Compound heat and ozone pollution in urban areas

The frequent occurrences of heat wave events and air pollution episodes have become pressing global concerns. Concurrent heat and ozone pollution events, in particular, have been widely documented across various regions and often result in more severe impacts compared to isolated stressors, leading to increased mortality and morbidity rates. However, our understanding of these compound events in urban environments, particularly their dynamics under different background climates and urban settings, remains limited. In this study, we systematically characterized the frequency, intensity, and duration of compound heat and ozone pollution events during warm seasons across all urban areas in the continental U.S. using long-term, high-resolution daily air pollution and air temperature datasets. Results suggest that urban heat waves, defined by daily maximum temperature, were more frequent, more intense, and longer lasting than their rural counterparts, primarily due to the urban heat island effect. In contrast, over half of the U.S. cities experienced fewer, less intense, and shorter ozone pollution episodes than surrounding rural environments. The spatially heterogeneous disparities in ozone pollution episodes among cities are mainly attributed to whether ozone production is limited by VOC or NOx, as revealed by time series analyses. Despite the overall decreasing trend of surface ozone concentrations during the last two decades, 89% of U.S. cities experienced more frequent compound heat and ozone pollution episodes than rural areas. Additionally, the cumulative heat and ozone intensities were higher in 91% and 88% of U.S. cities, respectively, than in their rural backgrounds. The duration of compound events tends to be shorter in urban areas. These findings highlight the dependence of such compound events on local and background conditions, emphasizing the need for locally tailored mitigation plans to reduce their impacts. This study also calls for detailed regional numerical simulations to elucidate the mechanisms driving these events.