Characterizing the Compound Extreme Temperature and PM_2.5 Pollution Events: A Multi-Country Analysis

The increasing frequency of climate extreme events and persistent air pollution challenges pose compound environmental risks, yet the spatiotemporal patterns and trends of compound extreme temperature-air pollution events remain poorly understood globally. In this paper, we quantify the temporal trends and spatial distributions of compound extreme temperature-PM_2.5 pollution events across 10,067 monitor locations in 43 countries spanning 2003-2023. We find that compound extreme heat and PM_2.5 pollution (heat + PM_2.5) events decreased in most countries in our sample (35/43), while compound extreme cold and PM_2.5 pollution (cold + PM_2.5) events showed diverse trends across different nations. Despite the increase in global temperature, cold + PM_2.5 events still represent a substantial component of the compound extreme temperature and PM_2.5 pollution events. Sensitivity analyses using alternative thresholds and MERRA-2 PM_2.5 data sets yield generally consistent results. The spatial heterogeneity in the compound events is linked to the diverse temperature-PM_2.5 relationships, for example, we find higher PM_2.5 concentrations during cold conditions in China, India, and Europe, while the USA and Australia show higher PM_2.5 concentrations during hot conditions. Concentrated populations in high-frequency regions further amplify the exposure burden, with India, Pakistan, and China exhibiting disproportionately higher exposure levels relative to the compound events. Given the likely health impacts of these compound events, our findings suggest that future policies should target temperature-dependent emission activities that will amplify pollution under extreme temperature conditions.