Ž . The Science of the Total Environment 287 2002 177201 Chemical composition of PM and PM in Mexico 2.5 10 City during winter 1997 Judith C. Chow a,  , John G. Watson a , Sylvia A. Edgerton b , Elizabeth Vega c a Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA b Battelle Pacific Northwest National Laboratory, Washington, DC, USA c Instituto Mexicano del Petroleo, Mexico City, Mexico ´ Received 13 February 2001; accepted 1 June 2001 Abstract PM and PM were measured over 24-h intervals at six core sites and at 25 satellite sites in and around Mexico 2.5 10 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. PM concentrations were highly variable, 10 with almost three-fold differences between the highest and lowest concentrations. Fugitive dust was the major cause of PM differences, although carbon concentrations were also highly variable among the sampling sites. Approxi- 10 mately 50% of PM was in the PM fraction. The majority of PM mass was comprised of carbon, sulfate, nitrate, 10 2.5 ammonium and crustal components, but in different proportions on different days and at different sites. The largest fine-particle components were carbonaceous aerosols, constituting 50% of PM mass, followed by 30% 2.5 secondary inorganic aerosols and 15% geological material. Geological material is the largest component of PM , 10 constituting 50% of PM mass, followed by 32% carbonaceous aerosols and 17% secondary inorganic 10 aerosols. Sulfate concentrations were twice as high as nitrate concentrations. Sulfate and nitrate were present as ammonium sulfate and ammonium nitrate. Approximately two-thirds of the ammonium sulfate measured in urban areas appears to have been transported from regions outside of the study domain, rather than formed from emissions in the urban area. Diurnal variations are apparent, with two-fold increases in concentration from night-time to daytime. Morning samples had the highest PM and PM mass, secondary inorganic aerosols and carbon 2.5 10 concentrations, probably due to a shallow surface inversion and rush-hour traffic.  2002 Elsevier Science B.V. All rights reserved. Keywords: PM ; PM ; Mexico City; Satellite sites 2.5 10  Corresponding author. Tel.: 1-775-674-7050; fax: 1-775-674-7009. Ž . E-mail address: judyc@dri.edu J.C. Chow . 0048-969702$ - see front matter  2002 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 8 - 9 6 9 7 01 00982-2