Smog related air pollutants (i.e., PM2.5, 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 U.S. (for example, NOx emissions from electricity generators in the Ohio Valley) into Ontario.

NOx 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.


Modelling transboundary impacts on Ontario’s air quality

The Community Multi-Scale Air Quality model (CMAQ) (USEPA, 2018), 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 PM2.5) in the atmosphere. Emissions of smog precursors from a region that included eastern North America was used along with meteorological conditions to assess ambient concentrations of pollutants for 2015. 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 (PM2.5)

Transboundary contributions to Ontario’s PM2.5 concentrations are mainly due to secondary PM2.5 formation (i.e., PM2.5 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. These transboundary contributions vary across the province, ranging from approximately 25% to 87%.

Along Canada-U.S. border areas of southwestern Ontario, transboundary sources of PM2.5 (mainly secondary PM2.5) dominate, contributing to over 70% of the annual PM2.5 concentrations, and decreasing further away from border areas as shown below.

Transboundary influences on Ontario’s annual average PM2.5 concentrations

This map shows the transboundary influences on Ontario’s annual average fine particulate matter concentrations.

In the Greater Toronto Area (GTA) and Ottawa, where there are many more local sources of directly emitted PM2.5, transboundary contributions of PM2.5 are much less (e.g. up to 25% in the GTA and 35% in Ottawa).

Outside of urban areas, transboundary influences increase in all directions. In much of northern Ontario the majority of the PM2.5 is dominated by transboundary sources (over 80% in most cases) as there are very few local sources of directly emitted PM2.5.


The influence of transboundary sources (U.S. and global background) on ozone levels in Ontario varies across the province. Transboundary contributions 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 95% of the ozone is attributable to transboundary sources, with the U.S. contributing to as much as 40% and the remainder attributable to global background, as shown below.

Transboundary contributions vary across the province, ranging from approximately 87% to 99% with the US contributions accounting for approximately 11% to 38%.

Influences on Ontario’s ozone concentrations on high concentration days

This map shows influences on Ontario’s ground-level ozone concentrations on high concentration days.

Global background plays a significant role in Ontario’s transboundary air pollution, contributing to 60-80% of ozone concentrations on poor air quality days across the province. In areas of northern and eastern Ontario, global background levels are much more significant.

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).