ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-961-2010 Microscopic characterization of carbonaceous aerosol particle aging in the outflow from Mexico City Moffet R. C. 1 Henn T. R. 1 2 Tivanski A. V. 1 7 Hopkins R. J. 1 Desyaterik Y. 3 8 Kilcoyne A. L. D. 1 Tyliszczak T. 1 Fast J. 4 Barnard J. 4 Shutthanandan V. 3 Cliff S. S. 5 Perry K. D. 6 Laskin A. 3 Gilles M. K. 1 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 01 02 2010 10 3 961 976 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/961/2010/acp-10-961-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/961/2010/acp-10-961-2010.pdf

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. At the T1 and T2 sites, located northeast of T0 (~29 km and ~65 km, respectively), the fraction of homogenously mixed organic particles increased in both size and number. These observations illustrate the evolution of the physical mixing state and organic bonding in individual particles in a photochemically active environment.

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