Anthropogenic Emission Largely Enhances Nocturnal Oxidation Chemistry in the Upper Mixing Layer of Megacities.

Nitrate radicals (NO3) are the dominant oxidants at night, shaping air quality, the climate, and ecosystems. However, the vertical variations of NO3 chemistry within the nocturnal boundary layer remain poorly understood, owing to the stratification of air masses and complex vertical chemistry. Here, we combined vertical and ground-based measurements of NO3 precursors from diverse atmospheric environments with model simulations to investigate the vertical structure of nocturnal NO3 oxidation and its long-term trend. Our results indicate that the nocturnal enhanced NO3 chemistry aloft event occurs far more frequently in megacities (64-72%) than in representative clean areas (16%), with a median enhancement factor of 2.7 relative to the surface. The event largely promotes the formation of secondary pollutants aloft rather than at the surface. We further show the event is more prevalent in China and India, attributed to intense ground-level NOx emissions, than in the United States and Europe. However, its frequency has declined rapidly in China in recent years, closely linked to the implementation of NOx emission controls. These findings highlight the critical role of vertical gradients in nocturnal NO3 chemistry on surface air pollution and underscore the need for comprehensive vertical measurements to support further improvement of urban air quality.