China’s stringent clean air policies have substantially reduced nitrogen oxides (NO
x) emissions, leading to a general decline in nitrogen dioxide (NO
2). However, surface ozone (O
3) pollution remains severe, creating a complex challenge due to the non-linear relationship between O
3 and its precursors. To disentangle the drivers behind these trends, this study quantifies the impacts of interannual variations in top-down constrained NO
x emissions on surface NO
2 and O
3 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 NO
x emission inventory derived from the assimilation of surface NO
2 measurements. Results reveal that NO
2 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 NO
x emissions and attributed NO
2 concentration variations, validating the methodology. In contrast, O
3 responses to NO
x 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)), NO
x reductions post-2018 showed limited effectiveness in mitigating summertime O
3 and even increased O
3 in spring and autumn, indicating a prevalent VOC-sensitive regime where NO
x reduction can disinhibit O
3 formation. Conversely, certain provinces (e.g., Anhui, Shanxi, Jilin) exhibited O
3 decreases, suggesting a NO
x-sensitive regime. The area benefiting from NO
x reductions expanded steadily in summer after 2017 but not in other seasons. This study confirms the efficacy of NO
x-focused policies for reducing primary NO
2 pollution but highlights that mitigating persistent O
3 requires a strategic shift to synergistic, region-specific control of volatile organic compounds alongside NO
x, informed by local chemical sensitivity.
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