Sources and processes of water-soluble and water-insoluble organic aerosol in cold season in Beijing, China

Water-soluble and water-insoluble organic aerosol (WSOA and WIOA) constitute a large fraction of fine particles in winter in northern China, yet our understanding of their sources and processes are still limited. Here we have a comprehensive characterization of WSOA in cold season in Beijing. Particularly, we present the first mass spectral characterization of WIOA by integrating online and offline organic aerosol measurements from high-resolution aerosol mass spectrometer. Our results showed that WSOA on average accounted for 59 % of the total OA and comprised dominantly secondary OA (SOA, 69 %). The WSOA composition showed significant changes during the transition season from autumn to winter. While the photochemical-related SOA dominated WSOA (51 %) in early November, the oxidized SOA from biomass burning increased substantially from 8 % to 29 % during the heating season. Comparatively, local primary OA dominantly from cooking aerosol contributed the major fraction of WSOA during clean periods. WIOA showed largely different spectral patterns from WSOA which were characterized by prominent hydrocarbon ions series and low oxygen-to-carbon (O/C = 0.19) and organic mass-to-organic carbon (OM/OC = 1.39) ratios. The nighttime WIOA showed less oxidized properties (O/C = 0.16 vs. 0.24) with more pronounced polycyclic aromatic hydrocarbons (PAHs) signals than daytime, indicating the impacts of enhanced coal combustion emissions on WIOA. The evolution process of WSOA and WIOA was further demonstrated by the triangle plot of f44 (fraction of m/z 44 in OA) vs. f43, f44 vs. f60, and the Van Krevelen diagram (H/C vs. O/C). We also found more oxidized WSOA and an increased contribution of SOA in WSOA compared with previous winter studies in Beijing, indicating that the changes in OA composition due to clean air act have affected the sources and properties of WSOA.