ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-2321-2006 Rapid ventilation of the Mexico City basin and regional fate of the urban plume de Foy B. 1 3 Varela J. R. 2 Molina L. T. 1 3 Molina M. J. 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA Departamento de Ingeniería de Procesos e Hidr\'aulica, Universidad Autónoma Metropolitana, Iztapalapa, Mexico current address: Molina Center for Energy and the Environment (mce2.org), California, USA 21 06 2006 6 8 2321 2335 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/6/2321/2006/acp-6-2321-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/2321/2006/acp-6-2321-2006.pdf

Urban areas can be large emitters of air pollutants leading to negative health effects and environmental degradation. The rate of venting of these airsheds determines the pollutant loading for given emission levels, and also determines the regional impacts of the urban plume. Mexico City has approximately 20 million people living in a high altitude basin with air pollutant concentrations above the health limits most days of the year. A mesoscale meteorological model (MM5) and a particle trajectory model (FLEXPART) are used to simulate air flow within the Mexico City basin and the fate of the urban plume during the MCMA-2003 field campaign. The simulated trajectories are validated against pilot balloon and radiosonde trajectories. The residence time of air within the basin and the impacted areas are identified by episode type. Three specific cases are analysed to identify the meteorological processes involved. For most days, residence times in the basin are less than 12 h with little carry-over from day to day and little recirculation of air back into the basin. Very efficient vertical mixing leads to a vertically diluted plume which, in April, is transported predominantly towards the Gulf of Mexico. Regional accumulation was found to take place for some days however, with urban emissions sometimes staying over Mexico for more than 6 days. Knowledge of the residence times, recirculation patterns and venting mechanisms will be useful in guiding policies for improving the air quality of the MCMA.

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