Objective To analyze the pollution characteristics of four water-soluble ions in atmospheric fine particulate matter (PM_2.5) in Xining, China.

Methods PM_2.5 samples were collected in Xining from the 10th to the 16th day of each month from 2018 to 2023. Ion chromatography was used to determine the concentrations of NO₃^-, SO₄^2-, Cl^-, and NH₄⁺ in PM_2.5; the Kruskal-Wallis H test was used to compare the concentrations of the four water-soluble ions across different years, seasons, and air quality grades. ρ(NO₃^-)/ρ(SO₄^2-), sulfur oxidation ratio (SOR), and nitrogen oxidation ratio (NOR) were used to analyze the characteristics of pollution sources of PM_2.5.

Results The median mass concentrations of the four water-soluble ions (SO₄^2-, NO₃^-, Cl^-, and NH₄⁺) in Xining from 2018 to 2023 were 3.71, 2.76, 0.73, and 1.92 μg/m³, respectively. There was a significant difference in the mass concentration of NO₃^-, NH₄⁺, Cl^-, and secondary inorganic aerosol (SIA) across different years (χ²=36.70-157.50, P < 0.05); the mass concentrations of SO₄^2- and SIA were the highest in 2018 (corrected P < 0.05); there was a significant difference in the mass concentrations of SO₄^2-, NO₃^-, Cl^-, NH₄⁺, and SIA across different seasons (χ²=109.85-412.70, P < 0.05). The mass concentrations of SO₄^2-, NO₃^-, Cl^-, NH₄⁺, and SIA were the highest in winter (corrected P < 0.05). There was a significant difference in the mass concentrations of SO₄^2-, NO₃^-, Cl^-, NH₄⁺, and SIA across different air quality grades (χ²=162.95-337.64, P < 0.05). The mass concentrations of SO₄^2-, NO₃^-, Cl^-, NH₄⁺, and SIA were higher on polluted days than on days with excellent or good air quality (corrected P < 0.05). The medians of SOR and NOR in PM_2.5 were 0.19 and 0.09, respectively, indicating that SO₄^2- in PM_2.5 was the main secondary pollutant. The medians of SOR and NOR were higher on polluted days than on days with excellent or good air quality. The median of ρ(NO₃^-)/ρ(SO₄^2-) was 0.88, indicating that compared with mobile source emissions, fixed source emissions contributed more to the mass concentration of PM_2.5 and the median of ρ(NO₃^-)/ρ(SO₄^2-) was higher on polluted days than on days with excellent or good air quality. The median of ρ(NO₃^-)/ρ(SO₄^2-) was more than 1 on polluted days in winter, indicating that compared with fixed source emissions, mobile source emissions contributed more to the mass concentration of PM_2.5 on polluted days in winter. There were no significant differences in SOR, NOR, and ρ(NO₃^-)/ρ(SO₄^2-) between weekdays and weekends (χ²=0.099-0.427, P>0.05).

Conclusion The pollution degree of water-soluble ions in atmospheric PM_2.5 in Xining is decreasing year by year. The pollution of water-soluble ions is more serious in winter, with mobile source emissions as the main pollutants in winter and fixed source emissions as the main pollutants in spring, summer, and autumn.