Megacity particulate matter airsheds redefine urban pollution exposure boundary

Urban air quality assessments typically focus on populations within municipal boundaries, despite atmospheric transport allowing pollution to affect large populations well beyond city limits. Here we develop a data-driven, reduced-complexity airshed framework to delineate the spatial domain relevant to fine particulate matter (particles no larger than 2.5 micrometers in diameter) exposure for 53 global megacities. The airshed definition combines satellite-derived pollution estimates, meteorological data, and geographic information. We find that megacity airsheds frequently extend beyond municipal boundaries and have a median area five times that of the corresponding municipal regions. Across the 53 megacities, approximately 44% of the exposed population resides outside municipal boundaries, corresponding to roughly 400 million people. Our results reveal a potential mismatch between administrative definitions and the extended population exposure footprint. By accounting for transboundary exposure, the airshed framework may improve the spatial interpretation of urban air pollution exposure and highlight the need for cross-jurisdictional pollution management. Megacity particulate matter airsheds frequently extend far beyond administrative boundaries, highlighting the need for cross-jurisdictional pollution management, according to analyses of population exposure for 53 global megacities.