Objective To investigate the pollution levels of NO₃^-, SO₄^2-, Cl^-, and NH₄⁺ in atmospheric fine particulate matter(PM_2.5) and the characteristics of pollution sources in Tongzhou District of Beijing, China.

Methods Atmospheric PM_2.5 samples were collected for 252 days on the 10th to 16th days of each month in 2016—2018. Ion chromatography was used to determine the mass concentration of NO₃^-, SO₄^2-, Cl^-, and NH₄⁺ in PM_2.5, and the Kruskal-Wallis H test was used to compare the concentration of the four water-soluble ions among different years, seasons, and air quality grades. NO₃^-/SO₄^2-, SOR, and NOR were used to analyze the characteristics of pollution sources of the four water-soluble ions in atmospheric PM_2.5.

Results The median mass concentrations of PM_2.5 and NO₃^-, SO₄^2-, NH₄⁺, and Cl^- in atmospheric PM_2.5 were 62, 7.36, 5.60, 4.16, and 0.84 μg/m³, respectively, in Tongzhou District in 2016—2018. There was a significant difference in the mass concentration of NO₃^-, NH₄⁺, and Cl^- among different years (P < 0.05). The concentration of NO₃^- in 2018 was higher than that in 2017, the concentration of NH₄⁺ in 2017 was lower than that in 2016, and the concentration of Cl^- in 2018 was lower than that in 2016. There was a significant difference in the mass concentration of NO₃^-, SO₄^2-, and Cl^- among different seasons (P < 0.05). Among the four water-soluble ions in atmospheric PM_2.5, NO₃^- had the highest mass concentration in spring, autumn, and winter, while SO₄^2- had the highest mass concentration in summer, and Cl^- had the highest mass concentration in winter and the lowest mass concentration in summer. The mass concentrations of the four water-soluble ions in atmospheric PM_2.5 showed varying degrees of increase with the aggravation of air pollution, and there was also a significant difference among different air quality grades (P < 0.05). The mass concentrations of NO₃^-, SO₄^2-, NH₄⁺, and Cl^- on heavily polluted days were increased by 27.4, 10.4, 15.0, and 21.4 times compared with those on clean days with good air quality. The medians of NO₃^-/SO₄^2-, SOR and NOR in atmospheric PM_2.5 were 1.29, 0.41, and 0.13, respectively, in Tongzhou District in 2016—2018, indicating that compared with fixed source emission, mobile source emission contributed more to the mass concentration of water-soluble ions in atmospheric PM_2.5, and the main source of NO₃^- and SO₄^2- in atmospheric PM_2.5 was the secondary pollutants produced by photochemical reaction.

Conclusion The mass concentration and secondary transformation of NO₃^-, SO₄^2-, NH₄⁺, and Cl^- in atmospheric PM_2.5 show different changing trends across years and seasons in Tongzhou District of Beijing in 2016—2018, and NO₃^- shows the most significant cumulative effect and makes great contributions to the pollution process. Compared with fixed sources, mobile sources such as automotive exhaust emission have greater contributions to air pollution. Mobile source emission should be further strictly controlled on the basis of strict control of coal-smoke pollution.