Amplified upward trend of the joint occurrences of heat and ozone extremes in China over 2013–2020

Climate change and air pollution are two intimately interlinked global concerns. The frequency, intensity and duration of heatwaves are projected to increase globally under future climate change. A growing body of evidence indicates that health risks associated with the joint exposure to heatwaves and air pollution can be greater than that due to individual factors. However, the co-occurrences of heat and air pollution extremes in China remain less explored in the observational records. Here we investigate the spatial pattern and temporal trend of frequency, intensity, and duration of co-occurrences of heat and air pollution extremes using China’s nationwide observations of hourly PM_2.5 and O₃, and the ERA5 reanalysis dataset over 2013–2020. We identify a significant increase in the frequency of co-occurrence of wet-bulb temperature (T_w) and O₃ exceedances (beyond a certain predefined threshold), mainly in the Beijing-Tianjin-Hebei (BTH) region (up by 4.7 days decade^-1) and the Yangtze River Delta (YRD). In addition, we find that the increasing rate (compared to the average levels during the study period) of joint exceedance is larger than the rate of T_w and O₃ itself. For example, T_w and O₃ co-extremes increased by 7.0% in BTH, higher than the percentage increase of each at 0.9% and 5.5%, respectively. We identify same amplification for YRD. This ongoing upward trend in the joint occurrence of heat and O₃ extremes should be recognized as an emerging environmental issue in China, given the potentially larger compounding impact to public health.