ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-6-7959-2006 Characterizing ozone production in the Mexico City Metropolitan Area: a case study using a chemical transport model Lei W. 1 de Foy B. 2 Zavala M. 1 Volkamer R. 1 3 Molina L. T. 1 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, MA, USA Molina Center for Energy and the Environment, CA, USA now at: Department of Chemistry and Biochemistry, University of California, San Diego, CA, USA 17 08 2006 6 4 7959 8009 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/6/7959/2006/acpd-6-7959-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/6/7959/2006/acpd-6-7959-2006.pdf

An episodic simulation is conducted to characterize ozone (O<sub>3</sub>) photochemical production and investigate its sensitivity to emission changes of ozone precursors in the Mexico City Metropolitan Area (MCMA) using the Comprehensive Air Quality Model with extensions (CAMx). High O<sub>x</sub> (O<sub>3</sub>+NO<sub>2</sub>) photochemical production rates of 10&ndash;80 ppb/h are predicted due to the high reactivity of volatile organic compounds (VOCs) in which alkanes, alkenes, and aromatics exert comparable contributions. The predicted ozone production efficiency is between 4&ndash;10 O<sub>3</sub> molecules per NO<sub>x</sub> molecule oxidized, and increases with VOC-to-NO<sub>2</sub> reactivity ratio. Process apportionment analyses indicate significant outflow of pollutants such as O<sub>3</sub> and peroxyacetyl nitrate (PAN) from the urban area to the surrounding regional environment. PAN is not in chemical-thermal equilibrium during the photochemically active periods. Sensitivity studies of O<sub>3</sub> production suggest that O<sub>3</sub> formation in the MCMA urban region with less chemical aging (NO<sub>z</sub>/NO<sub>y</sub>&lt;0.3) is VOC-limited. Both the simulated behavior of O<sub>3</sub> production and its sensitivities to precursors suggest that midday O<sub>3</sub> formation during this episode is VOC sensitive in the urban region on the basis of the current emissions inventory. More episodic studies are needed to construct a comprehensive and representative picture of the O<sub>3</sub> production characteristics and its response to emission controls.