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During the last 10 years, high atmospheric concentrations of airborne particles recorded in the Mexico City metropolitan area have caused concern because of their potential harmful effects on human health. Four monitoring campaigns have been carried out in the Mexico City metropolitan area during 2000 -2002 at three sites: (1) Xalostoc, located in an industrial region; (2) La Merced, located in a commercial area; and (3) Pedregal, located in a residential area. Results of gravimetric and chemical analyses of 330 samples of particulate matter (PM) with an aerodynamic diameter less than 2.5 m (PM 2.5 ) and PM with an aerodynamic diameter less than 10 m (PM 10 ) indicate that (1) PM 2.5 /PM 10 average ratios were 0.42, 0.46, and 0.52 for Xalostoc, La Merced, and Pedregal, respectively; (2) the highest PM 2.5 and PM 10 concentrations were found at the industrial site; (3) PM 2.5 and PM 10 concentrations were lower at nighttime; (4) PM 2.5 and PM 10 spatial averages concentrations were 35 and 76 g/m 3 , respectively; and (5) when the PM 2.5 standard was exceeded, nitrate, sulfate, ammonium, organic carbon, and elemental carbon concentrations were high. Twenty-four hour averaged PM 2.5 concentrations in Mexico City and

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