Ultrafine particle number size distributions in Rochester: Implication for nucleation events and application of advanced receptor modeling

Number concentrations and size distributions of particles in the size range of 0.01 to 0.5 mum were measured in Rochester, NY from December 2001 to December 2003. A total of 7,778 samples, each with 54 channels, were obtained and analyzed to assess the presence and/or formation of ultrafine particles. The relationships between the number concentrations and 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 mum < Dp < 0.050 mum). In the urban atmosphere nucleation events can be classified as traffic-related morning events, local SO2- related events, or regional nucleation and growth events based on their particle formation behavior and sources. There were significant seasonal and temporal variations in frequency and intensity of the nucleation types. Morning nucleation events typically showed peaks in the UFP number concentrations at around 8:00, and were particularly apparent in winter months correlated with CO increases. These particles appear to be formed following direct emissions of motor vehicles during the morning rush hour and low temperatures regularly enhanced the formation in urban area. There were also often observed increases in this smaller size range particles in the late afternoon during the afternoon rush hour particularly in winter when the mixing heights remain lower than in summer. Strong local nucleation events (>30,000 cm-3) peaking at around 13:00 were more likely to occur in the spring and summer months. During the prominent nucleation events, peaks of SO2 were strongly associated with the number concentrations of UFP, whereas there were no significant correlations between these events and PM2.5 and CO. Increased SO2 concentrations were observed when the wind direction was northwesterly where major SO2 sources were located. It is hypothesized that UFP formed during the events are sulfuric acid and water from the oxidation of SO 2. In general, regional nucleation events were frequently observed in the spring and winter and distinctly suppressed in summer. There were, however, a more limited number of regional nucleation events followed by particle growth up to sizes of approximately 0.1 mum over periods of up to 18 hours. The nucleation and growth events tended to be common in spring and summer months especially in April. The size distribution data for two winter months was analyzed by advanced Positive Matrix Factorization (PMF) receptor model to identify the sources of UFP. Size distributions of fine and ultrafine particles were more likely to be influenced by motor vehicles in winter.