Anthropogenic emission largely enhances nocturnal oxidation chemistry in the upper mixing layer of megacities

Nitrate radicals (NO ₃ ) play a crucial role in the removal of nitrogen oxides (NO _x ) from Earth's atmosphere and act as the dominant nocturnal oxidant in polluted regions, thereby influencing air quality, climate, and ecological systems. However, the vertical variations of NO ₃ chemistry within the planetary boundary layer during nighttime remain poorly understood due to the stratification of nocturnal air masses and complex chemical conditions. Here, we present vertical and ground-based observations of NO ₃ precursors across diverse atmospheric environments. Our results indicate that the enhanced NO ₃ chemistry aloft event, defined as the NO ₃ production rate above the canopy being higher than at surface levels, occurs frequently in megacities (64-72%) with a median enhancement factor of 2.7. This phenomenon likely largely promotes more rapid oxidation reactions and secondary pollutant formation in the aloft environment. We show this event is more prevalent in China and India than in the United States and Europe. However, a rapid decline in its frequency has been observed in China in recent years, closely linked to the implementation of stringent NO _x emission control measures. We demonstrate that this event is attributed to the interplay between intense ground-level NOx emissions and atmospheric stability. These findings highlight the critical role of vertical gradients in nocturnal NO ₃ chemistry on surface air pollution and underscore the need for comprehensive vertical measurement to support further improvement of urban air quality.