1Southern Ontario Centre for Atmospheric Aerosol Research, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada
2Chemistry Department and Centre for Atmospheric Chemistry, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada
3Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada
4Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,\newline Ontario M5S 3H6, Canada
Abstract. Ultrafine particle (UFP) number and size distributions were simultaneously measured at five urban and rural sites in Southern Ontario, Canada as part of the Border Air Quality and Meteorology Study (BAQS-Met 2007). Particle formation and growth events at these five sites were classified based on their strength and persistence as well as the variation in geometric mean diameter. Regional nucleation and growth events and local short-lived strong nucleation events were frequently observed at the near-border rural sites, upwind of industrial sources. Surprisingly, the particle number concentrations at one of these sites were higher than the concentrations at a downtown site in a major city, despite its high traffic density. Regional nucleation and growth events were favored at intense solar irradiance and less polluted cooler drier air. The most distinctive regional particle nucleation and growth event during the campaign was observed simultaneously at all five sites, which were up to 350 km apart. Although the ultrafine particle concentrations and size distributions generally were spatially heterogeneous across the region, a more uniform spatial distribution of UFP across the five areas was observed during this regional nucleation event. Thus, nucleation events can cover large regions, contributing to the burden of UFP in cities and potentially to the associated health impacts on urban populations. In addition, particle formation in southwestern Ontario appears to more often be related to anthropogenic gaseous emissions, although biogenic emissions may at times contribute. Local short-lived nucleation events at the near-border sites during this three-week campaign were associated with high SO2, which likely originated from US and Canadian industrial sources. These particle formation events may contribute to the production of cloud condensation nuclei, thus potentially influencing regional climate. Longer-term studies are needed to help resolve the relative contributions of anthropogenic and biogenic emissions to nucleation and growth in this region.