Long-Term Changes in Heatwave-Ozone Compound Events and Spatially Heterogeneous Ozone Associations across China

<title>Abstract</title> <p>Heatwaves and surface ozone pollution frequently co-occur during the warm season, posing compound environmental and health risks. However, the long-term evolution of different heatwave types and their influences on ozone pollution remain insufficiently characterized across China. Using CN05.1 daily maximum and minimum temperature data, the ChinaHighAirPollutants surface ozone product, ERA5 reanalysis variables, and selected socioeconomic and environmental indicators, this study examined daytime heatwaves (DHWs), nighttime heatwaves (NHWs), compound day-night heatwaves (CHWs), ozone pollution, and heatwave-ozone compound event days across China during 2000–2024. Conditional probability and risk-ratio analyses were conducted for six regional domains, while multiscale geographically weighted regression (MGWR) was used to characterize spatially varying ozone associations. Results showed significant nationwide increases in all heatwave types, ozone levels, and compound events, with NHWs and CHWs rising faster than DHWs. Ozone pollution exhibited a pronounced east-west gradient, with hotspots in the North China Plain and Yangtze River Delta, and compound events expanded substantially after 2015 beyond traditional hotspots. Heatwaves significantly elevated ozone probability, with DHWs exerting the strongest influence and CHWs prolonging ozone exposure through persistent favorable conditions. Beijing-Tianjin-Hebei region showed the highest conditional ozone probabilities, whereas the Sichuan Basin displayed the largest relative increase compared with non-heatwave conditions. MGWR revealed pronounced spatial heterogeneity: Maximum temperature and solar radiation generally showed positive associations with ozone, while relative humidity showed predominantly negative associations; temperature sensitivity of ozone increases markedly in Beijing-Tianjin-Hebei, Central China, and Eastern China between 2010 and 2020. Overall, climate warming intensifies both heatwave-ozone compound events and climatic sensitivity of ozone pollution, underscoring the need for integrated climate adaptation and air-quality management.</p>