Investigation of the effects of biomass burning aerosols on the surface irradiance were conducted as a part of the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment during August--September 1995. Measurements of broadband and spectral irradiance, in conjunction with measurements of aerosol physical and optical properties (optical depth, phase function, and size distribution) were made under varying conditions of aerosol loading during the SCAR-B field campaign. Estimates of aerosol single scattering albedo (ω0) were made from matching of the measured irradiance values to the model computed irradiances by varying ω0, for observations made under cloudless conditions. Values of ω0, at approximately 550 nm, estimated from this technique using broadband 400--700 nm irradiance measurements, ranged from approximately 0.82 to 0.94 for the dates and times of these SCAR-B measurements. Utilizing spectral irradiance data, the model retrieved values of ω0 decreased with increasing wavelength, with the change of ω0 as a function of wavelength differing on different days. Reductions in photosynthetically active radiation (PAR; 400--700 nm) incident at the surface were computed to range from about 20 to 45% compared to background aerosol conditions for the 2 month biomass burning season at several locations in the southern Amazon Basin. These large reductions in incident PAR at the surface due to the heavy aerosol loadings could have implications for primary production of sensitive ecosystems. In addition, reductions of total incident solar radiation from aerosol direct radiative effects may have significant impact on reducing surface heating and increasing aerosol layer heating from absorption. ¿ 1998 American Geophysical Union