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Volatile organic compounds (VOCs) and carbonaceous aerosol were measured ata sub-urban site near Mexico City in March of 2006 during the MILAGRO study(Megacity Initiative: Local and Global Research Objectives). Diurnalvariations of hydrocarbons, elemental carbon (EC) and hydrocarbon-likeorganic aerosol (HOA) were dominated by a high peak in the early morningwhen local emissions accumulated in a shallow boundary layer, and a minimumin the afternoon when the emissions were diluted in a significantly expandedboundary layer and, in case of the reactive gases, removed by OH. Incomparison, diurnal variations of species with secondary sources such as thealdehydes, ketones, oxygenated organic aerosol (OOA) and water-solubleorganic carbon (WSOC) stayed relatively high in the afternoon indicatingstrong photochemical formation. Emission ratios of many hydrocarbon speciesrelative to CO were higher in Mexico City than in the U.S., but we foundsimilar emission ratios for most oxygenated VOCs and organic aerosol.Secondary formation of acetone may be more efficient in Mexico City than inthe U.S., due to higher emissions of alkane precursors from the use ofliquefied petroleum gas. Secondary formation of organic aerosol was similarbetween Mexico City and the U.S. Combining the data for all measured gas andaerosol species, we describe the budget of total observed organic carbon(TOOC), and find that the enhancement ratio of TOOC relative to CO isconserved between the early morning and mid afternoon despite largecompositional changes. Finally, the influence of biomass burning isinvestigated using the measurements of acetonitrile, which was found tocorrelate with levoglucosan in the particle phase. Diurnal variations ofacetonitrile indicate a contribution from local burning sources. Scatterplots of acetonitrile versus CO suggest that the contribution of biomassburning to the enhancement of most gas and aerosol species was not dominantand perhaps not dissimilar from observations in the U.S

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