Brook

Abstract

The impact of various atmospheric transport directions on ambient fine particle (PM2.5) 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 PM2.5 mass concentrations measured using tapered element oscillating microbalances (TEOM). PM2.5 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 PM2.5 in the greater Toronto area (GTA) is approximately 30–35%. At each location, average PM2.5 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 PM2.5 was determined during two separate 2-week spring/summer measurement campaigns. Components identified included SO4 2–, NO3 –, NH4 +, 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, NO3 –, NaCl, and trace elements were found to be the most enriched over the rural/regional background levels. INTRODUCTION In Canada, the highest concentrations of fine particles (PM2.5) tend to occur over southern Ontario, southern Quebec, and the southern Atlantic provinces.1,2 This area is a large geographic region (referred to here as southeastern Canada) containing approximately three-quarters of the country’s population. High PM2.5 concentrations in rural locations throughout southeastern Canada suggest that regional-scale (>200 km) transport influences PM2.5 and that maximum levels will occur within or just downwind of urban areas. It is well known that nonurban areas throughout this region as well as in the northeastern United States experience acid deposition originating from relatively distant upwind sources.3,4 Southeastern Canadian levels of Hg deposition,5 ambient SO4 2–,4,6,7 O3, 8,9 trace elements,10 and regional haze11 have also all been shown to be influenced by distant upwind sources. Consequently, it is reasonable to expect that over southeastern Canada, the concentration of PM2.5 mass in general is impacted by regional transport, including transboundary transport, especially given the multiday lifetime of fine particles in the atmosphere. Chuersuwan et al.12 have shown that the long-range transport of PM is important to PM2.5 episodes over the northeastern United States (specifically New Jersey). The transport of PM2.5 trace elements for the border region between Texas and Mexico was examined for a short time period,13 but transport patterns for PM2.5 over southeastern Canada have not been studied. This is primarily because of the lack of rural measurements, which are better suited to such an assessment than are urban measurements. Three years of rural PM2.5 data are now available. The goal of this article is to examine the variability in rural PM2.5 levels by transport direction for locations in rural southeastern Canada, including sites near the Canada–United States border. This provides information on the relative importance of different upwind regions to high PM2.5 concentrations and on regional and continental background concentrations, which were expected to be influenced by regional-scale to long-range transport. Regional Transport and Urban Contributions to Fine Particle Concentrations in Southeastern Canada Jeffrey R. Brook, Carrie D. Lillyman, Marjorie F. Shepherd, and Alexandre Mamedov Atmospheric and Climate Sciences Division, Meteorological Service of Canada, Environment Canada, Downsview, Ontario, Canada IMPLICATIONS In June 2000, a Canada-wide standard for PM2.5 of 30 μg/m3 (24-hr average), 98th percentile over three years, was established. Current levels in parts of southeastern Canada, especially in the Windsor-to-Quebec City corridor, are greater than this standard. It is, therefore, important to investigate further the sources of high ambient concentrations. During periods of high PM2.5 concentration, regional transport is estimated to account for 65–70%, and local emissions for 30–35%, of PM2.5. This indicates a need for air-quality management strategies to address both domestic Canadian PM2.5 and precursor sources as well as sources in the eastern United States to fully address achievement of the Canada-wide standard for PM2.5. Brook, Lillyman, Shepherd, and Mamedov 856 Journal of the Air & Waste Management Association Volume 52 July 2002 In addition, the importance of locally emitted/produced PM2.5 relative to regional background concentrations was examined using measurements collected in the greater