• • Water-soluble ions averaged 26.04 μg m−3, comprising 52.1% of PM2.5 mass; SNA (NO3−, NH4+, SO42−) accounted for 87.6% of total ions, underscoring the dominance of secondary inorganic aerosols in Taiyuan's autumn-winter PM2.5.
• • The NO3−/SO42− mass ratio of 1.83 confirms a regime shift from sulfate to nitrate dominance, implying that controlling NOx emissions is now more critical than SO2 for mitigating PM2.5 pollution.
• • During moderate pollution, SO42− surged to 5.31 times clean-day levels, while during heavy pollution, NO3− surged to 5.37 times, indicating that nitrate formation is the key driver of severe haze episodes.
• • PMF source apportionment revealed that secondary sources contributed 45.4% on clean days but escalated to 64.7% and 63.4% on lightly and moderate-to-heavy polluted days, respectively, highlighting the increasing role of secondary formation in pollution exacerbation.