Mapping the Heterogeneity of NOx Emission Reductions across Chinese cities from 2018 to 2025

The reduction of nitrogen oxides (NO_x = NO + NO₂) emissions is crucial for air pollution control. Since 2011, China has experienced its most intensive and effective period of reducing NO_x emissions. Many previous studies have focused on emission trends, but they often lag by several years due to the lack of reliable and timely data. Leveraging the advantages of satellite-based NO₂ observations, this study uses tropospheric NO₂ vertical column densities (NO₂ TVCD) at 0.05° × 0.05° horizontal resolution from POMINO-TROPOMI products and employs a top-down emission inversion method (PHLET). The PHLET is a two-dimensional model that describes the quantitative relationship between NO_x emissions and NO₂ TVCD over a period, influenced by horizontal transport and nonlinear chemical processes. Then we estimate annual anthropogenic NO_x emissions at 0.05° × 0.05° for China from 2018 to 2025. First, it reveals that national anthropogenic NO_x emissions exhibit a declining trend, with the rate of decline slowing after 2020. Specifically, this trend is reflected at the provincial level, with 12 out of 31 provincial-level administrative regions showing similar declining trends and meeting their reduction targets in 2025. Second, most cities meeting reduction targets are distributed in North China and East China, while South China, Southwest China, and Northwest China still hold significant reduction potential. Third, the derived NO_x emissions show great consistency with ground-based surface NO₂ concentration measurements in most cities. In a few cities, discrepancies between emissions and observed concentrations stem from insufficient spatial representativeness of monitoring stations. Moreover, we further validated this conclusion through an atmospheric chemistry transport model (GEOS-Chem) simulation against ground-based measurements.