A long-term study of aerosol modulation of atmospheric and surface solar heating over Pune, India

Implications of aerosol characteristics, observed during a five-year (2004–2009) period over Pune (a tropical urban location), to short-wave radiation budget are reported. A discrete ordinate radiative transfer (DISORT) model with a code, namely, Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART), has been used to carry out the radiative transfer computations. The validity of the method is demonstrated using independent ground-based remote sensing observations. Uncertainties in the estimates are also quantified. Clear-sky forcing reveals the points that include (1) Large negative bottom-of-the-atmosphere (BOA) forcing (more than 30 Wm 2) in all the months with peaks during October, December and March when the surface forcing exceeds~ 40 Wm 2, and (2) Surface forcing values are higher for pre-monsoon months, while they are comparable for winter and post-monsoon months. The top-of-the-atmosphere (TOA) forcing is found to be negative during all the seasons. Large differences between TOA and BOA forcing during pre-monsoon, winter and post-monsoon indicate large absorption of radiant energy (~30 Wm 2) within the atmosphere during these seasons, thus increasing atmospheric heating by~1 K/d. These values imply that aerosols have considerable impact on the atmosphere–surface system by causing substantial warming/cooling at the atmosphere/surface. This persistent trend in strong atmospheric absorption is likely to alter atmospheric thermodynamic conditions and thus affects circulation considerably.