The main objectives of this study are to develop a systematic approach for the identification and classification of ultrafine particle (UFP) events and to analyze the events for implications of sources and meteorological conditions conducive to elevated UFP number concentrations. UFP events are prolonged periods with elevated UFP number concentrations. Particle number size distributions, gas pollutants, and meteorological parameters were concurrently measured during the summers of 2003 to 2005 in Detroit, Michigan, USA. Among the 74 identified UFP events, 40 (54%) are Aitken mode particle events and 34 (46%) are nucleation mode particle events. Correlation results show that 65 out of the 74 UFP events were associated with plumes of combustion sources, including all the Aitken mode events and 25 out of the 34 nucleation mode events. These in-plume particle events were positively correlated with elevated NO, CO, particle surface area, and occasionally high levels of SO2. The remaining 9 nucleation mode events, however, showed no such correlations, and hence referred to as secondary nucleation mode particle events. These secondary nucleation events occurred under relatively clean (i.e., low preexisting aerosols) and sunny conditions shortly after the breakup of nocturnal inversion and during midday. Overall, the results indicate that motor vehicles and industrial plumes are the major sources of elevated UFPs in urban/industrial air. Under favorable conditions, atmospheric secondary nucleation can occur in rather polluted urban/industrial air and become a major contributor of UFPs. The formation mechanisms, hence chemical composition, are likely different between the in-plume and secondary UFPs. Therefore, exposure assessments to ambient UFPs need to take into account the contributions from both types of particles.