Abstract
Abstract. 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.
Highlights
Ozone and particulate matter (PM) are two of the main air pollutants of concern in large urban areas
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 were removed by OH in case of the reactive gases
Diurnal variations of species with secondary sources such as the aldehydes, ketones and organic aerosol stayed relatively high in the afternoon indicating strong photochemical formation
Summary
Ozone and particulate matter (PM) are two of the main air pollutants of concern in large urban areas. We will focus on the emissions and chemistry of organic carbon in the combined gas and particle pha3s7es, and focus on the chemical transformation at the local scale. Several studies, including in Mexico City, showed that the observed SOA formation could not be explained quantitatively from the measured VOCs and their laboratory-determined particulate yields (de Gouw et al, 2005; Volkamer et al, 2006; de Gouw et al, 2008; Kleinman et al, 2008). We study organic carbon in the combined gas and aerosol phases using data from the sub-urban T1 site in Mexico City during MILAGRO. Previous estimates of the influence of biomass burning on the emissions in and around Mexico City are highly variable (Yokelson et al, 2007; DeCarlo et al, 2008; Moffet et al, 2008; Querol et al, 2008; Stone et al, 2008)
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