Meteorological Controls on Air Pollution in India

This study examines the large-scale circulation and thermodynamic anomalies associated with extreme air pollution events over India using a composite analysis based on detrended PM₂.₅ data. High- and low-pollution episodes are identified from monthly anomalies in near-surface air quality, and composites are constructed to reveal consistent dynamical and thermodynamic patterns. During high-pollution periods, anomalous upper-tropospheric anticyclonic circulation and positive height anomalies are observed, accompanied by suppressed vertical motion and warming, which inhibit ventilation and favor pollutant accumulation. In contrast, low-pollution events exhibit enhanced upper-level divergence, stronger ascent, and cooling throughout the troposphere, supporting efficient dispersion and wet removal of aerosols. The divergence and vertical velocity fields highlight the role of weakened overturning circulation and reduced convection in modulating stagnant conditions. Analysis of moist static energy (MSE) further distinguishes polluted and clean regimes: elevated MSE during high-pollution periods indicates enhanced stability and reduced convective potential, while lower MSE during cleaner phases reflects greater instability and active vertical exchange that promotes pollutant removal. At the surface, positive sea-level pressure anomalies and weakened low-level winds limit horizontal ventilation, whereas negative pressure anomalies and intensified winds enhance dispersion. Overall, the results highlight that large-scale circulation and thermodynamic variability strongly modulate monthly air pollution extremes over India. The detrended composite effectively isolates meteorological drivers, offering clearer insight into the processes governing severe pollution episodes.