Seasonal and regional sensitivities of secondary inorganic aerosols to NH₃ and NO_x emission reductions over China

Abstract. Secondary inorganic aerosols, comprising sulfate, nitrate, and ammonium (SNA), are major PM_2.5 pollution contributors in China, but the relative effectiveness of controlling their gaseous precursors (NH₃, NO_x, and SO₂) remains insufficiently quantified across seasons and regions. Here we use year-round WRF-Chem sensitivity simulations for 2017 to assess responses of surface SNA, population-weighted exposure, and PM_2.5-related premature mortality to independent 50 % reductions in anthropogenic NH₃, NO_x, and SO₂ emissions over China. We find that NH₃ and NO_x reductions each decrease national mean SNA by 24–36 % across seasons, whereas SO₂ reductions produce smaller decreases of 5–15 %. NH₃ reduction is most effective in winter, decreasing national mean SNA by 32.7 % (-3.88 μg m^-3), while NO_x reduction is more effective in spring and summer (up to -35.6 %). Chemical-regime analysis using a potential impact indicator reveals that NH₃-sensitive regimes dominate more than 70 % of China in winter, shifting to NO_x-sensitive regimes across eastern China in spring and summer, while SO₂-sensitive regimes appear mainly over the summertime arid west. Seasonal regime evolution shows a clear east–west contrast broadly aligned with the Hu Line. Halving either NH₃ or NO_x emissions could avert ~200,000 PM_2.5-related SNA-attributable premature deaths annually, with NO_x reductions offering larger national benefits and NH₃ reductions yielding greater benefits in several megacities. These results support seasonally and regionally differentiated NH₃ and NO_x control strategies for future PM_2.5 mitigation in China.