Joint occurrence of heatwaves and ozone pollution and increased health risks in Beijing, China: role of synoptic weather pattern and urbanization

Heatwaves (HWs) paired with higher ozone (O₃) concentration at the surface level pose a serious threat to human health. Their combined modulation of synoptic patterns and urbanization remains unclear. Using 5 years of summertime temperature and O₃ concentration observation in Beijing, this study explored potential drivers of compound HWs and O₃ pollution events and their public health effects. Three favorable synoptic weather patterns were identified to dominate the compound HWs and O₃ pollution events. These weather pat- terns contributing to enhance those conditions are characterized by sinking air motion, low boundary layer height, and high temperatures. Under the synergy of HWs and O₃ pollution, the mortality risk from all non-accidental causes increased by approximately 12.31 % (95 % confidence interval: 4.66 %, 20.81 %). Urbanization caused a higher risk of HWs and O₃ in urban areas than at rural stations. Particularly, due to O₃ depletion caused by NO titration at traffic and urban stations, the health risks related to O₃ pollution in different regions are charac- terized as follows: suburban stations > urban stations > rural stations > traffic stations. In general, favorable synoptic patterns and urbanization enhanced the health risk of these compound events in Beijing by 33.09 % and 18.95 %, respectively. Our findings provide robust evidence and implications for forecasting compound HWs and O₃ pollution events and their health risks in Beijing or in other urban areas all over the world that have high concentrations of O₃ and high-density populations.