Evidence of internal mixing of African dust and biomass burning particles by individual particle analysis using electron beam techniques

[1] In this investigation, the interaction between aerosols derived from biomass burning and dust aerosol is the focus of interest. We have examined the morphology and chemical composition of dust and biomass burning aerosol particles sampled on airborne filters using an environmental scanning electron microscope equipped with both an energy dispersive X-ray analysis and an image analysis system. Using these techniques, we were able to study both mineral dust and biomass burning particles in samples containing significant amounts of both biomass burning and dust aerosol, compared to samples dominated by biomass burning aerosol and a sample dominated by dust aerosol. Thirteen particle types were classified in the samples. Aluminosilicates were the dominant particle type in the sample dominated by dust aerosol. Many more particles were found to be rich in S, Na, and K in the sample dominated by biomass burning aerosols, and particles in these samples were also found to be completely devoid of Cl due to aging of the aerosol and chemical conversion during atmospheric transport. We suggest that biomass burning particles containing all three characteristic elements are internal mixtures. Particle imaging showed that up to a third of soot particles were internally mixed with mineral dust particles in samples containing significant numbers of both biomass burning and mineral dust particles. This internal mixing would change the optical properties of the particle ensemble and the direct radiative forcing caused by biomass burning significantly. © 2010 by the American Geophysical Union.