Impacts of mitigating regional and trans-regional anthropogenic emissions on aerosols and meteorology over the non-attainment cities of western India

Air quality management has been extremely challenging over India due to substantial role of trans-regional pollution transport. This is especially the case for non-attainment cities located in the western India (states of Rajasthan and Gujarat), where influx of pollution from the Indo-Gangetic Plain plays a key role. In this study, we have conducted high-resolution simulations using WRF-Chem model (v3.9.1) to quantify the impacts of mitigating regional and trans-regional anthropogenic emissions on surface fine particulate matter (PM2.5) and aerosol optical depth (AOD) over eight non-attainment cities located in western India. The analysis is carried out for winter and post-monsoon, when PM2.5 levels are typically high. The results show that phasing out the regional anthropogenic emissions reduces PM2.5 by 40–71% and AOD by 24–44%, while eliminating both regional and trans-regional emissions yields total reductions of 84–93% for PM2.5 and 45–77% for AOD across the non-attainment cities. AOD variations, to an extent, are governed by background processes (including domain-wide natural emissions, such as from dust and biogenic sources, along with fire emissions and pollutant transport from beyond the model domain boundaries). Impact of background sources is relatively stronger during the post-monsoon season, thereby reducing the sensitivity of AOD to anthropogenic emission mitigation. Number of days, when PM2.5 exceeds the National Ambient Air Quality Standards (NAAQS), have been analysed for 3 different scenarios: 50% regional anthropogenic emission reduction, 100% regional anthropogenic emissions reduction, 100% regional and trans-regional anthropogenic emission reduction. The results reveal clear spatial heterogeneity in mitigation effectiveness, with cities in eastern Rajasthan (Jaipur, Alwar, and Kota) showing higher sensitivity to trans-regional emissions. In these cities, 13–40% of NAAQS exceedance days (relative to exceedance days in the reference simulation) persist even in the absence of regional emissions, highlighting a greater challenge for air quality management. Impact of anthropogenic emissions on meteorological factors, such as net surface shortwave radiation (NSSR), 2 m temperature (T2) and planetary boundary layer height (PBLH), over the cities is also studied. It is revealed that trans-regional emissions have a higher impact on these variables as compared to regional emissions over all cities during both winter and post-monsoon. Mitigation of regional plus trans-regional emissions during winter and post-monsoon enhances the monthly mean NSSR by 18–45 W/m2 across the cities, with highest daily increment of 39 - 113 W/m2. As a result, monthly mean T2 (PBLH) enhances by ∼0.10-0.35 K (up to 73 m) across cities with the highest daily increment of 0.2 - 1.0 K (up to 241 m). The findings would be valuable for improving air quality and managing urban climate over non-attainment cities in western Indian region.