Extreme heat exacerbates atmospheric ozone pollution: Holistic data-based causal investigation in China

Climate change-induced extreme heat (EH) events intensify atmospheric surface ozone (O₃) pollution, posing growing threats to public health and environment. To address the lack of rigorous empirical confirmation regarding the causal impact of EH on O₃ despite their observed co-occurrence, this study develops a robust statistical framework to assess this relationship across China. The framework integrates three checkpoints: (1) correlational checkpoint based on linear mixed model; (2) causal checkpoint based on instrumental variable, Heckman two-step selection model, and propensity score methods with generalized bootstrap validation; and (3) directional (of causality) checkpoint based on transfer entropy. Results consistently demonstrate a statistically significant causal effect of EH on elevated O₃ levels, with the dominant influence flowing from EH to O₃ nationwide and across most provinces in China. These findings support integrated air quality management and climate adaptation strategies, guiding policy interventions to mitigate the compound risks of EH and O₃ pollution.