Perturbation of the northern hemisphere radiative balance by backscattering from anthropogenic sulfate aerosols*

Anthropogenic sulfate (SO4=) aerosol particles play two potential roles in the radiative climate of the earth. In cloud-free air, SO4= particles scatter sunlight, some of which is lost to space, thereby reducing solar irradiance at the ground. The same particles can act as cloud condensation nuclei (CCN), the number concentration of which is an important determinant of cloud albedo. This albedo effect, in turn, also influences incoming short-wave solar radiation. Development of a three-dimensional global model for estimating the SO4= aerosol mass concentration, along with previously-acquired information on the scattering and back-scattering coefficients per unit mass concentration allow calculation of the effects of anthropogenic SO4= aerosol on clear-sky optical depth. Subsequently, this can be used to estimate the change in hemispheric and global average reflected solar radiation. The conclusion is that the change of reflected solar flux due to anthropogenic SO4= averaged over the Northern Hemisphere is ca. - 1.1 Wm-2, which is comparable but opposite in sign to the present-day radiative forcing by anthropogenic CO2, + 1.5 Wm-2. Because of the spatial variability of the anthropogenic SO4= distribution, its meteorological effects must be studied regionally. That is, global models with regional resolution and regional data are required. Unlike the direct effect on solar irradiance, the relationship of CCN number concentration to mass concentration is not known. Thus it is not yet possible to make quantitatively reliable statements about anthropogenic forcing of cloud albedo, although there is qualitative evidence that the CCN effect may also be substantial.