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 savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August-September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun-sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (tau (a)), aerosol size distributions, and single-scattering albedos, During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, 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 retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single-scattering albedo to,) of the aerosols were retrieved using several approaches, yielding average values of omega (o) at similar to 550 nm during ZIBBEE varying from similar to0.82 to similar to0.85, thus showing good agreement within the retrieval uncertainty of similar to0.03 of these methods. In general, omega (o) was relatively constant as a function of aerosol loading, with very little change occurring for tau (a) at 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of omega (o) with increasing wavelength. Variations in the spectral change of the Augstrom wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.