Assessing sources of PM2.5 in cities influenced by regional transport.

The human health effects of fine particulate matter (PM2.5) have provided impetus for the establishment of new air quality standards or guidelines in many countries. This has led to the need for information on the main sources responsible for PM2.5. In urban locations being impacted by regional-scale transport, source-receptor relationships for PM2.5 are complex and require the application of multiple receptor-based analysis methods to gain a better understanding. This approach is being followed to study the sources of PM2.5 impacting southern Ontario, Canada, and its major city of Toronto. Existing monitoring data in the region around Toronto and within Toronto itself are utilized to estimate that 30-45% of the PM2.5 is from local sources, which implies that 55-70% is transported into the area. In addition, there are locations in the city that can be shown to experience a greater impact from local sources such as motor vehicle traffic. Detailed PM2.5 chemical characterization data were collected in Toronto in order to apply two different multivariate receptor models to determine the main sources of the PM2.5. Both approaches produced similar results, indicating that motor-vehicle-related emissions, most likely of local origin, are directly responsible for about 20% of the PM2.5. Gasoline engine vehicles were found to be a greater overall contributor (13%) compared to diesel vehicles (8%). Secondary PM2.5 from coal-fired power plants continues to be a significant contributor (20-25%) and also played a role in enhancing production of secondary organic carbon mass (15%) on fine particles. Secondary fine particle nitrate was the single most important source (35%), with a large fraction of this likely related to motor vehicle emissions. Independent use of different receptor models helps provide more confidence in the source apportionment, as does comparison of results among complementary receptor-based data analysis approaches.

Further interpretation of the acute effect of nitrogen dioxide observed in Canadian time-series studies.

In this paper, the pooled NO2 association with nonaccidental mortality is examined across 10 cities in Canada in single- and two-pollutant time-series models. The results reaffirm that NO2 has the strongest association with mortality, particularly in the warm season. Although attributing such effects to NO2 cannot be ruled out, it is plausible that NO2 is acting as an indicator for some other exposure affecting the population. This could include PM2.5, as has been suggested from some personal exposure data, but it could also be indicating a more specific type of PM2.5, such as traffic-related particles, given that in cities the main source of NO2 is motor vehicle exhaust. NO2 could also be acting as a surrogate for other pollutant(s) originating from motor vehicles or high-temperature combustion, such as volatile organic compounds (VOCs) or polycyclic aromatic hydrocarbons. Another possibility is other oxidized nitrogen species ("NO(z)") or photochemically produced pollutants that can co-vary with NO2, especially during urban stagnation events. Data to test these different possibilities across several Canadian cities are examined. The focus is on correlations in time or space between NO2 and other pollutants that are more strongly linked to vehicle emissions. The results support the hypothesis that NO2 is a better indicator than PM2.5 of a range of other toxic pollutants. This includes VOCs, aldehydes, NO(z) and particle-bound organics in motor vehicle exhaust. Thus, overall, the strong effect of NO2 in Canadian cities could be a result of it being the best indicator, among the pollutants monitored, of fresh combustion (likely motor vehicles) as well as photochemically processed urban air.

Contribution of Nitrate and Carbonaceous Species to PM2.5 Observed in Canadian Cities.

At a variety of Canadian monitoring sites, carbonaceous compounds were estimated to account for an average of 50% of fine particle mass. These estimates were determined by subtracting the total fine particle mass associated with inorganic compounds from the total fine mass determined gravimetrically. This approach, which yields an upper limit estimate of the total amount of carbon-related mass was necessary since particulate carbon was not measured in the Canadian National Air Pollution Surveillance (NAPS) network. In this paper, total carbon estimates are evaluated against organic and elemental carbon measurements at locations in the Greater Vancouver area and Toronto. In addition, particle nitrate measurements at seven Canadian locations are used to determine the importance of nitrate relative to total mass and to examine the sampling artifacts due to the loss of particle nitrate from Teflon filters used in the NAPS di-chotomous samplers. Measurements of organic and elemental carbon indicated that the total carbon estimation approach provides representative estimates of the average contribution by carbonaceous material to the total fine and coarse mass. The average total carbon among all Vancouver area measurements (N = 225) was 4.28 µg m-3, while the estimated value was 4.34 µg m-3. There was a larger discrepancy between Toronto total carbon measurements (12.1 µg m-3) and estimates (8.8 µg m-3), which is attributed in part to sampling of particles above 10 mm in diameter. However, the R2 relating the measurements and estimates was about 0.71 for both areas. Linear regression slopes of 0.98 for Vancouver and 0.78 for Toronto (nonsignificant intercepts) indicate little bias in the Vancouver estimates, but a tendency for underestimation as the observed total carbon concentration increased in Toronto. Annually, nitrate was responsible for 17% and 12% of the fine mass in the Vancouver area and Ontario, respectively. In contrast, at two rural locations in southern Quebec and Nova Scotia, only 6% of fine mass was associated with nitrate. Due to filter losses, nitrate concentrations determined through the NAPS dichot sampling were much lower than actual concentrations (0.44 µg m-3 vs. 2.63 µg m-3). As a result of these losses (attributed mostly to loss during laboratory storage), previous total carbon estimates for the Canadian NAPS sites were likely to have been overestimated on average by about 10%.

Observations and Interpretations from the Canadian Fine Particle Monitoring Program.

Canadian particle monitoring programs examining PM10, PM2.5, and particle composition have been in operation for over 10 years. Until recently, the measurements were manual/filter-based with 24-hr sample collection varying in frequency from daily to every sixth day, using GrasebyAnderson dichotomous samplers. In the past few years, these monitoring activities have been expanded to include hourly measurements using tapered element oscillating microbalances (TEOMs). This continuous monitoring program started operation focusing on PM10, but now emphasizes PM2.5 through the addition of more TEOMs and switching of the inlets of some of the existing units. The data from all of these measurement activities show that there are broad geographical differences and also local- to regional-scale spatial differences in mass and composition of PM2.5. Due to variations in sources, significantly different PM2.5 concentrations are not uncommon within the same city. Comparison of nearby urban and rural sites indicates that 30 and 40% of the PM2.5 is from local urban sources in Montreal and Toronto, respectively. Hourly PM2.5 measurements in Toronto suggest that vehicular emissions are an important contributor to urban PM2.5. There has been a decreasing trend in urban PM2.5, with annual average concentrations between the 1987-1990 and 1993-1995 periods decreasing by 11 to 39%, depending upon the site. The largest declines were in Montreal and Halifax, and the smallest decline was in Toronto. Comparison of 24-hr TEOM and manual dichotomous sampler PM2.5 measurements from a site in Toronto indicates that the TEOM results in lower concentrations. The magnitude of this difference is relatively small in the warmer months, averaging about 12%. During the colder months the difference averages about 23%, but can be as large as 50%.