Effects of photochemical and aqueous-phase oxidation on secondary formation during the 31st FISU Summer World...

Secondary inorganic and organic aerosols (SIA and SOA) constitute the predominant components of fine particles (PM_2.5). In this study, we investigated the effects of photochemical and aqueous-phase oxidation on secondary aerosol formation in Sichuan, China, during the Summer World University Games, 2023. A Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) was employed for PM_2.5 component analysis, while the multiple linear engine (ME-2) facilitated the source apportionment of organic aerosols (OA). The average PM_2.5 was 13.6 µg/m³, in which secondary aerosols accounted for 85.3 % over the whole observation. Nitrate formation relied on both photochemical and aqueous-phase oxidation, whereas sulfate was mainly formed through photochemical oxidation. Photochemical processes significantly promoted the SOA formation, especially less oxidized oxygenated OA (LO-OOA). However, as relative humidity (RH) increased, the proportion of more oxidized oxygenated OA (MO-OOA) in SOA increased, and O/C_SOA rose. This indicated that photochemical oxidation primarily drove SOA formation, while aqueous-phase oxidation significantly enhanced the oxidation degree of SOA. The formation mechanism of SOA remained consistent across before, during and after the control periods, but emission control measures significantly slowed the rise in SOA levels during the control period when compared to before the control period. This study is helpful in understanding the role of photochemical and aqueous-phase oxidations on secondary aerosol formation under high O_x conditions, as well as the impact of emission control on SOA composition and evolution.