Heavy air pollution episodes in Beijing during January 2013: inorganic ion chemistry and source analysis using Highly Time-Resolved Measurements in an urban site

Heavy air pollution episodes occurred in Beijing in January 2013 attracted intensively attention around the whole world. During this period, the authors conducted highly time-resolved measurements of water soluble ions associated with PM2.5 at an urban site, and attempted to distinguish the ion chemistry and potential sources. In this study, hourly mean concentrations of Cl−, NO3−, SO42−, Na+, NH4+, K+, Mg2+ and Ca2+ were measured during the air pollution episode in January 2013, and the ions were found to exist mainly in the form of (NH4)2SO4, NH4NO3, NaCl and KCl in aerosol particles by correlation and linear analysis. SO42− and NO3− were observed peak concentrations in 10–15, 18–20, 21–24, and 26–30 January during this monitoring campaign. The percentage of SO42− and NH4+ in total ions concentrations exhibited an increasing trend with the enhancement of PM2.5 concentration, indicating high concentrations of SO42− and NH4+ had played important roles in the formation of air pollution episodes. Ratio of [NO3−]/[SO42−] was calculated, finding the sources of SO42− would contribute more to the formation of PM2.5 than mobile sources. Diurnal variations of SO42−, NO3−, NH4+ were examined, and all of them exhibited similar pattern with high concentration in night and relative low level at daytime. Emission from coal combustion, remote transportation at night or impact of meteorological was likely to be responsible for the high level of SO42−, NH4+ andNO3−. Potential sources were identified by applying PMF. Secondary nitrate, secondary sulfate, coal combustion and biomass burning, as well as fugitive dust were considered as the major contributors to total ions.