Seasonal variation and secondary formation of size-segregated aerosol water-soluble inorganic ions during pollution episodes in Beijing

• Seasonal characteristics of size-segregated ions in pollution episodes were analyzed.
• Contribution of droplet-mode secondary ions was clearly elevated on polluted days.
• Relative humidity and temperature affect secondary formation of sulfate and nitrate.

Particulate matter (PM) pollution is a serious issue that has aroused great public attention in Beijing. To examine the seasonal characteristics of aerosols in typical pollution episodes, water-soluble inorganic ions (SO42 −, NO3−, NH4+, Cl−, K+, Na+, Ca2 + and Mg2 +) in size-segregated PM collected by an Anderson sampler (equipped with 50% effective cut-off diameters of 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.65, 0.43 μm and an after filter) were investigated in four intensive campaigns from June 2013 to May 2014 in the Beijing urban area. Pronounced seasonal variation of TWSIs in fine particles (aerodynamic diameter less than 2.1 μm) was observed, with the highest concentration in summer (71.5 ± 36.3 μg/m3) and the lowest in spring (28.1 ± 15.2 μg/m3). Different ion species presented different seasonal characteristics of mass concentration and size distribution, reflecting their different dominant sources. As the dominant component, SO42 −, NO3− and NH4+ (SNA) in fine particles appeared to play an important role in the formation of high PM pollution since its contribution to the TWSIs and PM2.1 mass increased significantly during pollution episodes. Due to the hygroscopic growth and enhanced secondary formation in the droplet mode (0.65–2.1 μm) from clean days to polluted days, the size distribution peak of SNA in the fine mode tended to shift from 0.43–0.65 μm to 0.65–2.1 μm. Relative humidity (RH) and temperature contributed to influence the secondary formation and regulate the size distributions of sulfates and nitrates. Partial correlation analysis found that high RH would promote the sulfur and nitrogen oxidation rates in the fine mode, while high temperature favored the sulfur oxidation rate in the condensation mode (0.43–0.65 μm) and reduced the nitrogen oxidation rate in the droplet mode (0.65–2.1 μm). The NO3−/SO42 − mass ratio in PM2.1 (73% of the samples) exceeded 1.0, suggesting that vehicle exhaust currently makes a greater contribution to aerosol pollution than stationary sources in Beijing.