AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Influence of Aqueous-Phase Processing on the Chemical Composition of Fog Droplets and Interstitial Aerosols in California’s San Joaquin Valley
QI ZHANG, Hwajin Kim, Sonya Collier, Jianzhong Xu, Xinlei Ge, Yele Sun, Pierre Herckes, Youliang Wang, University of California, Davis
Abstract Number: 597 Working Group: There Must be Something in the Water: Cloud, Fog and Aerosol Aqueous Chemistry for Aerosol Production
Abstract A fog chemistry study was conducted in Fresno, California’s San Joaquin Valley (SJV), in January 2010. Fog droplets were collected while interstitial submicron aerosol was characterized in real time with an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS). The fog water samples were later analyzed by HR-AMS, ion chromatography (IC) and total organic carbon analyzer (TOC). Compared to interstitial aerosol, dissolved substances in fog droplets are composed of a higher mass fraction of ammonium, nitrate, sulfate, and oxygenated organic compounds, indicating aqueous-phase formation of secondary species as well as dissolution of water-soluble gases under humid conditions. Sulfate shows the highest relative enhancement in fog water although its contribution to total dissolved mass is relatively low. The HR-AMS mass spectra of organic matter in fog water (F-OA) are highly similar to the spectrum of oxygenated organic aerosol (OOA) derived from positive matrix factorization (PMF) of the HR-AMS spectra of interstitial aerosol (r2 = 0.95), but there are also significant chemical differences between F-OA and OOA. For example, F-OA appears to contain a larger fraction of nitrogen-containing organic compounds, with an average N/C ratio ~4 times that of OOA. Most strikingly is the enhancement of the CxHyN2+ ions in F-OA spectra, indicating the presence of imidazole compounds which commonly result from the aqueous reactions of aldehydes with amino compounds. The results of this study demonstrate that aqueous reactions in fogs and clouds are likely an important source of oxygenated and nitrogen-containing organic aerosol in the atmosphere.