Five-year roadside measurements of ultrafine particles in a major Canadian city

This study reports the diurnal, seasonal and annual variation of ultrafine particles (UFP) in a large city. Particle number (PN) concentrations were measured in Toronto, Canada on a major arterial roadway between March 2006 and May 2011 using a Fast Mobility Particle Sizer. The PN in the size range of 8-300 nm decreased during the measurement period primarily due to changes in the vehicle fleet. PN50 (particles with diameters less than 50 nm) decreased by 21% between 2006 and 2010. Notably, the majority of the reduction occurred during the winter months. PN50 exhibited the strongest seasonality and diurnal trend. PN50-100 (particles between 50 and 100 nm) and PN100-300 (particles between 100 and 300 nm) decreased by 17% and 24%, respectively. Correlation analysis between gas phase criteria pollutants showed good correlation between PN50-100 and NO2, SO2, and PM2.5. In contrast, PN50 exhibited the highest correlation with temperature, NO and NO2. A multiple linear regression model was developed for each size fraction. The model adequately explained the annual, seasonal and day-to-day variability of PN50-100 (R-2 = 0.64) and PN100-300 (R-2 = 0.83). The model captured the annual and seasonal variability of PN50 but only partially explained the day-to-day variability (R-2 = 0.52). The long-term reductions in PN50 indicate that policy interventions are having some success in slowly decreasing UFT, concentrations in Toronto. (C) 2011 Elsevier Ltd. All rights reserved.