In Situ Observation of Multiphase Oxidation-Driven Secondary Organic Aerosol Formation during Cloud Processing at a Mountain Site in Southern China

Secondary organic aerosols (SOAs) account for a largefractionof atmospheric fine particles, but the mechanisms of their formationand evolution processes remain unclear. In this study, a ground-basedcounterflow virtual impactor was used in combination with an onlinetime-of-flight aerosol chemical speciation monitor to investigatethe multiphase (gaseous and aqueous) oxidation processes and the SOAinfluencing factors during cloud events at a mountain site in southernChina. Our results showed that the clouds promoted the formation oflow-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidizedoxygenated organic aerosol (MO-OOA). At night, ozone (O-3) and nitrate radicals (NO3 center dot) oxidized volatileorganic compounds (VOCs, mostly biogenic VOCs) to the precursors ofLO-OOA and were absorbed in the cloud droplets to form LO-OOA, whilethe hydroxyl radical (center dot OH) was the main oxidant for LO-OOAduring daytime. In the cloud droplets, LO-OOA was further oxidizedby center dot OH to form MO-OOA. We propose that isoprene can be oxidizedto form products in the gas phase and then absorbed by acidic clouddroplets to form 2-methylglyceric acid (2-MG), 2-MG-organosulfate,and 2-MG-organonitrate. This study improves our understanding of SOAformation, driven by multiphase oxidation during cloud events.