Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 4 2009 10.5194/acpd-9-16993-2009 http://www.atmos-chem-phys-discuss.net/9/16993/2009/ http://www.atmos-chem-phys-discuss.net/9/16993/2009/acpd-9-16993-2009.html http://www.atmos-chem-phys-discuss.net/9/16993/2009/acpd-9-16993-2009.pdf 16993 17033 2009-08-12 Microscopic characterization of carbonaceous aerosol particle aging in the outflow from Mexico City R. C. Moffet T. R. Henn A. V. Tivanski R. J. Hopkins Y. Desyaterik A. L. D. Kilcoyne T. Tyliszczak J. Fast J. Barnard V. Shutthanandan S. S. Cliff K. D. Perry A. Laskin M. K. Gilles mkgilles@lbl.gov Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720-8226, USA Department of Physics, University of Würzburg, Am Hu-bland, 97074 Würzburg, Germany W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, 99352, USA Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, 99352, USA Department of Applied Sciences, University of California, Davis, California, USA Department of Meteorology, University of Utah, Salt Lake City, Utah, USA now at: Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, USA now at: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, 80523-1371, USA This study was part of the Megacities Initiative: Local and Global Research Observations (MILAGRO) field campaign conducted in Mexico City Metropolitan Area during spring 2006. The physical and chemical transformations of particles aged in the outflow from Mexico City were investigated for the transport event of 22 March 2006. A detailed chemical analysis of individual particles was performed using a combination of complementary microscopy and micro-spectroscopy techniques. The applied techniques included scanning transmission X-ray microscopy (STXM) coupled with near edge X-ray absorption fine structure spectroscopy (NEXAFS) and computer controlled scanning electron microscopy with an energy dispersive X-ray analyzer (CCSEM/EDX). As the aerosol plume evolves from the city center, the organic mass per particle increases and the fraction of carbon-carbon double bonds (associated with elemental carbon) decreases. Organic functional groups enhanced with particle age include: carboxylic acids, alkyl groups, and oxygen bonded alkyl groups. At the city center (T0) the most prevalent aerosol type contained inorganic species (composed of sulfur, nitrogen, oxygen, and potassium) coated with organic material. 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