaquin Valley (California) on two flights during July 2007. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed chemical composition, particle size distribution, mixing state, sub- and supersaturated water uptake behavior, light scattering properties, and the interrelationship between these parameters and meteorology. Total PM1.0 levels and concentrations of organics. nitrate. and ammonium were enhanced in the plume from the source as compared to the background aerosol. Organics dominated the plume aerosol mass (similar to 56-64%), followed either by sulfate or nitrate. and then ammonium. Particulate amines were detected in the plume aerosol by a particle-into-liquid sampler (PILS) and via mass spectral inarkers in the Aerodvne C-ToF-AMS. Amines were found to be a significant atmospheric base even in the presence of arnmonia; particulate amine concentrations are estimated as at least 14-23% of that of ammonium in the plume. Enhanced sub- and supersaturated water uptake and reduced refractive indices were coincident with lower organic mass fractions, higher nitrate mass fractions, and the detection of amines. The likelihood of suppressed droplet growth owing to kinetic limitations from hydrophobic organic material is explored. After removing effects associated with size distribution and mixing state, the normalized activated fraction of cloud condensation nuclei (CCN) increased as a function of the subsaturated hygroscopic growth factor, with the highest activated fractions being consistent with relatively lower organic mass fractions and higher nitrate mass fractions. Subsaturated hygroscopic growth factors for the organic fraction of the aerosol are estimated based on employing the Zdanovskii-Stokes Robinson (ZSR) mixing rule. Representative values for a parameterization treating particle water uptake in both the sub- and supersaturated regimes are reported for incorporation into atmospheric models.

Others

• Sorooshian, A., & Feingold, G. (2013, JUN). Precipitation in warm clouds and its susceptibility to aerosol perturbations. GEOCHIMICA ET COSMOCHIMICA ACTA.