Estimation of aerosol optical properties and radiative effects in the Ganga basin, northern India, during the wintertime

[1] An aerosol model has been developed using mass size distributions of various chemical components measured at Kanpur (an urban location in the Ganga basin, GB, in northern India) and applied to estimate the radiative effects of the aerosols over the entire GB during the winter season. The number size distribution of various species was derived from the measured mass concentration, and the optical properties were calculated using Mie theory. The maximum anthropogenic contribution to the total extinction was estimated to be � 83%. The relative contributions of various species to the aerosol optical depth (AOD) at 0.5 mm are in the following order: (NH4)2SO4 (nss-SO4, 30%), nitrate (NO3 , 24%), salt (mainly NaCl and KCl, 18%), dust (17%) and black carbon (BC, 11%). Relative contribution of nss-SO4 ,N O3 and salt to the calculated AOD decreases with wavelength, and that of dust increases with wavelength, whereas BC contribution is spectrally insensitive. The extinction coefficient strongly depends on the RH, as the scattering by fine mode fraction, which contributes 88% to the total extinction, is enhanced at high ambient RH. The spectral variation of absorption coefficient indicates that the most likely source of BC in this region is fossil fuel. The spectral variation of single scattering albedo (SSA) in the coarse mode fraction suggests mixing of BC and dust particles. During the observational period, the mean shortwave (SW) clear sky top of the atmosphere (TOA) and surface forcing over Kanpur are estimated to be � 13 ± 3 and � 43 ± 8 W m � 2 , respectively. The corresponding longwave forcings are 3.6 ± 0.7 and 2.9 ± 0.6 W m � 2 , respectively. Mean AOD at 0.55 mm over the GB as derived from MODIS data is 0.36 ± 0.14. Extending our model over the entire GB, the net mean TOA and surface forcing become � 6.4 and � 30.2 W m � 2 (with overall � 15% uncertainty). This results in high atmospheric absorption (+23.8 W m � 2 ), translating into a heating rate of 0.67 K day � 1 . The SW surface to TOA forcing ratio (� 3.7) over the GB is 23% higher than the corresponding value for Indian Ocean. The aerosols reduce the incoming solar radiation reaching the surface by � 19%, which has significant effect on the regional climate.