Aerosol radiative forcing during dust events over New Delhi, India

We present results from sun/sky radiometer measurements of aerosol optical characteristics carried out in New Delhi during March-June, 2006, as part of the Indian Space Research Organization's Integrated Campaign for Aerosol Radiation Budget. For the first time at this site, derived are parameters such as aerosol optical depth (AOD), single scattering albedo (SSA), asymmetry parameter, A ngstrom exponent, and real and imaginary refractive indices in five spectral channels. During the campaign, a consistent increase in aerosol loading from March to June with monthly average AOD values at 0.5 mu m of 0.55, 0.75, 1.22 and 1.18, respectively, was observed. A ngstrom exponent gradually decreases from 1.28 (March) to 0.47 (June), indicating an increased abundance of coarse particles due to dust storms that transport desert dust from the Thar desert and adjoining regions. SSA at 0.5 mu m is found to be in the range of 0.84 to 0.74 from March to June, indicating an increasing contribution from the mixture of anthropogenic and desert dust absorbing aerosols. Optical properties derived during the campaign are used in a radiative-transfer model to estimate aerosol radiative forcing at the surface and at the top-of-the atmosphere. A consistent increase in surface cooling is evident, ranging from -39 W m(-2) (March) to -99 W m(-2) (June) and an increase in heating of the atmosphere from 27 W m(-2) (March) to 123 W m(-2) (June). Heating rates in the lower atmosphere (up to 5 km) are 0.6, 1.3, 2.1, and 2.5K/d from March, April, May, and June 2006, respectively. Higher aerosol induced heating in the premonsoon period has been shown to have an impact on the regional monsoon climate.