Characteristics and Source Apportionment of Water-Soluble Ions in PM2.5 in Taiyuan during Autumn and Winter 2023
To investigate the pollution characteristics and temporal variations of water-soluble ions in atmospheric fine particulate matter (PM2.5) during autumn and winter in Taiyuan City, continuous sampling and analysis were conducted using a Swiss Metrohm ion chromatograph in autumn and winter 2023. The results show that the daily mean concentration of nine water-soluble ions was 26.04 μg m−3, accounting for 52.1% of the average PM2.5 mass concentration. Ion concentrations ranked in descending order: NO3−, NH4+, SO42−, Cl−, K+, Na+, Ca2+, F−, and Mg2+. Secondary inorganic ions (SNA) constituted 87.6% of total water-soluble ions. The mass ratio of NO3− to SO42− reached 1.83, indicating a shift from sulfate-dominated to nitrate-dominated aerosol chemistry. During pollution episodes, water-soluble ion concentrations increased exponentially, with distinct ion-specific trends: on moderately polluted days, SO42− increased to 5.31 times that on clean days, whereas on heavily polluted days, NO3− increased to 5.37 times, highlighting nitrate as a primary driver of severe pollution. Comparison with historical data reveals a recent increase in the proportion of water-soluble ions in PM2.5, with higher proportions during more polluted periods. Analysis of NH4+ forms and PM2.5 acidity suggests that acidic components contribute more under heavier pollution. Positive matrix factorization (PMF) identified four major sources: secondary sources, combustion and motor vehicles, industrial sources, and dust. The secondary source contributions were 45.4%, 64.7%, and 63.4% on clean, lightly polluted, and moderate-to-heavy polluted days, respectively, indicating a significantly higher secondary contribution on polluted days.