Analysis of chemical characteristics of PM 2.5 in Beijing over a 1-year period

Beijing is one of the largest and most densely populated cities in China. PM 2.5 (fine particulates with aerodynamic diameters less than 2.5 μm) pollution has been a serious problem in Beijing in recent years. To study the temporal and spatial variations in the chemical components of PM 2.5 and to discuss the formation mechanisms of secondary particles, SO 2 , NO 2 , PM 2.5 , and chemical components of PM 2.5 were measured at four sites in Beijing, Dingling (DL), Chegongzhuang (CG), Fangshan (FS), and Yufa (YF), over four seasons from 2012 to 2013. Fifteen chemical components, including organic carbon (OC), elemental carbon (EC), K + , NH 4 + , NO 3 − , SO 4 2− , Cl − , Al, Ca, Fe, Mg, Na, Pb, Si, and Zn, were selected for analysis. Overall, OC, SO 4 2− , NO 3 − , and NH 4 + were dominant among 15 components, the annual average concentrations of which were 22.62 ± 21.86, 19.39 ± 21.06, 18.89 ± 19.82, and 13.20 ± 12.80 μg·m −3 , respectively. Compared with previous studies, the concentrations of NH 4 + were significantly higher in this study. In winter, the average concentrations of OC and EC were, respectively, 3 and 2.5 times higher than in summer, a result of coal combustion during winter. The average OC/EC ratios over the four sites were 4.9, 7.0, 8.1, and 8.4 in spring, summer, autumn, and winter, respectively. The annual average [NO 3 − ]/[SO 4 2− ] ratios in DL, CG, FS, and YF were 1.01, 1.25, 1.08, and 1.12, respectively, which were significantly higher than previous studies in Beijing, indicating that the contribution ratio of mobile source increased in recent years in Beijing. Analysis of correlations between temperature and relative humidity and between SOR ([SO 4 2− ]/([SO 4 2− ] + [SO 2 ])) and NOR ([NO 3 − ]/([NO 3 − ] + [NO 2 ])) indicated that gas-phase oxidation reactions were the major formation mechanism of SO 4 2− in spring and summer in urban Beijing, whereas slow gas-phase oxidation reactions and heterogeneous reactions both occurred in autumn and winter. NO 3 − was mainly formed through year-round heterogeneous reactions in urban Beijing.