Atmos. Chem. Phys. Discuss., 9, 24085-24143, 2009
© Author(s) 2009. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area
E. C. Apel1, L. K. Emmons1, T. Karl1, F. Flocke1, A. J. Hills1, S. Madronich1, J. Lee-Taylor1, A. Fried1, P. Weibring1, J. Walega1, D. Richter1, X. Tie1, L. Mauldin1, T. Campos1, B. Sive2, L. Kleinman3, S. Springston3, R. Zaveri4, J. Ortega4,*, P. Voss5, D. Blake6, A. Baker6, C. Warneke7, D. Welsh-Bon7, J. de Gouw7, J. Zheng8, R. Zhang8, J. Rudolph9, W. Junkermann10, and D. D. Riemer11
1Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
2Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH, USA
3Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, NY, USA
4Atmospheric Sciences & Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
5Picker Engineering Program, Smith College and the University of Massachusetts, Amherst, USA
6Department of Chemistry, University of California, Irvine, CA, USA
7Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, CO, USA
8Department of Atmospheric Sciences, Texas A&M, USA
9Centre for Atmospheric Chemistry, York University, Toronto, Canada
10Forschungszentrum Karlsruhe, Garmisch-Partinkirchen, Germany
11University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami, FL, USA
*currently at: Colorado School of Mines, Golden, CO, USA

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 reproduce the general features of the daytime cycle of the VOC OH reactivity distribution showing that NMHCs dominate the distribution except in the afternoon hours and that the VOC OH reactivity peaks in the early morning due to high morning emissions from the city into a shallow boundary layer. 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. In addition, a plume was studied in which air was advected out of the MCMA and intercepted downwind with the DOE G1 on 18~March and the 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 and many OVOCs were photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial and temporal extent of the 19~March plume and to help interpret the OH reactivity in the downwind plume. 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 downwind. 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.

Citation: Apel, E. C., Emmons, L. K., Karl, T., Flocke, F., Hills, A. J., Madronich, S., Lee-Taylor, J., Fried, A., Weibring, P., Walega, J., Richter, D., Tie, X., Mauldin, L., Campos, T., Sive, B., Kleinman, L., Springston, S., Zaveri, R., Ortega, J., Voss, P., Blake, D., Baker, A., Warneke, C., Welsh-Bon, D., de Gouw, J., Zheng, J., Zhang, R., Rudolph, J., Junkermann, W., and Riemer, D. D.: Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area, Atmos. Chem. Phys. Discuss., 9, 24085-24143, doi:10.5194/acpd-9-24085-2009, 2009.
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