SUMMARY
An analysis of the weekend air pollution episode of April 14-15, 2007 is presented using air quality modeling techniques. It is shown that the emissions of Toluca Metropolitan Area west of México City influencing the air quality of the city is a better explanation of the air pollution event on that weekend than the existence of a “weekend effect”. It is also shown that the enforcement of traffic rules curbing car circulation during that weekend would not have resulted in ozone concentration reductions over México City. The improvement in air quality in México City requires the implementation of comprehensive measurements at the regional scale.