Amplified upward trend of the joint occurrences of heat and ozone extremes in China over 2013-20

Climate change and air pollution are two intimately interlinked global concerns. The frequency, intensity, and duration of heat waves are projected to increase globally under future climate change. A growing body of evidence indicates that health risks associated with the joint exposure to heat waves and air pollution can be greater than that due to individual factors. However, the cooccurrences 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 cooccurrences of heat and air pollution extremes using China's nationwide observations of hourly PM2.5 and O-3, and the ERA5 reanalysis dataset over 2013-20. We identify a significant increase in the frequency of cooccurrence of wet-bulb temperature (w) and O-3 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 O-3 itself. For example, T-w and O-3 coextremes 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 O-3 extremes should be recognized as an emerging environmental issue in China, given the potentially larger compounding impact to public health.