Clear-sky direct aerosol radiative forcing variations over mega-city Delhi

The direct aerosol radiative forcing (DARF) has been estimated for the clear-sky conditions over Delhi from January 2006 to January 2007 using Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) in the wavelength range 300-3000 nanometer. The single scattering albedo (SSA) and the asymmetry parameter used in this model were estimated using the Optical Properties of Aerosol and Cloud (OPAC) model. The annual average AOD observed at 500 nm was similar to 0.86 +/- 0.42 with an average Angstrom exponent similar to 0.68 +/- 0.35. The average monthly AOD throughout the year over Delhi was found to be in the range 0.56 to 1.22 with the Angstrom exponent in the range 0.38 to 0.96. A high monthly average BC concentration in the range 4-15 mu g m(-3) led to monthly average SSA in the range 0.90 +/- 0.4 to 0.74 +/- 0.3 during the year. Consequently, the monthly average clear-sky DARF at the surface was found to vary in the range -46 +/- 8 W m(-2) to -110 +/- 20 W m(-2), at TOA in the range -1.4 +/- 0.4 to 21 +/- 2 W m(-2), whereas in the atmosphere it was in the range 46 +/- 9 W m(-2) to 115 +/- 19 W m(-2) throughout the year. As the dust concentration in the atmosphere was highest (May-June) the SSA showed an increase with wavelength however when dust concentration was low the SSA decreased with the wavelength.