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Urban areas can be large emitters of air pollutants leading tonegative health effects and environmental degradation.The rate of venting of these airsheds determines the pollutantloading for given emission levels, and also determines the regionalimpacts of the urban plume. Mexico City has approximately20 million people living in a high altitude basin withair pollutant concentrations above the health limits most daysof the year.A mesoscale meteorological model (MM5) and a particle trajectorymodel (FLEXPART) are used to simulate air flowwithin the Mexico City basin and the fate of the urbanplume during the MCMA-2003 field campaign. The simulated trajectories arevalidated against pilot balloon and radiosonde trajectories.The residence time of air withinthe basin and the impacted areas are identified by episodetype. Three specific cases are analysed to identifythe meteorological processes involved.For most days, residence times in the basin are less than 12 hwith little carry-over from day to day and little recirculationof air back into the basin. Very efficient vertical mixingleads 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 patternsand venting mechanisms will be usefulin guiding policies for improving the air quality of the MCMA

Rapid ventilation of the Mexico City basin and regional fate of the urban plume