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The impact of various atmospheric transport directions on ambient fine particle (PM 2.5 ) concentrations at several sites in southeastern Canada was estimated (for May-September) using back-trajectory analysis. Three-day back trajectories (four per day) were paired with 6-hr average PM 2.5 mass concentrations measured using tapered element oscillating microbalances (TEOM). PM 2.5 concentrations at rural locations in the region were affected by nonlocal sources originating in both Canada and the United States. Comparison of sites revealed that, on average, the local contribution to total PM 2.5 in the greater Toronto area (GTA) is approximately 30-35%. At each location, average PM 2.5 concentrations under south/southwesterly flow conditions were 2-4 times higher than under the corresponding northerly flow conditions. The chemical composition of both urban and rural PM 2.5 was determined during two separate 2-week spring/summer measurement campaigns. Components identified included SO 4 2-, NO 3 -, NH 4 + , black carbon and organic carbon (OC), and trace elements. Higher particle mass at the urban Toronto site was composed of a higher proportion of all components. However, black carbon, NO 3 -, NaCl, and trace elements were found to be the most enriched over the rural/regional background levels.

Canada has recently established standards for the management of particulate matter (PM) air quality. National networks currently measure PM mass concentrations and chemical speciation. Methods used in the U.S.

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