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PM_2.5 and PM₁₀ were measured over 24-h intervals at six core sites and at 25 satellite sites in and around Mexico City from 23 February to 22 March 1997. In addition, four 6-h samples were taken each day at three of the core sites. Sampling locations were selected to represent regional, central city, commercial, residential, and industrial portions of the city. Mass and light transmission concentrations were determined on all of the samples, while elements, ions and carbon were measured on approximately two-thirds of the samples. PM₁₀ concentrations were highly variable, with almost three-fold differences between the highest and lowest concentrations. Fugitive dust was the major cause of PM₁₀ differences, although carbon concentrations were also highly variable among the sampling sites. Approximately 50% of PM₁₀ was in the PM_2.5 fraction. The majority of PM mass was comprised of carbon, sulfate, nitrate …