Constrained Estimates of Anthropogenic NOx Emissions in China (2014–2021) from Surface Observations

China’s rapid urbanization has precipitated severe atmospheric pollution, drawing sustained scientific and policy attention. Although nationwide implementations of emission control measures have achieved measurable reductions in ambient NO2 concentrations, fundamental uncertainties persist in quantifying anthropogenic NOx emission and their interannual variability. In this study, NOx emissions over China are inferred using the Regional Air Pollutant Assimilation System (RAPAS) combined with ground-based hourly NO2 observations, and a detailed analysis of the spatiotemporal variation patterns of NOx emissions is also provided. Nationally, most sites display declining NO2 concentrations during 2014–2021, with steeper reduction trends in winter, particularly in pollution hotspots. The RAPAS-optimized NOx emission estimates demonstrate superior performance relative to prior inventories, with site-averaged biases, root mean square errors, and correlation coefficients improved substantially across all geographic regions in China. The trajectories of changes in NOx emissions exhibit marked regional disparities: South and Northeast China experienced more than 8.0% emission growth during 2014–2017, while NOx emissions in northwest and southwest China increased by 35% and 26%, significantly higher than those in East China. The reductions accelerated significantly post 2018, particularly in central and eastern regions (more than −20%). The interannual variation in NOx emissions in the five national urban agglomerations shows a similar trend of first rising and then decreasing. The NOx emissions of Anhui, Yunnan, Shanxi, Gansu and Xinjiang provinces increased significantly from 2014 to 2017, while the emissions of Shandong and Zhejiang decreased at a relatively high rate (more than 80 Gg per year). These findings are helpful to provide a more comprehensive understanding of current NOx pollution and provide scientific basis for policymakers to propose effective strategies.