Distinctions in source regions and formation mechanisms of secondary aerosol in Beijing from summer to winter

Abstract. To investigate the sources and evolution of haze pollution in different seasons, long-term (from 15 August to 4 December 2015) variations of chemical composition of PM 1 were characterized in Beijing, China. Positive matrix factorization (PMF) analysis with multi-linear engine (ME-2) resolved three primary and two secondary OA sources, including hydrocarbon-like OA (HOA), cooking OA (COA), coal combustion OA (CCOA), local secondary OA (LSOA) and regional SOA (RSOA). Distinctly different correlations between RSOA and sulfate were found in our study, with tight correlation (R 2  = 0.71) in late summer, decreased correlation (R 2  = 0.62) in autumn and almost no correlation (R 2  = 0.02) in early winter. This difference implies that sulfate was mainly transported at a large regional scale in late summer, while local and/or nearby sulfate formation may be more important in winter. Secondary aerosol species including SIA (sulfate, nitrate and ammonium) and SOA (LSOA and RSOA) dominated PM 1 during all three seasons. In particular, SOA contributed 46 % to total PM 1 (with 31 % as RSOA) in late summer, whereas SIA contributed 41 % and 45 % to total PM 1 in autumn and early winter, respectively. Enhanced contributions of secondary species (66–76 % of PM 1 ) were also observed in pollution episodes during all three seasons, further emphasizing the importance of secondary formation processes in haze pollution in Beijing. Combining chemical composition and meteorological data, our analyses suggest that photochemical oxidation dominated SOA formation during all three seasons, while for sulfate formation, gas-phase photochemical oxidation was the major pathway in late summer and heterogeneous processes were likely more important in autumn and early winter.