City-level Disparities in NOX Emission Trends and Their Inhibitory Effects on O3 Mitigation in China

As a major air pollutant and precursor of ozone (O₃), anthropogenic nitrogen oxides (NO_X = NO + NO₂) have been effectively controlled in China since peaking around 2012. However, the evolving contrast of emissions across cities and its impacts on secondary pollutants such as O₃ remain poorly understood, primarily due to the limitations of existing emission inventories. Here we track the historical high-resolution (5 km) NO_X emissions based on POMINO-OMI and POMINO-TROPOMI NO₂ VCDs, adopting our previously developed inversion, PHLET. The results demonstrate significantly weaker NO_X emission declines in economically small cities where environmental pollution received much less attentions, leading to a shift of emission burdens toward western and non-capital cities. Moreover, simulations based on GEOS-Chem indicate that such disparities in NO_X emission trends have inhibited the mitigation of O₃ mainly in the western China, and even added up to the O₃ increase in some areas of the North China Plain. Our study points to the value of satellite-based inversion to access historical environmental regulations, and emphasizes the importance of collaborative pollution control across regions for comprehensive pollution control in China and other Global South countries undergoing rapid emission changes.