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A severe regional haze problem in the megacity Beijing and surrounding areas,\udcaused by fast formation and growth of fine particles, has attracted much\udattention in recent years. In order to investigate the secondary formation\udand aging process of urban aerosols, four intensive campaigns were conducted\udin four seasons between March 2012 and March 2013 at an urban site in\udBeijing (116.31° E, 37.99° N). An Aerodyne high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) was\uddeployed to measure non-refractory chemical components of submicron\udparticulate matter (NR-PM1). The average mass concentrations of\udPM1 (NR-PM1+black carbon) were 45.1 ± 45.8, 37.5 ± 31.0, 41.3 ± 42.7, and 81.7 ± 72.4 µg m−3 in spring,\udsummer, autumn, and winter, respectively. Organic aerosol (OA) was the most\udabundant component in PM1, accounting for 31, 33, 44, and 36 %\udseasonally, and secondary inorganic aerosol (SNA, sum of sulfate, nitrate,\udand ammonium) accounted for 59, 57, 43, and 55 % of PM1\udcorrespondingly. Based on the application of positive matrix factorization\ud(PMF), the sources of OA were obtained, including the primary ones of\udhydrocarbon-like (HOA), cooking (COA), biomass burning OA (BBOA) and coal\udcombustion OA (CCOA), and secondary component oxygenated OA (OOA). OOA, which\udcan be split into more-oxidized (MO-OOA) and less-oxidized OOA (LO-OOA), accounted\udfor 49, 69, 47, and 50 % in four seasons, respectively. Totally, the\udfraction of secondary components (OOA+SNA) contributed about 60–80 %\udto PM1, suggesting that secondary formation played an important role in\udthe PM pollution in Beijing, and primary sources were also non-negligible.\udThe evolution process of OA in different seasons was investigated with\udmultiple metrics and tools. The average carbon oxidation states and other\udmetrics show that the oxidation state of OA was the highest in summer,\udprobably due to both strong photochemical and aqueous-phase oxidations. It\udwas indicated by the good correlations (r = 0.53–0.75, p < 0.01)\udbetween LO-OOA and odd oxygen (Ox =  O3 + NO2), and between\udMO-OOA and liquid water content in aerosols. BBOA was resolved in spring and\udautumn, influenced by agricultural biomass burning (e.g., field preparation\udburnings, straw burning after the harvest). CCOA was only identified in\udwinter due to domestic heating. These results signified that the\udcomprehensive management for biomass burning and coal combustion emissions\udis needed. High concentrations of chemical components in PM1 in\udBeijing, especially in winter or in adverse meteorological conditions,\udsuggest that further strengthening the regional emission control of primary\udparticulate and precursors of secondary species is expected

Seasonal variations in high time-resolved chemical compositions, sources, and evolution of atmospheric submicron aerosols in the megacity Beijing