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Characterization of water soluble inorganic ions and their evolution processes during PM2.5 pollution episodes in a small city in southwest China

Qiao, Baoqing, Chen, Yang, Tian, Mi, Wang, Huanbo, Yang, Fumo, Shi, Guangming, Zhang, Leiming, Peng, Chao, Luo, Qiong, Ding, Shimin
The Science of the total environment 2019 v.650 pp. 2605-2613
aerosols, ammonium, basins, emissions, inorganic ions, nitrates, nitrogen content, particulates, pollutants, pollution, sulfates, summer, water solubility, weather, winter, China
PM2.5 samples were collected in four segregate one-month periods, each representing one season, for analyzing their contents of water soluble inorganic ions (WSIIs) in a small city inside Sichuan Basin. Daily PM2.5 concentrations ranged from 23.2 to 203.1 μg m−3 with an annual mean of 66.9 ± 33.6 μg m−3. Annual mean concentrations of WSIIs was 28.8 ± 20.3 μg m−3, accounting for 43.1% of PM2.5. Seasonal mean concentrations of WSIIs ranged from 17.5 ± 9.3 μg m−3 in summer to 46.5 ± 27.6 μg m−3 in winter. Annual mean mass ratio of NO3−/SO42− was 0.49, demonstrating predominant stationary sources for secondary inorganic aerosols (SNA, including SO42−, NH4+ and NO3−); whereas annual mean molar ratio of [NH4+]/[NO3−] was 3.5, suggesting dominant agriculture emissions contributing to the total nitrogen. During a severe and long-lasting (13 days) winter pollution period when mean PM2.5 concentration reached to 132.5 μg m−3, PM2.5 concentration was enhanced by a factor of 2.6 while that of SNA by a factor of 2.9 compared to those before the pollution event, and the fraction of SNA in PM2.5 only increased slightly (from 46.7% to 50.6%). Thus, local accumulation of pollutants under poor diffusion conditions played a major role causing the extremely high PM2.5 concentration, besides the contributions from the enhanced SNA formation under specific weather conditions.