Aerosol Radiative Effects Over India from Direct Radiation Measurements and Model Estimates

Abstract

Multi-year measurements of surface-reaching solar (shortwave) radiation fluxes across a network of aerosol observatories (ARFINET) are combined, for the first time, with concurrent satellite (CERES)-based retrieval of top of the atmosphere (TOA) fluxes and radiative transfer simulations to estimate regional aerosol direct radiative forcing (ARF) over the Indian region. We observed that the synergistic approach improves the accuracy of ARF estimates, which is otherwise differed by as high as 9% (in the atmosphere) in the independent model (radiative transfer and aerosol model constraining measured values spectral aerosol optical properties) simulations. Especially, the model simulated TOA fluxes are found to differ significantly, which leads to the overestimation/ underestimation in the atmospheric forcing. During JJAS, an overestimation of ~ 2 W m-2 (corresponding heating rate ~ 0.15 K day-1) is noticed. The ARF estimated from the synergistic approach revealed significant spatial heterogeneity across distinct geographic regions of India, with surface (SUR) forcing as high as -48.5 W m-2 over the Indo Gangetic Plains, -45.1 W m-2 over northeast India and -34.4 W m-2 over the southern Peninsula and as low as -15.8 W m-2 in the oceanic regions of the Bay of Bengal. The influence of dust and anthropogenic sulfate and carbonaceous aerosols are crucial in modulating ARF over the northern part of India, which contributes up to 60% during their high emission periods. The effect of anthropogenic aerosols on ARF is also significant (~ 50%) over the peninsular and oceanic regions. In terms of clear sky surface reaching solar radiation fluxes (SWin), the reduction in SWin due to the effect of aerosols in the atmosphere is found to vary between 3 to 22%, being highest over the IGP during ON and DJF.

Keywords: SW-radiation, aerosol radiative forcing, heating rate, aerosol composition, aerosol sources, ARFINET, CERES, MERRA-2