Elucidating Contributions of Anthropogenic and Soil NOx Emissions Changes to O3 Trends Over China

Surface ozone (O3) pollution is a critical environmental challenge, but existing research predominantly focuses on its summer peaks. Our analysis shows that high-O3 episodes now expand into spring and autumn in China, with warm-season (April–September) maximum daily average 8-hr (MDA8) O3 concentrations rising at 5.2 μg m−3 yr−1 during 2014–2023. Current O3 mitigation focuses on anthropogenic NOx emissions (ANOx) while neglecting the contribution of soil NOx emissions (SNOx) to tropospheric O3 formation. Here, we developed an innovative framework combining the machine learning model (XGBoost) with the Unified Inputs for WRF-Chem (UI-WRF-Chem) to quantify the impacts of ANOx and SNOx on O3 increases during 2015–2019. Results show that although warm-season SNOx constitute only 23%–27% of total NOx emissions in China (2015–2022), they generally drive O3 increases, exhibiting distinct regional and seasonal heterogeneities. Compared to urban areas, the O3 formation regimes in rural areas of the Yangtze River Delta region predominantly shift to NOx-limited, making O3 highly sensitive to SNOx. SNOx changes lead to warm-season MDA8 O3 increases of 1.0–1.3 μg m−3 during 2016–2019 relative to 2015. Although similar regime shifts have occurred, rural areas in the Beijing-Tianjin-Hebei and Fenwei Plain regions still retain strong VOCs-limited characteristics; thus, SNOx perturbation impacts on O3 are smaller. Although summer SNOx are higher, their contribution to O3 increases in transitional seasons (April, May, and September) shows an upward trend, suggesting more attention should be paid to fertilization-driven SNOx. Our study highlights that future O3 control strategies should account for SNOx and their regional and seasonal differences.