ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-3425-2009 Emission and chemistry of organic carbon in the gas and aerosol phase at a sub-urban site near Mexico City in March 2006 during the MILAGRO study de Gouw J. A. 1 2 Welsh-Bon D. 1 2 Warneke C. 1 2 Kuster W. C. 1 Alexander L. 3 Baker A. K. 4 Beyersdorf A. J. 4 9 Blake D. R. 4 Canagaratna M. 5 Celada A. T. 6 Huey L. G. 7 Junkermann W. 8 Onasch T. B. 5 Salcido A. 6 Sjostedt S. J. 7 10 Sullivan A. P. 7 11 Tanner D. J. 7 Vargas O. 7 Weber R. J. 7 Worsnop D. R. 5 Yu X. Y. 3 Zaveri R. 3 NOAA Earth System Research Laboratory, Boulder, CO, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA Pacific Northwest National Laboratory, Richland, WA, USA University of California, Irvine, CA, USA Aerodyne Research Inc., Billerica, MA, USA Instituto de Investigaciones Eléctricas, Cuernavaca, Morelos, Mexico Georgia Institute of Technology, Atlanta, GA, USA Research Center Karlsruhe – Institute for Meteorology and Climate Research, Garmisch-Partenkirchen, Germany now with: NASA Langley Research Center, Hampton, VA, USA now with: Department of Chemistry, University of Toronto, Toronto, ON, Canada now with: Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA 28 05 2009 9 10 3425 3442 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/9/3425/2009/acp-9-3425-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/3425/2009/acp-9-3425-2009.pdf

Volatile organic compounds (VOCs) and carbonaceous aerosol were measured at a sub-urban site near Mexico City in March of 2006 during the MILAGRO study (Megacity Initiative: Local and Global Research Objectives). Diurnal variations of hydrocarbons, elemental carbon (EC) and hydrocarbon-like organic aerosol (HOA) were dominated by a high peak in the early morning when local emissions accumulated in a shallow boundary layer, and a minimum in the afternoon when the emissions were diluted in a significantly expanded boundary layer and, in case of the reactive gases, removed by OH. In comparison, diurnal variations of species with secondary sources such as the aldehydes, ketones, oxygenated organic aerosol (OOA) and water-soluble organic carbon (WSOC) stayed relatively high in the afternoon indicating strong photochemical formation. Emission ratios of many hydrocarbon species relative to CO were higher in Mexico City than in the U.S., but we found similar emission ratios for most oxygenated VOCs and organic aerosol. Secondary formation of acetone may be more efficient in Mexico City than in the U.S., due to higher emissions of alkane precursors from the use of liquefied petroleum gas. Secondary formation of organic aerosol was similar between Mexico City and the U.S. Combining the data for all measured gas and aerosol species, we describe the budget of total observed organic carbon (TOOC), and find that the enhancement ratio of TOOC relative to CO is conserved between the early morning and mid afternoon despite large compositional changes. Finally, the influence of biomass burning is investigated using the measurements of acetonitrile, which was found to correlate with levoglucosan in the particle phase. Diurnal variations of acetonitrile indicate a contribution from local burning sources. Scatter plots of acetonitrile versus CO suggest that the contribution of biomass burning to the enhancement of most gas and aerosol species was not dominant and perhaps not dissimilar from observations in the U.S.

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