Aerosol composition, sources and processes during wintertime in Beijing, China
Air pollution is a major environmental concern during all seasons in the\nmegacity of Beijing, China. Here we present the results from a winter\nstudy that was conducted from 21 November 2011 to 20 January 2012 with\nan Aerodyne Aerosol Chemical Speciation Monitor (ACSM) and various\ncollocated instruments. The non-refractory submicron aerosol (NR-PM1)\nspecies vary dramatically with clean periods and pollution episodes\nalternating frequently. Compared to summer, wintertime submicron\naerosols show much enhanced organics and chloride, which on average\naccount for 52% and 5%, respectively, of the total NR-PM1 mass. All\nNR-PM1 species show quite different diurnal behaviors between summer and\nwinter. For example, the wintertime nitrate presents a gradual increase\nduring daytime and correlates well with secondary organic aerosol (OA),\nindicating a dominant role of photochemical production over gas-particle\npartitioning. Positive matrix factorization was performed on ACSM OA\nmass spectra, and identified three primary OA (POA) factors, i.e.,\nhydrocarbon-like OA (HOA), cooking OA (COA), and coal combustion OA\n(CCOA), and one secondary factor, i.e., oxygenated OA (OOA). The POA\ndominates OA during wintertime, contributing 69 %, with the other 31%\nbeing SOA. Further, all POA components show pronounced diurnal cycles\nwith the highest concentrations occurring at nighttime. CCOA is the\nlargest primary source during the heating season, on average accounting\nfor 33% of OA and 17% of NR-PM1. CCOA also plays a significant role in\nchemically resolved particulate matter (PM) pollution as its mass\ncontribution increases linearly as a function of NR-PM1 mass loadings.\nThe SOA, however, presents a reverse trend, which might indicate the\nlimited SOA formation during high PM pollution episodes in winter. The\neffects of meteorology on PM pollution and aerosol processing were also\nexplored. In particular, the sulfate mass is largely enhanced during\nperiods with high humidity because of fog processing of high\nconcentration of precursor SO2. In addition, the increased\ntraffic-related HOA emission at low temperature is also highlighted.