10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

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CCN Activity of Secondary Organic Aerosol Largely Controlled by Molecular Weight

JIAN WANG, John Shilling, Jiumeng Liu, Alla Zelenyuk, David Bell, Markus Petters, Ryan Thalman, Fan Mei, Rahul Zaveri, Guangjie Zheng, Brookhaven National Laboratory

     Abstract Number: 1190
     Working Group: Clouds and Climate

Abstract
Aerosol particles strongly influence global climate by modifying the albedo and lifetime of clouds. An accurate assessment of the aerosol impact on climate requires the knowledge of the concentration of cloud condensation nuclei (CCN), a subset of aerosol particles that could activate and form cloud droplets in the atmosphere. Atmospheric particles often consist of hundreds, or even thousands of organic species, which collectively often dominate the composition of aerosol particles. As a result, CCN concentrations can be very sensitive to the hygroscopicity of the organics in the particles. Laboratory and field studies show organic hygroscopicity increases nearly linearly with the atomic O:C ratio (i.e., oxidation level). Such increase in hygroscopicity has long been thought a result of higher polarity and therefore increased water solubility for more oxygenated organics. In this study, we systematically characterized organic hygroscopicity of secondary organic aerosol (SOA) formed from representative precursors. We show that for the majority of SOA, organic hygroscopicity is mainly controlled by the molecular weight. Instead of increased water solubility, the increase of the organic hygroscopicity with O:C is to a large degree due to that (1) SOA formed from smaller VOC molecules tends to have lower molecular weight and higher O:C and (2) during oxidation, fragmentation produces organic molecules with lower molecular weight. A simple model of organic hygroscopicity based on molecular weight, O:C, and volatility is developed, and it successfully reproduces the variation of SOA κorg with O:C observed in the laboratory and field studies.