Assessment of aerosols optical properties and radiative forcing over an Urban site in North-Western India

The present work is aimed to analyze aerosols optical properties and to estimate aerosol radiative forcing (ARF) from January to December 2013, using sky radiometer data over Rohtak, an urban site in North-Western India. The results reveal strong wavelength dependency of aerosol optical depth (AOD), with high values of AOD at shorter wavelengths and lower values at longer wavelength during the study period. The highest AOD values of 1.07 +/- 0.45 at 500 nm were observed during July. A significant decline in Angstrom exponent was observed during April-May, which represents the dominance of coarse mode particles due to dust-raising convective activities. Aerosols' size distribution exhibits a bimodal structure with fine mode particles around 0.17 mu m and coarse mode particles with a radius around 5.28 mu m. Single scattering albedo values were lowest during November-December at all wavelengths, ranging from 0.87 to 0.76, which corresponds to the higher absorption during this period. Aerosols optical properties retrieved during observation period are used as input for SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) to estimate the direct ARF at the surface, in the atmosphere and at the top of the atmosphere (TOA). The ARF at the TOA, surface and in the atmosphere are found to be in the range of -4.98 to -19.35 W m(-2), -8.01 to -57.66 W m(-2) and +3.02 to +41.64 W m(-2), respectively. The averaged forcing for the whole period of observations at the TOA is -11.26 W m(-2), while at the surface it is -38.64 W m(-2), leading to atmospheric forcing of 27.38 W m(-2). The highest (1.168 K day(-1)) values of heating rate was estimated during November, whereas the lowest value (0.084 K day(-1)) was estimated for the February.