Ammonia chemistry and oxidation dynamics as dual driving factors of PM2.5 nitrate pollution: Insights from the spatiotemporal disparities in central China

Despite marked improvements in air quality across China, nitrate-driven PM_2.5 pollution in central China has exhibited divergent trends tied to urbanization gradients, with underlying mechanisms remaining poorly understood. Through six cold-season observational campaigns (2017/2018–2022/2023) across urban, rural, and suburban sites, we identified ammonia-dominated chemistry and oxidation dynamics as dual drivers of contrasting nitrate trends. Despite substantial NO₂ reductions (−34.7 %), urban nitrate increased by 20.4 %, driven by lack of coordinated NH₃ control (+16.1 %) and enhanced oxidation. The intensity of heterogeneous reactions increased the most during observations due to elevated aerosol water content and particle acidity. In contrast, rural and suburban regions achieved nitrate declines (12.4 % and 9.0 %, respectively) through synchronized NH₃–NO₂ co-reductions, which countered oxidation enhancement despite higher NH₃. De-weathered nitrate still rose 2.8 % at the urban site but decreased by 20.1 % and 23.3 % at rural and suburban sites, further confirming urban NO_X-only strategies fail to curb nitrate pollution due to ammonia buffering and enhanced oxidation. To address this, we propose a region-specific framework: 1) prioritizing agricultural NH₃ mitigation in rural zones, 2) targeting non-agricultural NH₃ and oxidant precursors in urban hotspots, and 3) implementing hybrid emission trading systems in transitional suburbs. This study provides actionable insights for managing multi-pollutant synergies in rapidly urbanizing regions where agricultural and industrial activities intersect.