A Multi-Agglomeration Assessment of Air Quality Responses to Top-Down NOx Emission Changes: Insights from Trends in Surface NO2 and O3 Across Urban China (2014–2021)

China’s stringent clean air policies have substantially reduced nitrogen oxides (NOx) emissions, leading to a general decline in nitrogen dioxide (NO2). However, surface ozone (O3) pollution remains severe, creating a complex challenge due to the non-linear relationship between O3 and its precursors. To disentangle the drivers behind these trends, this study quantifies the impacts of interannual variations in top-down constrained NOx emissions on surface NO2 and O3 concentrations from 2014 to 2021 across mainland China and five national urban agglomerations. We employed the WRF-CMAQ model with a fixed-emission simulation approach, using an observationally optimized NOx emission inventory derived from the assimilation of surface NO2 measurements. Results reveal that NO2 reductions were predominantly emission-driven (>80% post-2017), with declines most pronounced in winter. A strong linear consistency was found between interannual changes in top-down NOx emissions and attributed NO2 concentration variations, validating the methodology. In contrast, O3 responses to NOx reductions were spatially and seasonally heterogeneous, reflecting a non-linear photochemical regime. In major urban agglomerations (e.g., Beijing–Tianjin–Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD)), NOx reductions post-2018 showed limited effectiveness in mitigating summertime O3 and even increased O3 in spring and autumn, indicating a prevalent VOC-sensitive regime where NOx reduction can disinhibit O3 formation. Conversely, certain provinces (e.g., Anhui, Shanxi, Jilin) exhibited O3 decreases, suggesting a NOx-sensitive regime. The area benefiting from NOx reductions expanded steadily in summer after 2017 but not in other seasons. This study confirms the efficacy of NOx-focused policies for reducing primary NO2 pollution but highlights that mitigating persistent O3 requires a strategic shift to synergistic, region-specific control of volatile organic compounds alongside NOx, informed by local chemical sensitivity.