Meteorological normalization of surface ozone variability across warm and cold seasons in China during 2015–2024

Abstract. Surface ozone (O₃) concentrations in China have continued to increase despite substantial reductions in primary air pollutants, but the relative roles of meteorological variability and longer-term non-meteorological changes remain uncertain across seasons and regions. Here, we applied a LightGBM-based meteorological normalization framework combined with SHapley Additive exPlanations (SHAP) to investigate interannual variations in maximum daily 8 h average O₃ (MDA8 O₃) across China during warm and cold seasons from 2015 to 2024. The model reproduced daily MDA8 O₃ variability reasonably well, with mean testing R² values of 0.71 and 0.77 in the warm and cold seasons, respectively. Observed national mean MDA8 O₃ increased by 2.1 μg m^-3 yr^-1 in the warm season and 1.7 μg m^-3 yr^-1 in the cold season. After meteorological normalization, MDA8 O₃ still increased at 1.2 μg m^-3 yr^-1 in both seasons, indicating that the decadal O₃ increase was mainly associated with non-meteorological components. SHAP analysis revealed distinct seasonal and regional meteorological associations. During the warm season, temperature and solar radiation were more important in northern inland and basin regions, whereas relative humidity and wind-field variables were more important in southern coastal regions. During the cold season, solar radiation dominated across most regions, while relative humidity was more important in the Pearl River Delta. The net meteorological contribution shifted from generally negative during 2015–2019 to positive after 2019, indicating that the recent unfavorable meteorological conditions have amplified O₃ pollution in several regions.