Measurement report: Large contribution of biomass burning and aqueous-phase processes to the wintertime secondary organic aerosol formation in Xi’an, Northwest China

Abstract

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.