Driving factors of atmospheric nitrogen oxides emissions and pollution in China

Human-induced nitrogen oxide (NO_x) emissions have long dominated atmospheric nitrate (NO₃^-) pollution in China, yet linkages among anthropogenic drivers, emission trends, and deposition responses remain poorly quantified. Using δ¹⁵N of particulate and precipitation NO₃^- across eight Chinese regions for 2001-2022, we provide comprehensive isotopic constraints on NO_x sources and their atmospheric fate. Spatially, modelled source-fraction estimates suggest relatively higher coal-combustion contributions in Northeast and North China and larger microbial N cycles contributions in the Southwest and Northwest, though these regional differences lack statistical significance and cannot be robustly resolved with current datasets. Nationally, fossil-fuel combustion contributes ∼53% of total NO_x emissions, and non-fossil sources (biomass burning, microbial N cycles) account for the remaining 47%. These patterns correlate closely with population density, energy consumption, transport networks and agricultural activities. Temporally, a significant δ¹⁵N decline from 2001 to 2022 records a systematic source transition driven by stringent denitrification policies, vehicle emission standards, and shifting agricultural practices. Post-2011 controls caused a 35% drop in surface NO₂ but only a 19% reduction in NO₃^- deposition, demonstrating decoupling between surface NO₂ and NO₃^- deposition. Our isotope-derived total NO_x shows a non-significant 7% decline (p=0.19), precluding robust inference of emission-deposition decoupling. This NO₂-NO₃^- mismatch arises from enhanced atmospheric oxidants and sustained NH₃. We identify a NO_y-deposition saturation threshold of 15.0 kg N ha^-1 yr^-1, beyond which further NO_x cuts yield limited deposition reductions. Our results show NO_x-only mitigation is inadequate. Co-regulating NO_x, VOCs and NH₃ mitigates air-quality and ecological nitrogen risks for China and similar regions.