Mechanisms and attribution of frequent compound heatwave and ozone pollution events in eastern China

Compound heatwave and ozone pollution events (CHOEs) have increased sharply in eastern China, posing significant environmental risks. This study quantified the spatiotemporal evolution of CHOEs in eastern China from 1980 to 2024. The Liang–Kleeman information flow was applied to assess large-scale climate forcing based on climate indices. Nighttime light and precursor emission data were used to evaluate urbanization and emission contributions. The results show a sharp increase in CHOEs after 2016, with high-frequency areas concentrated in four urban clusters. Peak periods have expanded from summer to both ends of the warm season. Conditional probability analysis indicates a sustained rise in the probability of ozone exceedance during heatwave conditions. Liang–Kleeman information flow analysis identifies the South China Sea Subtropical High Ridge ( T = 0.405) and Indian Ocean Warm Pool Strength ( T = 0.381) as the leading climate factors closely associated with the interannual variability of CHOEs. Favorable meteorological conditions for CHOEs are established through the development of high-pressure anomalies, subsidence, and enhanced shortwave radiation. The former primarily affects the Yangtze River and Sichuan basins, whereas the latter primarily influences North China. Rapid urban expansion closely matches the spatiotemporal increase in CHOEs. Substantial NO x emission reductions, together with relatively lagged VOCs reductions, drove an increase in the VOC/NO x ratio. This weakened the dependence of regional ozone formation on NO x and enhanced ozone accumulation during heatwaves. These results highlight the synergistic roles of climate variability and anthropogenic emissions on CHOEs, providing a scientific basis for regional risk assessments.