Transboundary influences on Ontario’s smog

Smog related air pollutants (i.e., PM_2.5, ozone and their precursors) are generated both locally and regionally, and, with winds, can travel hundreds of kilometres, affecting areas far from the source of the pollution. The long-range transport and transboundary flow of air pollutants play a significant role in Ontario’s air quality. Typically, during the summer, elevated levels of these smog related pollutants are often associated with distinct weather patterns (e.g., slow-moving high-pressure systems originating from south of the lower Great Lakes) that results in the long-range transport of these pollutants into Ontario from neighbouring U.S. industrial and urbanized states during south to southwesterly flow conditions (Yap et al., 2005).

Transboundary sources from around the globe (global background) are also significant contributors to Ontario’s ozone levels.

Long-range transport and transboundary flow of air pollutants

During the smog season (May to September) prevailing southwesterly airflows results in the transport of smog related pollutants from the U.S. (for example, NO_x emissions from electricity generators in the Ohio Valley) into Ontario.

NO_x emissions from electricity generators and prevailing winds

This map shows the nitrogen oxides emissions from electricity generators located in eastern U.S. and Canada with prevailing winds during smog season.

Note:

• Canadian Data - National Pollutant Release Inventory, Environment and Climate Change Canada
• U.S. Data - Clean Air Markets Program Data, United States Environmental Protection Agency

Modelling transboundary impacts on Ontario’s air quality

The Community Multi-Scale Air Quality model (CMAQ) v5.4 (USEPA, 2022), developed by the U.S. Environmental Protection Agency, was used to characterize, and predict the formation, transportation, deposition and transformation of smog related pollutants (e.g., ozone and PM_2.5) in the atmosphere. Emissions of smog precursors from a region that included eastern North America was used along with 2023 meteorological conditions to assess ambient concentrations of pollutants using model-ready inventory data^{footnote 1[1]} as compiled and made available periodically by Environment and Climate Change Canada (ECCC) and the United States Environmental Protection Agency (U.S. EPA). The CMAQ model results quantify the impact of transboundary flows of smog related pollutants into and across Ontario.

Transboundary influences on Ontario’s smog

Fine particulate matter (PM_2.5)

Transboundary contributions to Ontario’s PM_2.5 concentrations are mainly due to secondary PM_2.5 formation (i.e., PM_2.5 is formed in the atmosphere through a series of complex reactions involving precursor emissions such as sulphur dioxide and nitrogen oxides) originating from U.S. sources. The U.S. specific transboundary contributions vary across the province, ranging from approximately 5% to 80%.

Along the Canada-U.S. border areas of southwestern Ontario, U.S. and global background transboundary sources of PM_2.5 (mainly secondary PM_2.5) dominate, contributing to over 60% of the annual PM_2.5 concentrations. The contribution of U.S. sources of transboundary PM_2.5 decreases with distance from border areas in southern Ontario as shown below.

Transboundary influences on Ontario’s annual average PM_2.5 concentrations

This map’s layers show the influences of U.S. sources, Canadian sources, and global background on Ontario’s annual average PM_2.5 concentrations.

In the Greater Toronto Area (GTA) and Ottawa, where there are many more local sources of directly emitted PM_2.5, the cumulative transboundary contributions of PM_2.5 (i.e. U.S. and global background combined) are much less (e.g., up to approximately 50% in the GTA and approximately 60% in Ottawa).

Outside of urban areas, transboundary influences increase in all directions. In much of northern Ontario, most of the PM_2.5 is from global background transboundary sources, with the contribution increasing up to 95% as you move further north. U.S. contributions are also more significant in these northern areas than Canadian contributions as there are very few local sources of directly emitted PM_2.5.

Ozone

The influence of U.S. and global background transboundary sources on ozone levels in Ontario varies across the province. Transboundary contributions from the U.S. are most notable in areas of southwestern Ontario, in close proximity to the U.S. border, and along the northern shore of Lake Erie and the eastern shore of Lake Huron/ Georgian Bay. In southwestern Ontario, when ozone levels are elevated, over 90% of the ozone is attributable to cumulative transboundary sources (i.e., U.S. and global background combined), with the U.S. contributing up to approximately 40%.

Cumulative transboundary contributions to ozone levels vary across the province, ranging from approximately 75% to 99%, with the U.S. contributions accounting for approximately 1% in Ontario’s Far North and up to 40% in southern Ontario.

Influences on Ontario’s ozone concentrations on high concentration days

This map’s layers show the influences of U.S. sources, Canadian sources, and global background on Ontario’s ground-level ozone concentrations on high concentration days^{footnote 2[2]}.

Global background plays a significant role in Ontario’s ozone levels on high concentration days, contributing approximately 60-99% across the province. In areas of northern Ontario, global background is the most significant source of ozone, which can be as high as 99%.

As ozone precursor emissions continue to decrease across North America, the rising global background levels play an increasingly important role in Ontario’s air quality (Environment and Climate Change Canada, 2013).