Model calculations of non-cloud radiative forcing due to anthropogenic sulphate aerosol

Anthropogenic sulphate aerosol particles scatter incoming solar radiation thereby perturbing the radiative budget, hence climate. We have used a three dimensional radiative transfer model together with the sulphate concentration fields simulated by two independent chemistry-transport models to calculate the annual cycle of the radiative forcing due to anthropogenic sulphate aerosol. The calculated forcing pattern shows large peaks over the eastern United States, southeast Europe and eastern China. The calculated annual global-mean radiative forcing is -0.50 Wm-2 for Langner and Rodhe (1991) data and -0.49 Wm-2 for Penner et el. [1994 (a&b)] data. The forcing was found to vary with season, with a larger forcing during northern hemispheric summer than winter. Sulphate aerosol also appreciably perturbs the lower tropospheric heating rates over northern hemispheric mid-latitudes. The forcing was also found to be sensitive to the global cloud cover and to the optical properties of the aerosol. The possible sources of the differences in magnitude with previous estimates are discussed. Over northern hemispheric mid-latitudes, the negative radiative forcing due to the direct effect of aerosols appreciably offsets the positive forcing due to increase in greenhouse gases. A 26-layer radiative-convective model (RCM) was also used to examine the equilibrium temperature profiles due to sulphate aerosols and increase in greenhouse gases. It was found that the effect of sulphate aerosols is the cooling of surface-troposphere system. Sulphate aerosols reduce the tropospheric warming and enhance the stratospheric cooling caused by increase in greenhouse gases.