Heatwaves trigger severe surface ozone pollution in India: regional hotspots, trends and health effects

Compound weather extremes are a growing concern under climate change. We present the first comprehensive long-term assessment of surface ozone (SfO₃) changes during heatwaves (HWs) in India using surface, satellite, and reanalysis data for 2004–2024, with SfO₃ from MERRA-2 reanalysis. In northern India, SfO₃ peaks at 85–110 µg/m³ during HWs and declines within 3–4 days after cessation. Strong positive summer trends are observed across regions, ranging from +0.07 to +0.98 (±0.03 to 0.17) µg/m³/yr in the Western Himalaya (WH), North west (NW), West Coast, and North Central (NC). During HWs, SfO₃ exceeds the WHO guideline (70 µg/m³) in all regions, with exceedance frequency of 39–45% in NW, NC, northeast, and WH (115%) in 2024. Associated COPD and IHD mortality during HWs is estimated at 26513 ± 4697 deaths in 2024. These findings underscore that coupled heat–ozone extremes are intensifying, requiring urgent integrated climate–air quality policy action.