In Situ Observation of Multiphase Oxidation-Driven Secondary Organic Aerosol Formation during Cloud Processing at a Mountain Site in Southern China
Min Gao, Shengzhen Zhou, Yao He, Guohua Zhang, Nan Ma, Li Yao, Fenghua Li, Yuxiang Yang, Long Peng, Jun Zhao, Xinhui Bi, Weiwei Hu, Yele Sun, Boguang Wang, Xuemei Wang
Abstract
Secondary organic aerosols (SOAs) account for a large fraction of atmospheric fine particles, but the mechanisms of their formation and evolution processes remain unclear. In this study, a ground-based counterflow virtual impactor was used in combination with an online time-of-flight aerosol chemical speciation monitor to investigate the multiphase (gaseous and aqueous) oxidation processes and the SOA influencing factors during cloud events at a mountain site in southern China. Our results showed that the clouds promoted the formation of low-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidized oxygenated organic aerosol (MO-OOA). At night, ozone (O 3 ) and nitrate radicals (NO 3 •) oxidized volatile organic compounds (VOCs, mostly biogenic VOCs) to the precursors of LO-OOA and were absorbed in the cloud droplets to form LO-OOA, while the hydroxyl radical (•OH) was the main oxidant for LO-OOA during daytime. In the cloud droplets, LO-OOA was further oxidized by •OH to form MO-OOA. We propose that isoprene can be oxidized to form products in the gas phase and then absorbed by acidic cloud droplets to form 2-methylglyceric acid (2-MG), 2-MG-organosulfate, and 2-MG-organonitrate. This study improves our understanding of SOA formation, driven by multiphase oxidation during cloud events.