Direct radiative effects of aerosols over South Asia from observations and modeling

Quantitative assessment of the seasonal variations in the direct radiative effect (DRE) of composite aerosols as well as the constituent species over the Indian sub continent has been carried out using a synergy of observations from a dense network of ground based aerosol observatories and modeling based on chemical transport model simulations. Seasonal variation of aerosol constituents depict significant influence of anthropogenic aerosol sources in winter and the dominance of natural sources in spring, even though the aerosol optical depth doesn't change significantly between these two seasons. A significant increase in the surface cooling and atmospheric warming has been observed as season changes from winter (DRESUR = -28 +/- 12 W m(-2) and DREATM = +19.6 +/- 9 W m(-2)) to spring (DRESUR = -33.7 +/- 12 W m(-2) and DREATM = +27 +/- 9 W m(-2)). Interestingly, springtime aerosols are more absorptive in nature compared to winter and consequently the aerosol induced diabatic heating of the atmosphere goes as high as similar to 1 K day(-1) during spring, especially over eastern India. The atmospheric DRE due to dust aerosols (+14 +/- 7 W m(-2)) during spring overwhelms that of black carbon DRE (+11.8 +/- 6 W m(-2)) during winter. The DRE at the top of the atmosphere is mostly governed by the anthropogenic aerosols during all the seasons. The columnar aerosol loading, its anthropogenic fraction and radiative effects shows a steady increase with latitude across Indian mainland leading to a larger aerosol-induced atmospheric warming during spring than in winter.