Calculated Effects of Aerosols on Sunlight

Abstract

Solar fluxes have been calculated in a 57-level plane-parallel atmosphere, defined by Elterman's cross sections, using a two-stream approximation to multiple scattering. In this technique, only the integrals of the scattering functions are used so the results were checked with a Monte Carlo code which handles the angular scattering explicitly. The results of both codes agreed with tabulated fluxes computed without aerosols. With the sun at various zenith angles, there appear to be critical values AcritE of local earth albedo AE. For values of AE below AcritE aerosols do not decrease the total earth-atmosphere albedo below AE but above AcritE sufficient aerosol absorption leads to less solar flux leaving the atmosphere, hence a net warming trend. This result is sensitive to the ratio of aerosol absorption to scattering as well as to the ratio of back to forward scattering. When sufficient scattering aerosols are present, calculations at several wavelengths show the possibility that the downward solar flux can be increased near the earth's surface, while being decreased at the earth's surface. This effect can be negated when sufficient aerosol absorption is taken into account.