Synoptic circulation control on aerosol loading over the Indo-Gangetic Plain: Implications for regional air quality

The Indo-Gangetic Plain (IGP), home to over 900 million people, experiences some of the world's worst air pollution, with PM2.5 concentrations routinely exceeding WHO guidelines by factors of 5-10. While seasonal patterns of aerosol loading are well documented, driven by monsoon rainfall cycles and emission variations, the synoptic meteorological controls governing day-to-day pollution extremes remain poorly understood. This limits our ability to project future air quality under changing atmospheric circulation patterns and to evaluate whether climate models accurately represent the circulation-aerosol coupling essential for reliable near-term climate projections over South Asia.

We identify and characterise distinct synoptic circulation regimes over the IGP and quantify their control on aerosol loading and air quality. Using circulation classification applied to reanalysis data combined with satellite-derived aerosol observations and pollution measurements, we isolate how atmospheric circulation variability modulates PM2.5 and aerosol optical depth independently of emission changes. Given the IGP's unique valley topography, we find air quality shows distinct responses to meteorological variability across seasons, with implications for both climate model evaluation and future projections. We extend this analysis to the UK Earth System Model to assess whether current generation climate models capture the observed sensitivity of aerosol loading to circulation patterns. This is critical because future air quality depends on both emission pathways and changes to circulation regime frequency under climate change.

This work has important implications for climate risk assessment in South Asia. As the monsoon system responds to global warming, shifts in circulation patterns could amplify or offset emission-driven air quality trends, creating pollution hotspots even under declining emissions, or provide ventilation that moderates pollution despite stable emissions. The results could inform emission reduction strategies by clarifying when and how meteorological conditions determine pollution outcomes, and establish process based constraints for models projecting future climate risks in South Asia.