Aerosol direct radiative effects over Kanpur in the Indo-Gangetic basin, northern India: Long-term (2001-2005) observations and implications to regional climate

We present 5-year (2001-2005) monthly mean estimates of direct radiative effects (DRE) due to aerosols over Kanpur region in the Indo-Gangetic basin for the first time. Further, the monthly and annual heterogeneity of aerosol DRE has been evaluated on the basis of the anthropogenic and natural aerosol contribution. An optically equivalent model has been formulated on the basis of the surface measurements, altitude profiles of aerosol properties in conjunction with remotely retrieved aerosol parameters, and the optical properties are used to estimate the aerosol DRE at the top-of-atmosphere (TOA), surface and atmosphere in the shortwave (SW) and longwave (LW) region. Water-solubles, black carbon (BC), and dust in fine (dust(f)) and coarse (dust(c)) mode are considered to be the main aerosol components on the basis of the chemical composition measurements. Anthropogenic components (scattering water-solubles and absorbing BC) contribute more than 80% to the composite aerosol optical depth, AOD (at 0.5 mu m) in the winter, whereas the natural dusts contribute more than 55% in the summer months. Aerosols induce large negative surface forcing (more than -20Wm(-2)) with higher values (more than -30 W m(-2)) during the premonsoon season, when the transported natural dusts add to the anthropogenic aerosol pollution. The SW surface cooling is partially (maximum up to 11%) compensated by LW surface heating. The SW cloudy-sky aerosol DRE values are + 1.4 +/- 6.1, -23.3 +/- 9.3 and + 24.8 +/- 9.7 W m(-2) for TOA, surface and atmosphere, respectively. Annually similar to 5% BC mass fraction contributes similar to 9% to total AOD(0.5), but similar to 40% to the total aerosol surface DRE. The annual mean (+/- SD) TOA, surface and atmospheric clear-sky SW anthropogenic aerosol DRE over Kanpur are + 0.3 +/- 2.5, -19.9 +/- 9 and + 20.2 +/- 9.9 W m-(2), respectively. Large negative surface forcing and positive atmospheric forcing in the Kanpur region raise several climatic issues. Anthropogenic aerosols contribute 65.4% to the mean (+/- SD) annual heating rate of 0.84 +/- 0.3 K d(-1) over Kanpur. A persistently large reduction of net surface radiation would affect the regional hydrological cycle through changes in evaporation and sensible heat flux. Our study assesses the aerosol direct radiative effects in Kanpur region for a 5 year period, which would provide a baseline to more thoroughly address these climate-related issues in the future.