Air pollution to atmospheric warming: effects of increasing anthropogenic aerosols on South Asia’s climate

Abstract

Surface cooling by aerosols partially offsets GHG warming. Understanding and evolution of aerosol forcing over a global aerosol hotspot, South Asia, is crucial for regional and global climate. For the first time, utilizing multi-source data from satellites (Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), Ozone Monitoring Instrument (OMI), and Clouds and the Earth's Radiant Energy System (CERES)), and models (Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2) and Copernicus Atmospheric Monitoring Service (CAMS)), climatology and trends in aerosols and radiative effects on seasonal and annual scales are derived and quantified. Aerosol optical depth (AOD) exhibits significant spatiotemporal variability across South Asia. Annual mean AOD is mostly > 0.3 with AOD over Indo-Gangetic Plain (IGP) being > 0.5. AOD shows a significant increasing trend (≥ 0.01 yr‒1; ≥ 50%) during 2005–2019 across India except over northwest India. Annual trend in AOD is driven by post-monsoon and winter trends when anthropogenic aerosol emissions dominate, and by small- and medium-size AODs. AOD trends are negative over western India during pre-monsoon and monsoon which is dominated by coarse mode particles (dust, large-size AOD). On an annual scale, sulfate, black carbon, and organic carbon AODs have increased, whereas dust AOD does not exhibit any significant trend, confirming that increase in AOD is due to the increase in fine-mode aerosols dominantly emitted by anthropogenic activities, corroborated by the increasing trend of anthropogenic AOD. Trends in single scattering albedo are quite small and not statistically significant. Aerosol direct radiative effect (DRE) at the surface (DRESFC) decreases by ≤ − 1 Wm‒2 yr‒1. Aerosol-induced heating rate (HR) in the atmosphere exhibits an increasing trend (~ 0.01 K day‒1 yr‒1). Increasing trends of AOD, DRESFC (magnitude), and HR are higher over central and south India. Winter air quality across the IGP could worsen as a result of increases in anthropogenic aerosols amplified by declining trends in winds and rising relative humidity. Since 2005, there has been a robust and persistent evidence of increasing radiative influence of anthropogenic aerosols on climate (surface cooling and atmospheric warming) over South Asia. In light of the projected global warming, this additional aerosol-induced atmospheric warming across South Asia would be critical.