SIZE DISTRIBUTION AND SCATTERING PHASE FUNCTION OF AEROSOL-PARTICLES RETRIEVED FROM SKY BRIGHTNESS MEASUREMENTS

Ground-based measurements of the solar transmission and sky radiance i n a horizontal plane through the Sun are taken in several geographical regions and aerosol types: dust in a desert transition zone in Israel , sulfate particles in Eastern and Western Europe, tropical aerosol in Brazil, and mixed continental/maritime aerosol in California. Stratos pheric aerosol was introduced after the eruption of Mount Pinatubo in June 1991. Therefore measurements taken before the eruption are used t o analyze the properties of tropospheric aerosol; measurements from 19 92 are also used to detect the particle size and concentration of stra tospheric aerosol. The measurements are used to retrieve the size dist ribution and the scattering phase function at large scattering angles of the undisturbed aerosol particles. The retrieved properties represe nt an average on the entire atmospheric column. A comparison between t he retrieved phase function for a scattering angle of 120-degrees, wit h phase function predicted from the retrieved size distribution, is us ed to test the assumption of particle homogeneity and sphericity in ra diative transfer models (Mie theory). The effect was found to be small (20% +/- 15%). For the stratospheric aerosol (sulfates), as expected, the phase function was very well predicted using the Mie theory. A mo del with a power law size distribution, based on the spectral dependen ce of the optical thickness, alpha, cannot estimate accurately the pha se function (up to 50% error for lambda = 0.87 mum). Before the Pinatu bo eruption the ratio between the volumes of sulfate and coarse partic les was very well correlated with alpha. The Pinatubo stratospheric ae rosol destroyed this correlation. The aerosol optical properties are c ompared with analysis of the size, shape, and composition of the indiv idual particles by electron microscopy of in situ samples. The measure d volume size distributions before the injection of stratospheric aero sol consistently show two modes, sulfate particles with r(m) < 0.2 mum and coarse particles with r(m) > 0.7 mum. The ''window'' in the tropo spheric aerosol in this radius range was used to observe a stable stra tospheric aerosol in 1992, with r(m) approximately 0.5 mum. A combinat ion of such optical thickness and sky measurements can be used to asse ss the direct forcing and the climatic impact of aerosol. Systematic i nversion for the key aerosol types (sulfates, smoke, dust, and maritim e aerosol) of the size distribution and phase function can give the re lationship between the aerosol physical and optical properties that ca n be used to compute the radiative forcing. This forcing can be valida ted in dedicated field experiments.