Canada–United States Transboundary Particulate Matter Science Assessment 2013 - Executive Summary

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This document provides an update of the findings from the 2004 Canada–United States Transboundary Particulate Matter Science Assessment (the 2004 Assessment). Its goal is to present a scientific and technical basis for discussions of adding a particulate matter (PM) annex to the 1991 Canada–United States Air Quality Agreement (the Agreement), to assess the potential impacts of a PM annex and, ultimately, to help determine whether such an annex is currently warranted.

This assessment focuses on the fine particle fraction of PM, or PM2.5, because this size fraction can remain suspended in the air for several days to weeks, can be transported by winds over large distances, and thus is subject to atmospheric transboundary transport in North America.

This document is organized around five key science questions:

  1. What are the impacts of PM2.5 on human/ecosystem health and public welfare, and what are the current air quality standards to protect human and ecosystem health in Canada and the U.S.?
  2. What are the recent levels of PM2.5 in Canada and the U.S.?
  3. What are the emissions and emission trends of the pollutants that contribute to ambient PM2.5 concentrations in Canada and the U.S.?
  4. What is the evidence that transboundary flow of PM2.5 occurs across the Canada–U.S. border, and what changes are projected, given future emission rates in both countries?
  5. Are there emerging science issues that could affect the understanding of PM2.5 formation, PM2.5 levels, and its impacts on human and ecosystem health?

Key Findings

PM2.5 and its precursors have significant effects on the health of humans and ecosystems.

The already extensive body of studies providing evidence on the effects of fine particles on health has grown significantly since the 2004 Assessment. These studies provide evidence of consistent increases in premature mortality and morbidity related to ambient PM2.5 concentrations, with the strongest evidence being reported for cardiovascular-related effects. Furthermore, the ubiquity of PM2.5 implies that exposure to ambient PM2.5 concentrations can have a substantial public health impact, even with recent reductions. In addition, although deposition (wet and dry) of acidifying sulphur and nitrogen compounds related to PM2.5 in Canada and the U.S. has been reduced since 2004, recent deposition in both countries continues to exceed thresholds (termed critical loads) in some geographic areas, thus posing a risk of harmful effects to terrestrial and aquatic ecosystems. Finally, although significantly reduced in most border areas, PM2.5 continues to contribute to visibility impairment in both Canada and the U.S., particularly in highly populated regions of southern Ontario and Quebec in Canada and in the Midwest and Montana in the U.S. In response, both Canada and the U.S. recently lowered ambient air quality standards to protect human and ecosystem health from the harmful impacts of PM2.5.

Recent levels of ambient PM2.5 have been declining in both Canada and the U.S.

In Canada and the U.S., ambient concentrations of PM2.5 have diminished significantly from the levels reported in the 2004 Assessment. More specifically, between 2000 and 2012 the U.S. national average annual and 24-hour (h) concentrations of PM2.5 decreased 33% and 37%, respectively. Data from Canadian PM2.5 speciation sites indicate that, between 2003 and 2010, average annual concentrations of PM2.5 declined about 4 µg/m3 in eastern Canada while average levels across western Canada remained fairly constant. In 2012, ambient concentrations reported at most monitoring sites in the U.S. along the Canadian border would meet the annual and 24-h National Ambient Air Quality Standards (NAAQS) for PM2.5 set in 2012. In both eastern and western Canada, data from the filter-based monitoring network indicate that average annual concentrations (2008–2010) would meet the Canadian Ambient Air Quality Standard (CAAQS) set for 2015.

The decline of most PM2.5 precursors is expected to continue, while direct emissions of PM2.5 and ammonia (NH3) have and are expected to remain relatively stable.

National emission inventories in both Canada and the U.S. show that emissions of the PM2.5 precursors sulphur dioxide, nitrogen oxides and volatile organic compounds (SO2, NOx and VOCs) declined between 2002 and 2010. However, total direct emissions of anthropogenic PM2.5 have remained fairly stable in both Canada and the U.S. during this period, as have emissions of ammonia (NH3).

Projections based upon known policies established in Canada and the U.S. for governing future emissions indicate that emissions of PM2.5 and its precursors will follow recent trends. In Canada, primary emissions of PM2.5 are expected to remain stable through 2020, while emissions of sulphur oxides (SOx)and NOx are projected to decline by 33% and 13%, respectively, between 2006 and 2020. By contrast, Canadian VOC and NH3 emissions are not projected to change significantly in this period. In the U.S., emissions of SO2, NOx and VOCs are forecast to decrease 65%, 42% and 21% from 2008 to 2020, while emissions of PM2.5 are projected to decrease modestly (8%). NH3 emissions in the U.S. are expected to be 2% higher in 2020 than in 2008.

Projections are that the influence of transboundary transport between Canada and the U.S. will be reduced and that current and planned PM2.5 ambient air standards will likely not be exceeded.

Modelling analyses of the impact of future emission projections show notable anticipated reductions in ambient PM2.5 concentrations between 2006 and 2020 in both Canada and the U.S. Significant declines in ambient PM2.5 concentrations are expected to occur in most border region cities, with percentage reductions ranging up to 35% in major U.S. cities near the border and up to 25% in their Canadian counterparts.

There is ongoing evidence that PM2.5 is transported across the Canada–U.S. border. However, for most cities in both countries, the dominant sources of PM2.5 in 2020 continue to be domestic emissions; overall, transboundary influence is projected to be less in 2020 than in 2006. The influence of U.S. emissions on PM2.5 concentrations in Canadian cities near the border is projected to decrease by about 2–10%, with the largest reductions occurring in eastern Ontario and southwestern Quebec. The exception is Abbotsford, B.C., where there is a small projected increase in U.S. influence. The influence of Canadian emissions on select U.S. cities is also projected to decrease, but by less, in the range of 1–3%, with the exceptions of Seattle, Wash., Buffalo, N.Y., and Rochester, N.Y., where the Canadian influence is projected to increase slightly.

In the U.S., no areas in the border region are predicted to exceed the current annual or 24-h PM2.5 NAAQS (12 μg/m3) in 2020, including areas with projected increases in Canadian influence. In Canada, the predicted 2006–2020 decreases in PM2.5 are expected to result in rural/regional background PM2.5 concentrations over the region near the southern Ontario and southern Quebec border that will be below the 2015 and 2020 annual and 24-h CAAQS. However, these levels are close enough to the CAAQS that some populated areas with relatively large local emissions may experience PM2.5 above the CAAQS. In the border regions of western and Atlantic Canada, 2015 and 2020 CAAQS levels are not projected to be exceeded.

Emerging air quality issues could influence future concentrations of PM2.5 in both countries; thus there is a continued need to improve our scientific understanding of health and ecological effects, the impacts of air quality management activities, and the magnitude of transboundary transport.

The following emerging science issues will probably affect future ambient PM2.5 concentrations and/or how air quality management activities are developed to address PM2.5:

  • the need for improved understanding of the health effects of PM2.5 and its components in the context of exposure to other pollutants and how these combined effects might affect air quality standards and management strategies
  • the need for increased understanding of the impacts of climate change on PM2.5 concentrations and of the effects of PM2.5 and its components on climate change
  • the effects of changes in the mix of energy generation and end-use technologies on the concentrations of PM2.5 and the impacts of growing domestic fossil fuel extraction activities, such as the oil sands, and unconventional oil and gas development, such as the use of hydraulic fracturing (fracking)
  • changes in the relative importance of natural sources and intercontinental transport that could affect the management of ambient PM2.5 concentrations in Canada and the U.S.

As the science continues to evolve on these issues, air quality management activities in both Canada and the U.S. may need to adjust in order to continue to effectively protect public health and the environment.


This updated Transboundary PM Science Assessment cites a number of studies (including those on which the U.S. NAAQS and Canadian CAAQS are based) documenting that exposure to PM2.5 leads to significant impacts on human health, ecosystem health and public welfare; and that additional reductions of PM2.5 below the NAAQS and CAAQS would have additional public health benefits. To address PM2.5 standards as well as other important air quality issues, regulatory and non-regulatory actions are being implemented in both countries to reduce emissions of PM2.5 and its precursor pollutants. Air quality modelling analyses indicate that future concentrations of ambient PM2.5 are not expected to exceed the current PM2.5 NAAQS or CAAQS along the Canada–U.S. border, with a few possible exceptions limited to cities near the border with significant local contributions to PM2.5.

Because of the important health and environmental effects associated with PM2.5, it would be beneficial for both countries to track progress and exchange information relevant to achieving PM2.5-related emissions reductions, air quality improvement and program implementation over time. Doing so would provide confidence that future PM2.5 concentrations in the border region will be below the NAAQS and CAAQS; it would also afford an ongoing opportunity to determine how emerging issues, such as climate change and an evolving energy landscape, may be impacting PM2.5 concentrations and transboundary transport over time.

Given the science reported in this assessment and the important role of PM2.5 in air quality management activities in both Canada and the U.S., there would be value in addressing PM2.5 in some manner under the Agreement.

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