The significant and largely unregulated presence of ultrafine particles in the atmosphere is a growing environmental concern. Understanding the formation and sources of ambient ultrafine particles represents a major goal, but analyzing such small particles is a considerable analytical challenge. This study demonstrates how real-time measurements of particle size and chemistry, coupled with gas-phase measurements, can provide unique insight into the daily and seasonal variations of the sources and chemistry of ultrafine particles. Measurements of ambient ultrafine particles are compared from three different single particle techniques which provide high temporal resolution during the 2005 summer (August) and fall (November) seasons in Riverside, CA in conjunction with the Study of Organic Aerosols at Riverside (SOAR) field campaign. During both seasons, vehicle exhaust emissions strongly contributed to the ultrafine particle concentrations during the weekday morning rush hour periods. SMPS measurements during the summer season showed high ultrafine particle concentrations during the afternoons most likely formed by photochemical events. In this study, different sources (and hence chemistry) contributed to the particles during periods of high ultrafine particle concentrations. Therefore, it is important to obtain simultaneous information on ultrafine particle sources as well as concentrations and advance beyond relying just on ultrafine particle concentrations as a proxy in future studies on health effects.