Urban O₃ is produced by photochemical chain reactions that amplify background O₃ in mixtures of gaseous nitrogen oxides (NO_x) and organic molecules. Current thinking treats NO_x and organics as independent variables that limit O₃ production depending on the NO_x to organic ratio; in this paradigm, reducing organics either has no effect or reduces O₃. We describe a theoretical counterexample where NO_x and organics are strongly coupled and reducing organics increases O₃ production, and illustrate the example with observations from Mexico City. This effect arises from chain termination in the HO_x and NO_x cycles via organic nitrate production. We show that reductions in VOC reactivity that inadvertently reduce organic nitrate production rates will be counterproductive without concurrent reductions in NO_x or other organics

Abstract. Volatile organic compound (VOC) mixing ratios were measured with two different instruments at the T1 ground site in Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign in March of 2006. A gas chromatograph with flame ionization detector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while a proton-transfer-reaction iontrap mass spectrometer (PIT-MS) quantified 12 VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separation system was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluate PIT-MS measurements and to aid in the identification of unknown VOCs. The VOC measurements are also compared to simultaneous canister samples and to two independent proton-transfer-reaction mass spectrometers (PTR-MS) deployed on a mobile and an airborne platform during MILAGRO. VOC diurnal cycles demonstrate the large influence of vehicle traffic and liquid propane gas (LPG) emissions during the night and photochemical processing during the afternoon. Emission ratios for VOCs and OVOCs relative to CO are derived from early-morning Correspondence to: J. A. de Gouw (joost.degouw@noaa.gov) measurements. Average emission ratios for non-oxygenated species relative to CO are on average a factor of ∼2 higher than measured for US cities. Emission ratios for OVOCs are estimated and compared to literature values the northeastern US and to tunnel studies in California. Positive matrix factorization analysis (PMF) is used to provide insight into VOC sources and processing. Three PMF factors were distinguished by the analysis including the emissions from vehicles, the use of liquid propane gas and the production of secondary VOCs + long-lived species. Emission ratios to CO calculated from the results of PMF analysis are compared to emission ratios calculated directly from measurements. The total PIT-MS signal is summed to estimate the fraction of identified versus unidentified VOC species.

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 watersoluble 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.

Abstract. Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ∼30 km down wind of the city center. Ammonium nitrate (NH 4 NO 3 ) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. Between the hours of 08:00-12:45, particulate water-soluble organic carbon (WSOC) concentrations increased and decreased in a manner consistent with that of NO − 3 , and the two were highly correlated (R 2 =0.88) during this time. A box model was used to analyze these behaviors and showed that, for both NO − 3 and WSOC, the concentration increase was caused primarily (∼75-85%) by secondary formation, with a smaller contribution (∼15-25%) from the entrainment of air from the free troposphere. For NO − 3 , a majority (∼60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (∼40%) of the NO − 3 loss was due to particle evaporation. The WSOC concentration decrease was due largely to dilution (∼75%), but volatilization did have a meaningful impact (∼25%) on the decrease, as well. The results provide an estimate of ambient SOA evaporation losses and suggest that a significant fraction (∼35%) of the fresh MCMA secondary organic aerosol (SOA) measured at the surface volatilized.

Abstract. Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ~30 km down wind of the city center. Ammonium nitrate (NH4NO3) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. The concentration increase was due to both secondary formation and entrainment from the free troposphere. A majority (approximately two-thirds) of the midday concentration decrease was caused by dilution from boundary layer expansion, however a significant fraction (approximately one-third) of the nitrate loss was due to particle evaporation. The water-soluble organic carbon fraction of fine particles (WSOC) and nitrate were highly correlated (R2=0.80) for the entire three-day analysis period, however the WSOC-nitrate correlation was highest (R2=0.88) between the hours of 08:00–12:45, indicating similar sources and processing during this period. The results show that WSOC also experienced evaporation losses and that a significant fraction of the MCMA secondary organic aerosol (SOA) measured at the surface was semi-volatile.