A year-long comparison of particle formation events at paired urban and rural locations

Ultrafine particle size distribution data were collected in downtown Toronto and rural Egbert from May 2007 to May 2008. Particle formation events were observed in both locations and contributed to increased concentrations of particles less than 25 nm in diameter. These events were more frequent in spring and fall and rarely occurred in winter. Stronger solar radiation and drier air were correlated with the occurrence of formation events at both locations. Nucleation events occurred simultaneously at both sites on 10% of the days, and these events involved a shared air mass. Half of these simultaneous events were associated with northern air masses and only a quarter with southerly air masses. The higher loading of aged particles in southerly air masses transported from upwind industrial sectors appeared to limit the occurrence of nucleation events. Formation events occurred less frequently in downtown Toronto than at the rural site, and the frequency was lower on weekdays. It is hypothesized that vehicular emissions were responsible for the suppression of nucleation events in downtown Toronto.