Changes in Urban–Nonurban Ozone Disparities across China during 2000–2024

Surface ozone formation mechanisms differ between urban and nonurban areas because of distinct meteorological conditions and emission profiles, often leading to higher ozone concentrations in nonurban regions. Recent changes in climate and emissions have modified the ozone disparity between urban and nonurban areas in China; however, the temporal evolution of this difference remains debated, and its underlying causes are not yet fully understood. Here, using a high-spatiotemporal-resolution maximum daily 8-h average (MDA8) ozone data set for 2000–2024, we show that the ozone disparity in China first declined and then increased after 2013. Using machine learning approaches, including extreme gradient boosting (XGBoost) and Shapley additive explanations (SHAP), we identified the key factors influencing these changes across three major city clusters: Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), and Pearl River Delta (PRD). From 2013 to 2019, reductions in anthropogenic nitrogen oxide emissions and fine particulate matter (PM2.5) concentrations were the primary contributors to the changing ozone disparity. Meteorological contribution to the variations in urban–nonurban ozone disparities increased after 2020. In the context of climate change, urban ozone mitigation in China faces escalating challenges, especially in the PRD, which is more susceptible to climatic shifts and exhibits more pronounced urban–nonurban ozone disparities.