Driving factors of atmospheric nitrogen oxides emissions and pollution in China.

Human-induced nitrogen oxide (NOx) 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 NOx 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 NOx 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 NOx 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 NOy-deposition saturation threshold of 15.0 kg N ha-1 yr-1, beyond which further NOx cuts yield limited deposition reductions. Our results show NOx-only mitigation is inadequate. Co-regulating NOx, VOCs and NH3 mitigates air-quality and ecological nitrogen risks for China and similar regions.