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

PM 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 PM concentrations ranged from 23.2 to 203.1 μg m with an annual mean of 66.9 ± 33.6 μg m. Annual mean concentrations of WSIIs was 28.8 ± 20.3 μg m, accounting for 43.1% of PM. Seasonal mean concentrations of WSIIs ranged from 17.5 ± 9.3 μg m in summer to 46.5 ± 27.6 μg m in winter. Annual mean mass ratio of NO/SO was 0.49, demonstrating predominant stationary sources for secondary inorganic aerosols (SNA, including SO, NH and NO); whereas annual mean molar ratio of [NH]/[NO] 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 PM concentration reached to 132.5 μg m, PM 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 PM 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 PM concentration, besides the contributions from the enhanced SNA formation under specific weather conditions.