Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 3 2008 10.5194/acpd-8-9179-2008 http://www.atmos-chem-phys-discuss.net/8/9179/2008/ http://www.atmos-chem-phys-discuss.net/8/9179/2008/acpd-8-9179-2008.html http://www.atmos-chem-phys-discuss.net/8/9179/2008/acpd-8-9179-2008.pdf 9179 9207 2008-05-22 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, AZ, USA Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA Soot particles are major aerosol constituents that result from emissions of burning of fossil fuel and biomass. Because they both absorb sunlight and contribute to cloud formation, they are an influence on climate on local, regional, and global scales. It is therefore important to evaluate their optical and hygroscopic properties and those effects on the radiation budget. Those properties commonly change through reaction with other particles or gases, resulting in complex internal mixtures. Using transmission electron microscopy, 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. More than 50% of the particles consist of internally mixed soot, organic matter, and sulfate. Imaging combined with chemical analysis of individual particles show that many are coated, consist of aggregates, or both. Coatings on soot particles can amplify their light absorption, and coagulation with sulfates changes their hygroscopic properties, resulting in shorter lifetime. 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