Rapid ventilation of the Mexico City basin and regional fate of the urban plume B. de Foy1,*, J. R. Varela2, L. T. Molina1,*, and M. J. Molina1 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA 2Departamento 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
Abstract. 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.
Citation: de Foy, B., Varela, J. R., Molina, L. T., and Molina, M. J.: Rapid ventilation of the Mexico City basin and regional fate of the urban plume, Atmos. Chem. Phys., 6, 2321-2335, doi:10.5194/acp-6-2321-2006, 2006.