Aerosol Radiative Forcing Over Indian Subcontinent for 2000-2021 using satellite observations

Aerosols directly affect Earth’s radiation budget by scattering and absorbing incoming solar radiation and outgoing terrestrial radiation. While the uncertainty in aerosol radiative forcing (ARF) has decreased over the years, it is still higher than that of greenhouse gas forcing, particularly in the South Asian region, due to high heterogeneity in aerosols’ chemical properties. India has been identified as an aerosol hotspot. Understanding the Spatiotemporal heterogeneity of aerosol composition is critical in improving ARF estimation.                            

In this study, we have taken aerosol data from Multi-angle Imaging Spectro-Radiometer (MISR) level-2 version 23 aerosol products retrieved at 4.4 km and radiation data from Clouds and the Earth’s Radiant Energy System (CERES, spatial resolution=1^ox1^o), for 21 years (2000-2021) over the Indian subcontinent. MISR aerosol product includes size and shapes segregated aerosol optical depth (AOD), Angstrom exponent (AE), and single scattering albedo (SSA). Additionally, 74 aerosol mixtures included in version 23 data are used for aerosol speciation. In addition, we have used CERES radiation data in four different atmospheric conditions: all-sky (AS), clear-sky (CS), pristine (PR) and no aerosol (NAER), for estimating aerosol radiative forcing in different aerosol-cloud conditions.

We have seasonally mapped aerosol optical and microphysical properties from MISR for India at quarter degrees resolution. Results show strong Spatio-temporal variability, with a constant higher value of AOD for the Indo-Gangetic Plain (IGP). The fractional contribution of small-size particles to AOD is higher (>0.4) throughout the year, spatially during post-monsoon and winter seasons (October to February). SSA is found to be overestimated by MISR, where absorbing particles are present. The climatological map of short wave (SW) ARF at the top of the atmosphere (TOA) shows a strong cooling except in only a few places (values ranging from +2.5W/m² to -22.5 W/m²). Cooling due to aerosols is higher in the absence of clouds. Higher aerosol cooling is found over the IGP region, given the high aerosol concentration over the region. Aerosols are causing a surface cooling effect over our study domain, which is higher in clear conditions. The results strongly correlate with AOD from MISR and ARF from CERES.