Abstract
Secondary organic aerosol (SOA) plays an important role in particulate air pollution, but its formation mechanism is still not fully understood. The chemical composition of non-refractory particulate matter with a diameter ≤ 2.5 µm (NR-PM2.5), OA sources, and SOA formation mechanisms were investigated in urban Xi’an during winter 2018. The fractional contribution of SOA to total OA mass (58 %) was larger than primary OA (POA, 42 %). A biomass burning-influenced oxygenated OA (OOA-BB) was resolved in urban Xi’an, which was formed from the photochemical oxidation and aging of biomass burning OA (BBOA). The formation of OOA-BB was more favorable in the days with larger OA fraction and higher BBOA concentration. In comparison, the aqueous-phase processed oxygenated OA (aq-OOA) was more dependent on secondary inorganic aerosol (SIA) content and aerosol liquid water content (ALWC), and increased largely to 50 % of OA during SIA-enhanced periods. Further Van Krevelen (VK) diagram analysis suggests the increased aq-OOA contributions during SIA-enhanced periods were likely from alcohol or peroxide addition in the OA aqueous-phase oxidation processes.
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