Abstract: This study investigated the impact of gaseous precursor emission reduction over key source regions on inorganic aerosols (IAs) and PM2.5 in Tianjin during a typical regional haze process with the NAQPMS (Nested Air Quality Prediction Modeling System). Based on high-precision simulations, the high IA concentration originating from the North China Plain (NCP) was transported to East China and subsequently returned to the NCP region in a clockwise pattern, resulting in two pollution periods in Tianjin. The online pollutant source-tagging method coupled with the NAQPMS was used to quantify the contribution of different source regions to IAs in Tianjin, and the NCP region was identified as the key source area, with a daily contribution of 57.6%–100%. Sensitivity experiments were performed to analyze the impact of reducing precursor emissions by 30% in the NCP region a day before a pollution event and during the pollution days in Tianjin. NH₃ control notably reduced IA and PM2.5 concentrations in Tianjin by 30.8% and 13.3%, respectively, which were 16 and 26.6 times that of SO₂ reduction and 7 and 6.4 times that of NO_x reduction, respectively. SO₂ reduction increased nitrate concentration by 3.5% in Tianjin because SO₂ control increased gaseous NH₃, and excessive NH₃ in the atmosphere can neutralize HNO₃ to nitrate production based on the thermodynamic effect. NO_x reduction exhibited positive and negative effects on nitrate in different areas of the NCP, and transport of the two effects occurring in upstream areas caused the concentrations of nitrate, IAs, and PM2.5 in Tianjin to decrease first and then increase. This study highlights that transporting positive effects in key source areas can contribute to PM2.5 reduction while transporting negative effects harms the joint prevention and control of air pollution.