Number concentrations and size distributions of particles in the size range of 0.010-0.500 μm were measured in Rochester, NY, from December 2001 to December 2002. The relationships between the number concentrations, gaseous pollutants, and meteorological parameters were examined during particle nucleation events. More than 70% of measured total number concentration was associated with ultrafine particles (UFP, 0.011-0.050 μm). Morning nucleation events typically peaking UFP number concentrations at around 08:00 were apparent in winter months with CO increases. These particles appear to be formed following direct emissions from motor vehicles during morning rush hour. There were also often observed increases in this smaller-sized range particles in the late afternoon during the afternoon rush hour, particularly in winter when the mixing heights remain lowerthan in summer. Strong afternoon nucleation events (>30,000 cm{sup}(-3)) peaking at around 13: 00 were more likely to occur in spiring and summer months. During the prominent nucleation events, peaks of SO{sub}2 were strongly associated with the number concentrations of UFP, whereas there were no significant correlations between these events and PM{sub}2.5 and CO. Increased SO{sub}2 concentrations were observed when the wind direction was northwesterly where three SO{sub}2 sources were located. It is hypothesized that UFP formed during the events are sulfuric acid and water from the oxidation of SO{sub}2. There were also a more limited number of nucleation events followed by particle growth up to approximately 0.1 μm over periods of up to 18 h. The nucleation and growth events tended to be common in spring months especially in April.
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