Comparison of continuous and filter based mass measurements in Mexico City

US EPA federal equivalent method (FEM), the tapered element oscillating microbalance (TEOM) (Rupprecht and Pattachnick, Albany, NY) for measuring continuous hourly PM10, was collocated with a filter-based federal reference method (FRM), the PM10 sequential filter sampler (SFS), at five sites in Mexico City during February and March, 1997. A PM10 mass comparison showed significant differences between instruments that exceeded the expected uncertainties. In general, the TEOM measured higher and more variable PM10 than the SFS. It was found that when averaging 24 h of PM10 concentrations, exposures to very high levels were missed. The TEOM measurements exhibited much less spatial variations across different sites than the similar comparison of SFS measurements. The mass and chemical composition of the SFS measurements showed that the majority of the PM10 mass can be explained by the measured elemental, ionic, and carbon concentrations. This analysis shows that TEOM and filter-based PM10 cannot be used interchangeably to determine temporal and spatial distributions in Mexico City during 1997.