Fates of secondary organic aerosols in the atmosphere identified from compound-specific dual-carbon isotope analysis of oxalic acid

Secondary organic aerosols (SOAs) are important components of fine particulates in the atmosphere. However, the sources of SOA precursor and atmospheric processes affecting SOAs are poorly understood. This limits our abilities to improve air quality and model aerosol-mediated climate forcing. Here, we use novel compound-specific dual-carbon isotope fingerprints (Δ14C and δ13C) for individual SOA tracer molecules (i.e., oxalic acid) to investigate the fates of SOAs in the atmosphere at five emission hotspots in China. Coal combustion and vehicle exhaust accounted for ∼ 55 % of the sources of carbon in oxalic acid in Beijing and Shanghai, but biomass burning and biogenic emissions accounted for ∼ 70 % of the sources of carbon in oxalic acid in Chengdu, Guangzhou, and Wuhan during the sampling period. The dual-carbon isotope signatures of oxalic acid and bulk organic carbon pools (e.g., water-soluble organic carbon) were compared to investigate the fates of SOAs in the atmosphere. Photochemical aging and aqueous-phase chemical processes dominate the formation of oxalic acid in summer and in winter, respectively. The results indicated that SOA carbon sources and chemical processes producing SOAs vary spatially and seasonally, and these variations need to be included in Chinese climate projection models and air quality management practices.