Abstract: To evaluate the effectiveness of air pollution control policies in Tianjin, we collected PM_2.5 samples over six consecutive winters from 2017 to 2022 and measured the concentration of water-soluble inorganic ions (SO₄^2-, NO₃^-, NH₄⁺, Cl^-, K⁺, Ca²⁺, Na⁺, Mg²⁺). The results revealed a marked decline in the mass concentrations of SO₄^2-, NO₃^- and NH₄⁺ in PM_2.5 from 2017 to 2022, with reductions of 83.9%, 46.2%, and 55.2%, respectively. However, the proportion of NO₃^- within water-soluble inorganic ions (WSIIs) has been rising annually, with the dominant form of secondary inorganic ions (SO₄^2-, NO₃^- and NH₄⁺) transitioning from (NH₄)₂SO₄ to NH₄NO₃. The ratios of NO₃^-/SO₄^2-, NO₃^-/Cl^- and NO₃^-/K⁺ have been increasing annually as well, indicating that compared to stationary sources such as coal combustion and biomass burning, the influence of mobile sources such as motor vehicle exhaust on air quality in Tianjin was increasing. Backward trajectory analyses showed that the PM_2.5 in Tianjin was mainly influenced by air masses from the northwest, the southern part of Tianjin, and the central Hebei, without interannual differences in potential source areas. Furthermore, compared to short-distance air masses around Tianjin, SO₄^2- carried by longdistance air masses from the northwest showed a significant decrease. The reductions in NO₃^- and NH₄⁺ concentrations were relatively modest. Notably, NO₃^- and NH₄⁺ levels associated with short-distance air masses displayed an increasing trend in 2020, indicating the need to strengthen vehicle exhaust emissions control in Tianjin and the surrounding areas.

Key words: Tianjin, PM^2.5, water-soluble inorganic ion, winter