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Abstract (summary): The number and size distributions of particles between 10 and 400 nm were measured in Toronto and rural Egbert during May 2007 to May 2008 to compare nucleation and growth at paired urban and rural locations. Particle formation and growth were observed in Egbert more frequently than in Toronto, and simultaneous events occurred on 34 out of 368 days. In contrast, formation and growth rates were both higher in Toronto. Further, a linear regression analysis suggested that compounds contributing to nucleation and growth processes were different in Toronto and Egbert. Vehicular emissions seemed to suppress particle formation in downtown Toronto. Nucleation also appeared to be suppressed by long-range transported pollutants originating from industrial regions in southwestern Ontario and northern Ohio in the United States. A Nucleation Indicator (NI) was developed by combining relevant parameters, and it was found to provide a reasonable measure of the probability of nucleation events occurring.