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

by E C Apel, L K Emmons, T Karl, F Flocke, A J Hills, S Madronich, J Lee-Taylor, A Fried, P Weibring, J Walega, D Richter, X Tie, L Mauldin, T Campos, A Weinheimer, D Knapp, B Sive, L Kleinman, S Springston, R Zaveri, J Ortega, P Voss, D Blake, A Baker, C Warneke, D Welsh-Bon, J de Gouw, J Zheng, R Zhang, J Rudolph, W Junkermann, D D Riemer show all authors
Atmospheric Chemistry & Physics ()

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}/{MIRAGE}-Mex 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 absolute 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.\nA 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 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}.

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