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In January 2013, Beijing experienced several serious haze events. To achievea better understanding of the characteristics, sources and processes ofaerosols during this month, an Aerodyne high-resolution time-of-flightaerosol mass spectrometer (HR-ToF-AMS) was deployed at an urban site between1 January and 1 February 2013 to obtain the size-resolved chemicalcomposition of non-refractory submicron particles (NR-PM1). During thisperiod, the mean measured NR-PM1 mass concentration was89.3 ± 85.6 μg m−3, and it peaked at 423 μg m−3. Positivematrix factorization (PMF) differentiated the organic aerosol into fivecomponents, including a highly oxidized, low-volatility oxygenated organicaerosol (LV-OOA), a less oxidized, semi-volatile oxygenated OA (SV-OOA), acoal combustion OA (CCOA), a cooking-related OA (COA), and ahydrocarbon-like OA (HOA), which on average accounted for 28%, 26%,15%, 20% and 11% of the total organic mass, respectively. A detailed comparison between the polluted days and unpolluted days foundmany interesting results. First, the organic fraction was the most importantNR-PM1 species during the unpolluted days (58%), while inorganicspecies were dominant on polluted days (59%). The OA composition alsoexperienced a significant change; it was dominated by primary OA (POA),including COA, HOA and CCOA, on unpolluted days. The contribution ofsecondary OA (SOA) increased from 35% to 63% between unpolluted andpolluted days. Second, meteorological effects played an important role inthe heavy pollution in this month and differed significantly between the twotypes of days. The temperature and relative humidity (RH) were all increasedon polluted days and the wind speed and air pressure were decreased. Third,the diurnal variation trend in NR-PM1 species and OA components showedsome differences between the two types of days, and the OA was more highlyoxidized on polluted days. Fourth, the effects of air masses weresignificantly different between the two types of days; air was mainlytransported from contaminated areas on the polluted days. The comparisonalso found that the aerosol was more acidic on polluted days. Additionally,the variation trends of the mass concentration and mass fractions ofNR-PM1 species and OA components were more dramatic when theNR-PM1 mass loading was at a higher level. The serious pollutionobserved in this month can be attributed to the synergy of unfavorablemeteorological factors, the transport of air masses from high-pollutionareas, emission by local sources, and other factors

Characterization of submicron aerosols during a month of serious pollution in Beijing, 2013