Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 21 2008 10.5194/acp-8-6469-2008 http://www.atmos-chem-phys.net/8/6469/2008/ http://www.atmos-chem-phys.net/8/6469/2008/acp-8-6469-2008.html http://www.atmos-chem-phys.net/8/6469/2008/acp-8-6469-2008.pdf 6469 6481 2008-11-13 Internally mixed soot, sulfates, and organic matter in aerosol particles from Mexico City K. Adachi P. R. Buseck pbuseck@asu.edu School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, USA Soot particles, which are aggregated carbonaceous spherules with graphitic structures, are major aerosol constituents that result from burning of fossil fuel, biofuel, and biomass. Their properties commonly change through reaction with other particles or gases, resulting in complex internal mixtures. Using a transmission electron microscope (TEM) for both imaging and chemical analysis, we measured ~8000 particles (25 samples) with aerodynamic diameters from 0.05 to 0.3 μm that were collected in March 2006 from aircraft over Mexico City (MC) and adjacent areas. Most particles are coated, consist of aggregates, or both. For example, almost all analyzed particles contain S and 70% also contain K, suggesting coagulation and condensation of sulfates and particles derived from biomass and biofuel burning. In the MC plumes, over half of all particles contained soot coated by organic matter and sulfates. The median value of the soot volume fraction in such coated particles is about 15%. In contrast to the assumptions used in many climate models, the soot particles did not become compact even when coated. 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