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This paper presents the results of the first reported study on fine particulate matter (PM) chemical composition at Salamanca, a highly industrialized urban area of Central Mexico. Samples were collected at six sites within the urban area during February and March 2003. Several trace elements, organic carbon (OC), elemental carbon (EC), and six ions were analyzed to characterize aerosols. Average concentrations of PM with aerodynamic diameter of less than 10 m (PM 10 ) and fine PM with aerodynamic diameter of less than 2.5 m (PM 2.5 ) ranged from 32.2 to 76.6 g m Ϫ3 and 11.1 to 23.7 g m Ϫ3, respectively. OC (34%), SO 4 ϭ (25.1%), EC (12.9%), and geological material (12.5%) were the major components of PM 2.5 . For PM 10 , geological material (57.9%), OC (17.3%), and SO 4 ϭ (9.7%) were the major components. Coarse fraction (PM 10 Ϫ PM 2.5 ), geological material (81.7%), and OC (8.6%) were the dominant species, which amounted to 90.4%. Correlation analysis showed that sulfate in PM 2.5 was present as ammonium sulfate. Sulfate showed a significant spatial variation with higher concentrations to the north resulting from predominantly southwesterly winds above the surface layer and by major SO 2 sources that include a power plant and refinery. At the urban site of Cruz Roja it was observed that PM 2.5 mass concentrations were similar to the submicron fraction concentrations. Furthermore, the correlation between EC in PM 2.5 and EC measured from an aethalometer was r 2 ϭ 0.710. Temporal variations of SO 2 and nitrogen oxide were observed during a day when the maximum concentration of PM 2.5 was measured, which was associated with emissions from the nearby refinery and power plant. From cascade impactor measurements, the three measured modes of airborne particles corresponded with diameters of 0.32, 1.8, and 5.6 m.

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