Objective To analyze the pollution characteristics and source apportionment of water-soluble ions in atmospheric fine particulate matter (PM_2.5) in Donghe district and Qingshan district of Baotou city, China.

Methods On the 10th-16th days of each month and haze days in 2022, atmospheric PM_2.5 samples were collected at two surveillance sites in Donghe district and Qingshan district of Baotou city, and the mass concentrations of five water-soluble ions (SO₄^2-, NO₃^-, Cl^-, F^-, and NH₄⁺) in atmospheric PM_2.5 were determined by ion chromatography. The secondary conversion of SO₂ and NO₂ in the atmosphere was determined by sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR). Whether the air pollution was caused by stationary pollution sources or mobile pollution sources was determined by NO₃^-/SO₄^2- ratio.

Results The mass concentrations M(P₂₅, P₇₅) of SO₄^2-, NO₃^-, Cl^-, F^-, and NH₄⁺ in atmospheric PM_2.5 at the surveillance sites in Donghe district and Qingshan district were 3.282 (2.055, 4.620), 1.510 (0.747, 3.284), 0.916 (0.268, 1.750), 0.228 (0.097, 0.473), 1.553 (0.829, 3.019) μg/m³ and 2.856 (1.720, 4.963), 1.451 (0.747, 2.940), 0.679 (0.171, 1.860), 0.177 (0.076, 0.423), and 1.408 (0.672, 2.272) μg/m³, respectively. The SOR of atmospheric PM_2.5 was the highest in the first quarter in Donghe district and the highest in the third quarter in Qingshan district. The NOR of atmospheric PM_2.5 in both Donghe district and Qingshan district was the highest in the first quarter. The annual NO₃^-/SO₄^2- ratios in Donghe district and Qingshan district were 0.49 and 0.48, respectively.

Conclusion In two urban areas of Baotou city, there are some differences in the pollution levels of five water-soluble ions in atmospheric PM_2.5. Stationary pollution sources are the main sources of air pollution in this city.