Aerosol composition, sources and processes during wintertime in Beijing, China
Air pollution is a major environmental concern during all seasons in the megacity of Beijing, China. Here we present the results from a winter study that was con-ducted from 21 November 2011 to 20 January 2012 with an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) and various collocated instruments. The non-refractory sub-micron aerosol (NR-PM 1) species vary dramatically with clean periods and pollution episodes alternating frequently. Compared to summer, wintertime submicron aerosols show much enhanced organics and chloride, which on average ac-count for 52 % and 5 %, respectively, of the total NR-PM 1 mass. All NR-PM 1 species show quite different diurnal be-haviors between summer and winter. For example, the win-tertime nitrate presents a gradual increase during daytime and correlates well with secondary organic aerosol (OA), indicat-ing a dominant role of photochemical production over gas– particle partitioning. Positive matrix factorization was per-formed on ACSM OA mass spectra, and identified three pri-mary OA (POA) factors, i.e., hydrocarbon-like OA (HOA), cooking OA (COA), and coal combustion OA (CCOA), and one secondary factor, i.e., oxygenated OA (OOA). The POA dominates OA during wintertime, contributing 69 %, with the other 31 % being SOA. Further, all POA components show pronounced diurnal cycles with the highest concentrations occurring at nighttime. CCOA is the largest primary source during the heating season, on average accounting for 33 % of OA and 17 % of NR-PM 1 . CCOA also plays a significant role in chemically resolved particulate matter (PM) pollution as its mass contribution increases linearly as a function of NR-PM 1 mass loadings. The SOA, however, presents a reverse trend, which might indicate the limited SOA formation dur-ing high PM pollution episodes in winter. The effects of me-teorology on PM pollution and aerosol processing were also explored. In particular, the sulfate mass is largely enhanced during periods with high humidity because of fog process-ing of high concentration of precursor SO 2 . In addition, the increased traffic-related HOA emission at low temperature is also highlighted.