An analysis of data collected in the St. Louis Regional Air Pollution Study indicates that about 72% of the annual average PM-15 aerosol is background material, presumably of regional origin. The fine and coarse fractions are composed largely of sulfate and crustal material, respectively. About 29% of the particle mass is unaccounted for assuming the measured elements are present as stable oxides. The ratio between PM-15 and TSP concentrations varied considerably among sites depending on averaging time and percentile. Dispersion model results corroborate the regional origin of the bulk of the aerosol. Annual average dispersion modeling can reproduce the observed annual concentrations from local sources emitting trace elements; however only about 14% of the PM-15 mass can be accounted for by local point sources. A sensitivity analysis indicates that incorporating settling and deposition processes in dispersion models is unlikely to significantly alter predicted concentrations from local sources.