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.

Automated summary

In this study, the formation and evolution processes of secondary organic aerosols (SOAs) during cloud events were investigated. Results showed that clouds promoted the formation of low-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidized oxygenated organic aerosol (MO-OOA). Ozone and nitrate radicals oxidized volatile organic compounds (VOCs) to the precursors of LO-OOA at night, while the hydroxyl radical was the main oxidant for LO-OOA during daytime. In the cloud droplets, LO-OOA was further oxidized by the hydroxyl radical to form MO-OOA. Isoprene was proposed to be oxidized in the gas phase and then absorbed by acidic cloud droplets to form specific organic compounds.