ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-3061-2008 Vertical distribution of ozone and VOCs in the low boundary layer of Mexico City Velasco E. 1 2 Márquez C. 3 Bueno E. 3 Bernabé R. M. 3 Sánchez A. 3 Fentanes O. 3 Wöhrnschimmel H. 3 Cárdenas B. 3 Kamilla A. 4 Wakamatsu S. 4 5 Molina L. T. 1 2 Molina Center for Energy and the Environment (MCE2), La Jolla, CA, USA Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA Centro Nacional de Investigación y Capacitación Ambiental del Inst. Nacional de Ecología (CENICA-INE), D.F., Mexico Japanese International Cooperation Agency (JICA), Tokyo, Japan Atmospheric Environmental Science Laboratory, Faculty of Agriculture, Ehime University, Ehime, Japan 20 06 2008 8 12 3061 3079 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.net/8/3061/2008/acp-8-3061-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/3061/2008/acp-8-3061-2008.pdf

The evolution of ozone (O<sub>3</sub>) and 13 volatile organic compounds (VOCs) in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of O<sub>3</sub> and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h). VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O<sub>3</sub> from the previous day that contributes to the background concentration in surrounding regions. At the same time the release of heat stored in the urban surface forms a shallow unstable layer close to the ground, where the nocturnal emissions are trapped. After sunrise an O<sub>3</sub> balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with nitric oxide, delaying the ground-level O<sub>3</sub> rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.

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