Cities as hotspots of compound heat and fine particulate matter pollution: A 23-year urban-rural comparison across the United States.

Heat stress and fine particulate matter (PM2.5) pollution are major stressors that threaten public health and environmental quality. When heat waves and PM2.5 pollution episodes co-occur as compound events, their impacts intensify, often leading to increased mortality and morbidity. This study provides a 23-year (2000-2022) analysis of heat waves, PM2.5 pollution episodes, and their compound occurrences during the warm season across urban and surrounding rural areas of the contiguous U.S. using reconstructed daily minimum air temperature and PM2.5 datasets. Results show that urban areas generally experienced more frequent, prolonged, and intense events than rural surroundings. Specifically, 98.8 % of cities had more frequent heat waves, 88.6 % experienced longer durations, and 98.8 % were more intense, primarily due to the nighttime urban heat island effect. Similarly, 85.4 % of cities had more frequent PM2.5 pollution episodes, 52.8 % experienced longer durations, and 80.5 % exhibited higher cumulative pollution intensity, largely driven by urban emissions. Compound heat and PM2.5 pollution episodes were more frequent and intense in ∼98 % of urban areas, with more than half experiencing longer durations. The spatial patterns of compound events closely resembled those of PM2.5 pollution episodes, suggesting that air pollution plays a dominant role in their occurrence. Notably, after a declining trend through ∼2016, the number of PM2.5 pollution and compound event days increased in the western U.S. in recent years due to rising wildfire emissions. These findings highlight the heightened environmental risks experienced by urban populations and emphasize the need for city- and region-specific heat and pollution mitigation strategies.