Air quality monitoring on the Fort Hall Indian Reservation has revealed numerous exceedances of the National Ambient Air Quality Standard (NAAQS) for 24-h average PM10 mass. Wind-directional analysis coupled with PM10 measurements have identified the FMC elemental phosphorus plant as the major source of PM10. This study attempts to identify specific sources or operations within the FMC complex that contribute to the PM10 violations. Data used to identify FMC emission sources were collected at two downwind and one background monitoring sites and include 24-h PM10 concentrations, met data (wind speed and wind direction), 24h PM2.5 and coarse (PM10 minus PM2.5) mass and elemental concentrations, organic and elemental carbon data on selected ambient samples, continuous PM10 and PM2.5 measurements, and electron microscopy characterization of individual source and ambient particles. A limited source sampling effort was conducted, which yielded qualitative source profiles of selected FMC sources or processes. Ambient data show that PM10 is dominated by the fine faction (PM2.5) but that both fine and coarse-fraction aerosols are generally required to produce an exceedance. PM10 mass during exceedance events is split approximately evenly (with significant mass unaccounted for) between fine-faction phosphate, believed to be emitted predominantly from the ground and elevated CO flares and coarse-faction Ca and Si-rich dust believed to come from numerous sources at FMC.