Sphericity and morphology of smoke particles from biomass burning in Brazil

The degree of nonsphericity of smoke particles from biomass burning in Brazil was measured aboard the University of Washington C‐131A aircraft during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) project for several regions, types of fuel, and combustion. The nonsphericity (αo) of the particles was obtained from electrooptical light‐scattering measurements, using an aerosol asymmetry analyzer, and from scanning electron microscopy (SEM) images of the particles. The electrooptical measurements provide a measure of the nonsphericity based on the difference between the light scattering coefficient for aligned and randomly oriented particles. The SEM photographs provide information on the geometric shapes of the particles. The maximum value of αo obtained during the SCAR‐B for biomass burning in Brazil was below 13%. The degree of nonsphericity of the particles is shown to be related to the combustion efficiency, the mass absorption efficiency, and the fraction of black carbon to total particle mass. It is concluded after smoke particles from biomass burning in Brazil have been in the atmosphere for more than about 1 hour that the spherical approximation (and therefore Mie theory) is reasonably valid for estimating the physical and optical properties of the particles.