Mexico City belongs to a number of cities in the subtropical and tropical regions which suffer from severe air pollution episodes. High concentrations of ozone and related photooxidants as well as of particulate matter are causing health effects, ecological damages and economic losses. Although rigorous abatement measures were implemented by the city authorities during the past 10 years the air pollution situation in the Mexico City Metropolitan Area (MCMA) is still a severe problem and the concentrations of ozone and its precursors as well as aerosol particles still exceed significantly the threshold values. The effect of present day emissions as well as projected future emissions on primary pollutant on ozone distributions in the Mexico City Metropolitan Area was investigated using the coupled meteorology chemistry model MCCM (Grell et al., 2000). A comparison of the modelled ozone concentrations with measured values at different locations within Mexico City showed that present day maximum ozone mixing ratios occur in the south west of the city in the afternoon, which is downwind of the city centre, as an uphill air flow is prevailing during that time of the day. Minimum ozone levels are in the centre of the city, where the NO emitted by traffic titrates the ozone, and north of the city at places where the NO emissions of power plants locally reduce the ozone concentrations. Taken the 2010 baseline emissions, realistic emission reductions result in a decrease of daytime ozone concentrations between 0.005 and 0.025 ppm at most locations. However, in the centre of the city and for the locations with power plants switched off, the noontime ozone levels are higher for the mitigation scenario than for the baseline case. The model results indicate that taken the projected emissions for year 2010, extremely strong emission reduction measures for Mexico City would be necessary in order to significantly improve the air quality in Mexico City. However, simulations over longer time periods including emission data bases for larger areas to account also for long term and long range transport effects are necessary for sound predictions of the efficiency of emission reductions on air quality in Mexico City.

Literature: Grell, G. S. Emeis, W.R. Stockwell, T. Schoenemeyer, R. Forkel, J. Michalkes, R. Knoche, W. Seidl, 2000: Application of a Multiscale, Coupled MM5/Chemistry Model to the Complex Terrain of the VOTALP Valley Campaign. Atmospheric Environment, 34, 1435-1453