The concentration and composition Of PM2.5 has been measured with a variety of continuous and integrated samplers at the Hawthorne EPA Environmental Monitoring for Public Awareness and Community Tracking (EMPACT) sampling site in Salt Lake City, UT, and at an EPA Science to Achieve Results (STAR) sampling site in Bountiful, UT. Data are considered at both sites during a 10-day winter period with high PM2.5 concentrations due to winter inversions, and at the Hawthorne site only during a fourteen day summer period when the site was impacted by smoke from wildfires in the Wasatch Mountains. The PM2.5 was dominated by organic material and ammonium nitrate in the winter and by organic material in the summer. In both cases, substantial amounts of sem-volatile material, SVM, was present which was not measured by a Tapered Element Oscillating Microbalance (TEOM) monitor but was detected by a Real-Time Ambient Mass Sampler (RAMS). The PM2.5 data have been combined with concentrations of particulate soot and soil corrected potassium, and with gas-phase concentrations of NOx, CO, and SO2 to apportion the PM2.5 to primary emissions emitted by and secondary particulate material formed from emissions from mobile sources, wood smoke combustion (including the forest fire emissions), and oil refineries located near the Bountiful site on an hourly basis. Wood smoke and mobile source emissions dominated the contribution to the non-volatile fraction of the PM2.5, with about equal contributions for each, but very different diurnal patterns for the two contributions. The diurnal patterns for the attribution were consistent with the emission patterns for these two sources. In addition, the oil refineries contributed about 10% of the primary PM2.5 at Bountiful. The SVM could not be directly attributed to any source but appeared to be secondary ammonium nitrate and organic material formed from gaseous emissions from the various sources. The dominant contributors to this SVM were NOx and gas-phase organic compounds in emissions from both mobile sources and wood smoke.