Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign

The physical and optical properties of biomass burning aerosols in a savann a region in south central Africa (Zambia) were analyzed from measurements m ade during the Zambian International Biomass Burning Emissions Experiment ( ZIBBEE) during August-September 1997. Due to the large spatial extent of Af rican savannas and the high frequency of occurrence of burning in the annua l dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiativ e processes and for remote sensing of both surface and atmospheric properti es in these regions. Aerosol Robotic Network Sun-sky radiometer spectral me asurements of direct Sun observations and directional sky radiances were ut ilized to infer spectral aerosol optical depths (tau (a)), aerosol size dis tributions, and single-scattering albedos, During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the regi on, there was a high correlation between the measured tau (a) and the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrieval s of the biomass burning smoke show that the accumulation mode dominated an d a comparison with smoke from Amazonia (Bolivia) shows a shift toward smal ler particles for African savanna smoke. This may be the result of differen ces in mode of combustion (flaming versus smoldering), fuel type and moistu re content, and the aging processes of the aerosol. The single-scattering a lbedo to,) of the aerosols were retrieved using several approaches, yieldin g average values of omega (o) at similar to 550 nm during ZIBBEE varying fr om similar to0.82 to similar to0.85, thus showing good agreement within the retrieval uncertainty of similar to0.03 of these methods. In general, omeg a (o) was relatively constant as a function of aerosol loading, with very l ittle change occurring for tau (a) at 440 nm ranging from 0.7 to 1.7. Afric an savanna smoke exhibits significantly higher absorption than smoke from A mazonian forested regions and also a greater rate of decrease of omega (o) with increasing wavelength. Variations in the spectral change of the Augstr om wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions .