National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Ambient Levels

A national PM₁₀ and PM_2.5 monitoring program has been in operation since 1984 under the auspices of the National Air Pollution Surveillance (NAPS) network. This is primarily an urban network with few rural sites. In addition to the national network, British Columbia, Ontario and Québec operate particulate matter monitors.

PM data are typically collected over a 24 hour sampling period on a one-day-in-six sampling regime. By operating on this schedule, given a long enough sampling period, each day of the week is equally well sampled, and hence all conditions during the week are represented. It should be noted, however, that this sampling frequency does not permit the extremes of the concentration distribution to be accurately quantified. The one-in-six-day schedule has the likelihood of underestimating the frequency and magnitude of high concentration PM₁₀ events (by 20-30%), because the nearest days to the event day, and/or the event day itself, may be excluded by the sampling schedule.

PM levels in the atmosphere are a function of both natural and anthropogenic sources. 'Background' PM is generally defined as the distribution of PM concentrations that would be observed in the absence of anthropogenic emissions of PM and precursor emissions of VOC, NOx and SO₂. The actual magnitude of background PM for a given location is difficult to determine because of the influence of long range transport of anthropogenic particles and precursors. The range of expected background concentrations on an annual or long term basis is from 4 µg/m³ to 11 µg/m³ 3 for PM₁₀ and 1-5 µg/m for PM_2.5 for remote sites in North America. The range of expected background concentrations on a short term basis is much broader given the episodic nature of such natural events as wildfires and prairie dust storms, that can result in short term PM levels comparable to those in polluted urban atmospheres.

Twenty-four hour PM data typically exhibit a strongly skewed distribution dominated by a large number of low values. PM concentrations also typically exhibit variation on a number of temporal scales: diurnal, hebdomadal (day of week), seasonal and annual. The causes of these variations are multi-faceted and are related both to emission variability and to variations in geophysical variables such as mixed layer depth, wind speed and humidity levels.

Mean 24 hour PM₁₀ concentrations across Canada range from 15-42 µg/m³, with most sites in the range of 20-30 µg/m³. These levels are substantially above background levels, indicating that anthropogenic activities make a significant contribution to ambient PM₁₀ loadings. The highest 24 hour PM₁₀ concentrations recorded by the NAPS monitoring network were observed in Québec and Ontario (at sites in Montréal, Windsor, Hamilton and Walpole Island) and at a single site in Calgary, Alberta. However, even within cities, there may be sites that experience comparatively low ambient 24 hour PM₁₀ levels, as is the case in Montréal and Calgary. The three rural sites of Kejimkujik, Sutton and Egbert recorded mean 24 hour PM₁₀ concentrations of 11, 11 and 17 µg/m³ respectively, although observations were only available for 1992-1995 for Kejimkujik and Egbert and for May-September, 1993 for Sutton.

The season of maximum 24 hour PM₁₀ concentrations is regionally variable, reflecting variations in dominant sources of PM₁₀ (especially secondary aerosols) and synoptic meteorology. The sites that exhibit the highest degree of seasonality are in Windsor (a summertime maximum) and Victoria (a wintertime maximum). Many of the sites in British Columbia seem to exhibit a late winter spring maximum of both mean and median PM₁₀ concentrations and the upper quartile of the distribution, indicating that both average and extreme 24 hour PM₁₀ concentrations are typically higher during the months of January, February and March. Sites in Ontario seem to exhibit daily summertime maximum PM₁₀ concentration, which may reflect the greater abundance of secondary aerosols in the Windsor-Quebec City Corridor, where precursor concentrations are known to be high.

A hebdomadal cycle of 24 hour PM₁₀ concentrations is evident at most urban sites. Typically, weekend concentrations of PM₁₀ are lower than those observed during the work week. This difference is magnified for roadway sites, where up to a 50% increase in PM₁₀ was noted midweek relative to Sunday concentrations (all sites). This suggests a substantial contribution to PM₁₀ concentrations from transportation sources.

Yearly variations in PM₁₀ concentrations during the 1984 to 1995 sampling period show an apparent decrease at most sites with a complete data record. The largest percentage decreases occurred at the Montréal-Duncan/ Decarie, Edmonton and Vancouver sites. A trend analysis of annual PM₁₀ data for 1984 through 1993 showed a statistically significant (p < 0.001) decreasing trend in [ PM₁₀ ] on a national basis averaging 2 percent per year.

Twenty-four hour mean concentrations of PM_2.5 at the NAPS urban sites ranged from 8.5 to 20.2 µg/m³ . PM_2.5 concentrations are more spatially homogeneous than PM₁₀ but there are still significant site to site differences even within the same urban area. The highest PM_2.5 concentrations (in terms of means and 90th percentiles) were measured at sites in Montréal, Toronto, Hamilton, Windsor, Walpole Island and Vancouver. These were almost the same sites that recorded the highest PM₁₀ concentrations. The three rural sites of Kejimkujik, Sutton and Egbert recorded mean PM_2.5 24 hour concentrations of 7.0, 7.7 and 10.5 µg/m ³ respectively, although again, observations were only available for 1992-1995 for Kejimkujik and Egbert and for May-September, 1993 for Sutton.

The seasonal variability of PM_2.5 is more pronounced than that of PM₁₀ ; however, there is no discernible geographic pattern to this variability. The Montréal, Ottawa, Edmonton, Calgary and Vancouver/Victoria sites record higher PM_2.5 concentrations in the winter months and in particular during January and February. Other Ontario sites record the highest daily concentrations in the summer months with a peak median in August. The Maritime sites show variable seasonal variation in PM_2.5 concentrations with Saint John and Kejimkujik showing a strong summer maximum and Halifax a winter maximum.

Most urban sites show minimum 24 hour PM_2.5 concentrations on Sunday and maximum concentrations during the middle of the week. Again, this difference is magnified for roadway sites, where up to a 60% increase in PM_2.5 midweek relative to Sunday was noted (all sites). This indicates that there are large day-of-week differences in anthropogenic emissions and significant contributions from motor vehicles.

A trend analysis of PM_2.5 data for the period 1984-93 showed a statistically significant (p < 0.001) decreasing trend in PM_2.5 on a national basis averaging 3.3% per year. For the Ontario sites, there was no significant change in PM_2.5 between 1987 and 1993.

In 1994, ten sites (all but two in the Lower Fraser Valley) reported hourly PM₁₀ concentrations to the NAPS network using TEOM instruments. A maximum 1 hour PM₁₀ concentration of 255 µg/m ³ was measured at the Abbotsford site (in the LFV) and a maximum 1 hour concentration of 204 µg/m at the Edmonton site. Analysis of the diurnal variations in PM₁₀ have shown that a substantial increase in PM₁₀ levels occur during the morning rush hour, with a secondary peak during the late evening. Minimum values occur during the mid-afternoon and in the early hours of the morning (12-6:00 am.)

Relationships Among TSP, PM₁₀ and PM_2.5 and Inorganic Constituents of PM

Fourteen urban sites in the NAPS dichotomous sampler network operating from 1986 to 1994 had simultaneous measurements of TSP, PM10 , PM2.5 and sulphate (SO4 2- ). This data set is valuable in that it allows exploration of the composition of these different PM fractions at the 14 sites. On average across the 14 sites, PM10 accounted for approximately 50% of the TSP, while PM2.5 accounted for approximately 25% of TSP. Both fine and coarse particles accounted for approximately equal portions (about 50%) of the PM10 . Most of the sulphate was found on fine particles, where it comprised on average approximately 17% of the fine PM. However, considerable variation within and among sites exists for these ratios. The relationships between TSP, PM10 , PM2.5 are dependent on concentration, with ratios of PM10 and PM2.5 to TSP decreasing with increasing TSP concentration (i.e., more of the TSP mass is comprised of very coarse PM).

Other data from the NAPS network corroborate both the variability in PM_2.5 / PM₁₀ ratios and the overall finding that on average across Canada, approximately 50% of PM₁₀ is made up of fine particles (53% in this case).

These data, collected from 1984-1993 from 19 sites (16 locales), show that the median PM_2.5/ PM₁₀ ratios for most sites fall within a fairly narrow range of 0.4-0.6; that is, at least half of the time, 40-60% of PM₁₀ at a site iscomposed of fine particles (-2.5µm in diameter). Although there is clearly temporal variability in PM_2.5/ PM₁₀ ratios at a site, about 50% of the time, the ratios do not vary by much more than ±10% as indicated by the inter-quartile ranges (25th-75th percentiles).

There are relatively strong correlations (r² ) between PM₁₀ and PM_2.5 at each of the19 sites, which is consistent with the belief that temporal variations in fine particles have a significant influence on the observed variability in PM ₁₀. At a majority of the sites, the daily variability in fine particle mass had a stronger influence on the variations in PM₁₀ than did the coarse particle mass. This was most evident at the rural locations and at sites not heavily impacted by urbanization (i.e., traffic and construction). The exceptions to this pattern were the Prairie sites, where coarse mass dominated PM₁₀, and a site inMontréal that is heavily impacted by traffic.

Comparisons of TSP, PM₁₀, PM_2.5 and sulphate mass distributions at sites across Canada have shown a couple of key trends. Sites in the three Prairie cities of Winnipeg, Calgary and Edmonton have large and variable TSP concentrations, but their PM_2.5 and sulphate concentrations are small relative to the other sites and exhibit less variability. Much of the airborne particulate matter observed in these areas is expected to be mechanically derived and likely consists of local crustal material. Secondly, there is an obvious decrease in sulphate levels from the sites located east of the upper Great Lakes to those located west of the lakes. This pattern has been repeatedly observed and is a direct reflection of the magnitude and spatial density of SO₂ emissions within and upwind of these two areas.

Comparisons of urban and rural sites in close proximity to one another demonstrate, not surprisingly, that urban PM concentrations are greater than rural ones, particularly for coarse PM. This is mirrored by an enrichment in urban areas in the concentration of all inorganic elements and ions assayed for. There are several elements/ions for which the urban-rural difference is disproportionately greater than the total mass difference, however, indicating that these constituents are particularly enriched in urban areas (Ca, ^-, Fe, Al, Mg, Zn, Ti, Mn, V, Pb, Ni). This pattern is Si, NO₃ most likely attributed to the greater suspension of road dust, and more intensive industrial and combustion activity in urban areas.

Estimates of the amount of fine and coarse particle mass attributable to carbonaceous material (organic and elemental carbon) were made using a mass balance approach. Depending upon site, only about 37 to 61 percent of the PM_2.5 could be explained given themeasured concentrations of several inorganic ions and elements. Thus, carbonaceous material, which was likely to have been predominantly organic in nature, was responsible for about half of the overall fine particle mass. This fraction was higher in Alberta and British Columbia (~65%) than it was on the east coast (40-45%). ^- + dominate the identifiable Sulphate, NO₃ and NH₄ components of the fine PM mass, consistent with the results of many studies. Due to the increased importance of crustal material, a greater portion of the coarse particle mass (~70%) was explained by the inorganic constituents. This mass balance approach to partitioning PM mass into organic and inorganic fractions should be complemented by more detailed analytical studies of the carbonaceous component.