Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area

Abstract

Abstract. The volatile organic compound (VOC) distribution in the Mexico City Metropolitan Area (MCMA) and its evolution as it is uplifted and transported out of the MCMA basin was studied during the 2006 MILAGRO/MIRAGEMex field campaign. The results show that in the morning hours in the city center, the VOC distribution is dominated by non-methane hydrocarbons (NMHCs) but with a substantial contribution from oxygenated volatile organic compounds (OVOCs), predominantly from primary emissions. Alkanes account for a large part of the NMHC distribution in terms of mixing ratios. In terms of reactivity, NMHCs also dominate overall, especially in the morning hours. However, in the afternoon, as the boundary layer lifts and air is mixed and aged within the basin, the distribution changes as secondary products are formed. The WRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART (Model for Ozone and Related chemical Tracers) were able to approximate the observed MCMA daytime patterns and abCorrespondence to: E. C. Apel (apel@ucar.edu) solute values of the VOC OH reactivity. The MOZART model is also in agreement with observations showing that NMHCs dominate the reactivity distribution except in the afternoon hours. The WRF-Chem and MOZART models showed higher reactivity than the experimental data during the nighttime cycle, perhaps indicating problems with the modeled nighttime boundary layer height.A northeast transport event was studied in which air originating in the MCMA was intercepted aloft with the Department of Energy (DOE) G1 on 18 March and downwind with the National Center for Atmospheric Research (NCAR) C130 one day later on 19 March. A number of identical species measured aboard each aircraft gave insight into the chemical evolution of the plume as it aged and was transported as far as 1000 km downwind; ozone was shown to be photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial extent and temporal evolution of the plume and to help interpret the observed OH reactivity. The model results generally showed good agreement with experimental results for the total VOC OH reactivity downwind and gave insight into the distributions of VOC chemical classes. A box model with Published by Copernicus Publications on behalf of the European Geosciences Union. 2354 E. C. Apel et al.: Chemical evolution of volatile organic compounds detailed gas phase chemistry (NCAR Master Mechanism), initialized with concentrations observed at one of the ground sites in the MCMA, was used to examine the expected evolution of specific VOCs over a 1-2 day period. The models clearly supported the experimental evidence for NMHC oxidation leading to the formation of OVOCs downwind, which then become the primary fuel for ozone production far away from the MCMA.

Measurements of volatile organic compounds at a suburban ground site (T1) in Mexico City during the MILAGRO 2006 campaign: measurement comparison, emission ratios, and source attribution

“…In Table 3 we report both the 1-sigma uncertainty in the regression slope and the calibration uncertainty. We note that the emission ratios presented here for ethane and propane differ substantially from those calculated by Apel et al (2010), which were derived from lsided linear regression fits. Because of the poor correlation of ethane and propane with CO, their ratios versus CO have a limited value and a very large uncertainty.…”

“…Here, these measurements are compared with canister sample analyses and with proton-transfer-reaction mass spectrometry (PTR-MS) measurements made from the Aerodyne mobile laboratory and from the US Department of Energy G1 aircraft. Because the Aerodyne mobile laboratory and the G1 aircraft also sampled near other surface sites and because canister samples were collected at many surface sites and from the NASA DC-8 and NCAR C-130 aircraft, these comparisons can be used to evaluate the consistency of VOC data obtained throughout the campaign (Kleinman et al, 2008;Fortner et al, 2009;Karl et al, 2009;Apel et al, 2010). In addition, this study supplements our understanding of the specificity of proton-transfer-reaction mass spectrometry (PTR-MS) measurements in a dense megacity with a complex VOC composition that challenges the analytical capabilities of this technique (de Gouw and Warneke, 2007).…”

“…During MILAGRO a variety of different instruments and techniques were used to quantify VOCs from both fixed sites and mobile platforms (Heald et al, 2008;Apel et al, 2010). VOC measurements were made by two different groundbased instruments at the sub-urban T1 site (Fast et al, 2007): an in-situ gas chromatograph with flame ionization detection (GC-FID) was used to measure light hydrocarbons and a proton-transfer-reaction ion-trap mass spectrometer (PIT-MS) was used to measure acetonitrile, aromatics and oxygenated VOCs .…”

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…In terms of enhancement ratios, many hydrocarbon species relative to CO were higher in Mexico City than in the US Apel et al, 2010), and similar enhancement ratios were found for most oxygenated VOCs ). The higher hydrocarbon enhancement ratios in Mexico City compared to the US are due to the widespread use of LPG and higher industrial and evaporative emissions of aromatics.…”

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

“…HONO was measured using the Long-Path Absorption Photometer (LOPAP) technique described in Villena et al (2011) with a reported 10 % measurement uncertainty. Finally, a large suite of organic compounds was measured both in situ by fast GC-MS (Apel et al, 2010) and via whole air canister samples with offline GC-MS (Russo et al, 2010). Those species that were directly used in this analysis are listed in Table 5.…”