Estimation of the U.S. Influence on Mean PM10, PM2.5 and Particle Sulphate Concentrations in Eastern Canada
Jeff Brook, Atmospheric Environment Service, Environment Canada
Nearly 20 years of research on acidic deposition has demonstrated that a significant amount of the acidifying chemical species that are deposited in eastern Canada originate in the U.S. This is due to the prevailing wind directions over the eastern half of North America in conjunction with the spatial distribution of pollutant sources. As a result of these emission and weather patterns the U.S. is also a significant contributor to airborne particulate matter (PM) in rural parts of eastern Canada.
Multiple years of daily 24 hour PM10, PM2.5 and particle sulphate measurements from several eastern Canadian Sites have been examined to determine how much of the PM in eastern Canada originates in the U.S. These measurements were paired with 72 hour back-trajectories and then sorted into three groups.
Group 1: Transport was not from the U.S.
Group 2: Transport was from the U.S.
Group 3: Undefined origin (transport was from the U.S. and Canada).
In this initial analysis two regions in Canada have been considered. These are the southeastern part of Quebec and the southern Maritimes. Particle data from the rural Canadian Air and Precipitation Monitoring (CAPMoN) sites of Sutton, Quebec, and Kejimkujik, NS, were used. In the case of Kejimkujik, trajectories were already in the database for Saint John, NB, and thus, these trajectories were used to sort the Kejimkujik PM measurements. This is not expected to significantly affect the results due to the relatively short distance between the two locations and the coarse resolution of the rawinsonde network. Sample plots of the trajectories associated with Groups 1 and 2 are shown for Sutton and Kejimkujik in Figures 1 and 2, respectively. Although the trajectory sorting routine was designed to minimize the crossing-over between groups, the figures show that there were a small number of cases when this occurred. This tended to occur further upwind (i.e., t>60 hr.) and if less than 10 of the 12 six hour trajectory segment end points associated with a 72 hour trajectory were not in a given group then the PM measurement was assigned to Group 3.
Average and median concentrations of PM10, PM2.5 and sulphate derived from Group 2 observations are assumed to provide an estimate of the amount of PM coming into Canada from the U.S. These concentrations are given in Table 1. For each of the three PM measures the concentrations are higher when transport is from the U.S. (Group 2). Given that there are virtually no Canadian sources between Kejimkujik and the U.S. or between Sutton and the U.S. it likely that the Group 2 PM is from U.S. sources, with a small contribution from the global background. In terms of PM2.5, the average concentration in the air masses coming from the U.S. to Canada is 11.5
mg m-3. This is close to the proposed U.S. standard of 15 mg m-3 for an annual average. Some of the 24 hour measurements that make up this average of 11.5 mg m-3 are well in excess of the proposed Canadian PM2.5 reference level1 of 15 mg m-3 for a 24 hour average. Regional PM measurements are also available for southern Ontario and trajectory analyses are in progress. The situation is more complex because there is a greater potential for Canadian sources to influence the measurements. Therefore, a more detailed analysis is planned.
1
Reference Level corresponds to the 24 hour air concentration below which no statistically significant association with health effects can be detected (hospital admissions in Ontario were relied upon heavily for this Canadian research [Burnett R.T., Dales R.E., Krewski D., Vincent R., Dann T. and Brook J.R., 1995: Associations between ambient particulate sulfate and admissions to Ontario hospitals for cardiac and respiratory diseases, American Journal of Epidemiology, 142, 15-22.]). The Reference Level is intended to be a "science-based" level.
Table 1 Average, median and standard deviation of the 24 hour PM10, PM2.5 and sulphate (SO42-) measurements sorted by group (G). N corresponds to the number of PM2.5 samples used in the calculation.
|
|
|
|
PM10 ( m g m-3) |
PM2.5 ( m g m-3) |
SO42- ( m g m-3) |
||||||
|
Site |
G |
N |
ave. |
med. |
s.d. |
ave. |
med. |
s.d. |
ave. |
med. |
s.d. |
|
Sutton |
1 |
100 |
10.3 |
8.1 |
7.6 |
6.6 |
5.9 |
4.3 |
1.8 |
1.3 |
2.0 |
|
Sutton |
2 |
27 |
15.6 |
15.9 |
6.3 |
12.0 |
11.5 |
6.1 |
4.4 |
4.3 |
2.8 |
|
Kejimkujik |
1 |
145 |
8.7 |
7.0 |
6.7 |
5.3 |
4.0 |
4.5 |
1.4 |
1.0 |
1.9 |
|
Kejimkujik |
2 |
56 |
13.8 |
11.0 |
9.8 |
8.9 |
7.0 |
7.3 |
3.5 |
2.1 |
4.5 |
Figure 1:
Group 1
Sutton, Quebec
Group 2
Sutton, Quebec
Figure 2:
Group 1
Saint John (applied to Kejimkujik PM measurements)
Group 2
Saint John (applied to Kejimkujik PM measurements)(note shifting the end point to Kejimkujik, which is ~100 km southeast, will move the trajectories across southern Ontario over or south of the Great Lakes)
