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

Abstract

Abstract. To investigate the sources and evolution of haze pollution in different\nseasons, long-term (from 15\xa0August to 4\xa0December\xa02015) variations in\nchemical composition of PM 1 were characterized in Beijing, China.\nPositive matrix factorization (PMF) analysis with a multi-linear engine (ME-2)\nresolved three primary and two secondary organic aerosol (OA) sources, including\nhydrocarbon-like OA (HOA), cooking OA (COA), coal combustion OA (CCOA),\nlocal secondary OA (LSOA) and regional SOA (RSOA). The sulfate source\nregion analysis implies that sulfate was mainly transported at a large\nregional scale in late summer, while local and/or nearby sulfate formation\nmay be more important in winter. Meanwhile, distinctly different\ncorrelations between sulfate and RSOA or LSOA (i.e., better correlation with\nRSOA in late summer, similar correlations with RSOA and LSOA in autumn, and\nclose correlation with LSOA in early winter) confirmed the regional\ncharacteristic of RSOA and local property of LSOA. Secondary aerosol species\nincluding secondary inorganic aerosol (SIA – sulfate, nitrate, and ammonium) and SOA (LSOA and RSOA)\ndominated PM 1 during all three seasons. In particular, SOA contributed\n46\u2009% to total PM 1 (with 31\u2009% as RSOA) in late summer, whereas SIA\ncontributed 41\u2009% and 45\u2009% to total PM 1 in autumn and early winter,\nrespectively. Enhanced contributions of secondary species (66\u2009%–76\u2009% of\nPM 1 ) were also observed in pollution episodes during all three seasons,\nfurther emphasizing the importance of secondary formation processes in haze\npollution in Beijing. Combining chemical composition and meteorological\ndata, our analyses suggest that both photochemical oxidation and\naqueous-phase processing played important roles in SOA formation during all\nthree seasons, while for sulfate formation, gas-phase photochemical\noxidation was the major pathway in late summer, aqueous-phase reactions were\nmore responsible during early winter and both processes had contributions\nduring autumn.