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Canadian Smog Science Assessment Highlights and Key Messages

Sources of Smog Pollutants

Emissions of most smog precursors have decreased over the 1985–2006 period, except for ammonia (NH3) (Figure 9). Smog precursor emissions projections for the year 2015 indicate continued reductions into the future, with the exception of NH3 and volatile organic compounds (VOC).

Emissions estimates contain uncertainties which vary considerably from one sector to another and from one chemical to the next. Diffuse or open sources in particular are difficult to quantify, as the emissions can vary quite significantly both spatially and temporally. Sector-specific emissions are often estimates based on an emission factors. Efforts are in place to reduce uncertainties through continuous monitoring and comparison to observations, testing and collaboration with industry. Low to high confidence rankings of emissions estimates are listed in a NARSTO (a cooperative public-private sector organization of Canada, Mexico and the United States) PM Science Assessment15.

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Figure 9 Historical (1985 to 2006) and projected (2007 to 2015) anthropogenic emissions (including open sources) of smog-forming pollutants (Environment Canada, 2010)

Primary Particulate Matter (PM)

The largest sources of primary (directly emitted) fine particulate matter (PM2.5) are road dust and construction/demolition activity, both characterized as open sources, amounting to approximately 67% of the national total. Other important anthropogenic sources are residential wood combustion, transportation and some industrial activities such as wood processing and pulp and paper plants (Figure 10). One area of high PM2.5 emissions density is the Windsor–Quebec City corridor resulting mainly from industrial activities and from the transportation, and residential wood combustion sectors (Figure 11). Major urban centres in western Canada and along the Edmonton–Calgary corridor are also shown as areas of high PM2.5 emissions density, again likely the result of emissions from the transportation sector. Figure 11 includes the emissions from open anthropogenic sources, illustrating the impact of these sectors such as in the interior of British Columbia. In this area, primary PM2.5 is a major issue of concern associated with residential woodstoves, agricultural and controlled burning, and road dust.

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Note: Total 2006 national emissions of 1123 kt, not including natural sources.

Figure 10 Key sectors contributing to the 2006 PM2.5emissions inventory

Figure 11 Density map of PM2.5 emissions (kg km-2) including open sources, in Canada for 2006

Total anthropogenic PM2.5 emissions have remained fairly stable from 1985–2006 (Figure 9). Excluding open sources, PM2.5 emissions have decreased by approximately 50% over that period due to reductions in emissions from the wood, pulp and paper and electricity generation sectors. Overall, anthropogenic PM2.5 emissions (including open sources) are projected to slightly increase to 2015 due to the high proportion of road and construction dust and residential wood combustion.

Natural sources are also important contributors to primary PM2.5emissions. They include forest fires, windblown soil, sea salt spray, and volcanic dust. Forest fires can contribute to primary PM2.5emissions in the boreal forest and sea salt is an important influence along the coast.

Nitrogen Oxides (NOX)

In Canada, transportation accounts for approximately half of national nitrogen oxides (NOX) emissions.  Upstream oil and gas and electric power generation are also important source sectors, collectively accounting for 31% of the national total (Figure 12). The highest density of NOX emissions is in Alberta and the Windsor–Quebec City corridor (Figure 13), where the oil and gas and transportation sectors are the most prominent sources, respectively.

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Note: Total 2006 national emissions of 2307 kt, not including natural sources.

Figure 12 Key sectors contributing to the 2006 NOXemissions inventory

NOX emissions have decreased by approximately 8% from 1985–2006 (Figure 9). This decrease is attributable to more stringent emissions regulations on the transportation and electric power generation sectors. Some of the decreases are currently being offset by greater emissions from the upstream and downstream petroleum sectors; however, an overall decreasing trend is projected to continue to 2015 (Figure 9).

Figure 13 Density map of NOX emissions (kg km-2) in Canada for 2006, not including open or natural sources

Sulphur Dioxide (SO2)

Non-ferrous smelting is the largest source sector of national sulphur dioxide (SO2) emissions followed by electricity generation and the upstream and downstream petroleum sectors. Collectively they account for approximately 80% of Canada’s total SO2 emissions (Figure 14). As with NOX, the highest density of emissions occurs in the Prairie Provinces and the Windsor–Quebec City corridor (Figure 15). Sulphur dioxide emissions have decreased by over 47% from 1985–2006, with the largest decline (30%) occurring prior to 1995 (Figure 9) under the 1985 Eastern Canadian Acid Rain Program. Past reductions in SO2 emissions are due to the regulation of the sulphur content of fuels, the phase out of coal-fired electricity generation units, and changes in industrial processes such as smelting. Since 1995, while some sectors have continued to see decreases in emissions, some of these improvements have been offset at the national level by increasing emissions from the petroleum sectors in recent years. This trend is expected to continue to 2015 (Figure 9), resulting in a gradual increase from 2006 levels although decreasing overall over the 1985–2015 period.

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Note: Total 2006 national emissions of 1972 kt, not including natural sources.

Figure 14 Key sectors contributing to the 2006 SO2emissions inventory

Figure 15 Density map of sulphur oxide (primarily SO2 and minimal contributions of H2SO3) emissions (kg km-2) in Canada for 2006, not including open or natural sources

Volatile Organic Compounds (VOC)

Anthropogenic volatile organic compounds (VOC) emissions in Canada are primarily emitted by the upstream petroleum sector and the transportation sector, accounting for approximately half of national emissions (Figure 16). Emissions sources are concentrated in Alberta, southern Saskatchewan (primarily due to the petroleum industry and transportation sources) and in major urban areas such as along the Windsor–Quebec City corridor (Figure 17). VOC emissions have decreased by about 18% from 1995 to 2006, mainly due to reductions in the transportation and solvents and printing sectors (Figure 9). However, as is the case with NOX emissions, some of the more recent decreases have been offset by increases in other sectors, particularly upstream petroleum. This trend is projected to continue leading to an overall increase in VOC emissions through to 2015 (Figure 9).

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Note: Total 2006 national emissions of 2210 kt, not including natural sources.

Figure 16 Key sectors contributing to the 2006 VOC emissions inventory

Natural sources, including vegetation and forest fires, also contribute significantly to ambient VOC levels. This is especially the case in rural or forested areas, where VOC of natural origin are many times higher than those of anthropogenic sources. However, the importance of natural versus anthropogenic VOC as an O3 precursor, even in rural areas, is dependent upon overall reactivity, in other words how efficiently the individual VOC reacts to form O3. VOC reactivity may have implications for emissions reductions strategies, where individual compounds are considered rather than treating all VOC equally. While this approach has been considered, more research and monitoring of individual VOC species are required.

Figure 17 Density map of VOC emissions (kg km-2) in Canada for 2006, not including open or natural sources

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Figure 18 Key sectors contributing to the 2006 NH3emissions inventory

Figure 19 Density map of ammonia (NH3) emissions (kg km-2) in Canada for 2006 with open but no natural sources

Ammonia (NH3)

The agricultural sector is the most important contributor of ammonia (NH3), accounting for 90% of national emissions (Figure 18). Areas of intense agricultural activity include southern Ontario and Quebec, southern British Columbia, Alberta and Saskatchewan (Figure 19). In Canada, NH3 emissions have increased by about 22% over the period of 1985–2006 and are generally expected to increase through 2015 as a result of economic growth and the demand for agricultural outputs, including food and biomass for energy production and fuels (Figure 9).

Emissions Sources in the U.S.

Emissions sources in the U.S. are also important contributors to smog in Canada. In general, primary PM2.5 and precursor emissions in the U.S. have decreased over the period of 1990–2006 and are projected to continue to decrease. U.S. emissions of primary PM2.5 have decreased by about 65% since 1990 through reductions achieved in most source sectors. NOX emissions have been reduced by 36%, primarily through reductions from transportation, electrical utility fuel and industrial fuel combustion sources. Much of the decrease can be attributed to the implementation in 1990 of the NOX Budget Trading Program in the northeastern U.S. SO2 emissions have decreased by approximately 50% from 1990–2006, primarily from electrical utility and industrial fuel combustion sources, which account for the majority of the emissions. VOC emissions have decreased by 34%, with the majority of the reductions from transportation and solvent sources.

Projections of U.S. emissions to 2015 also show a general decrease in gaseous precursor emissions in most regions of the country. In the northeastern states, there is a projected overall decrease in emissions densities in most of the areas, which would have a beneficial impact on ambient levels of smog and precursor gases in Canada.


15.  Hidy, G., Niemi, D. Pace, T., 2003. Chapter 4: Emission characterization, in: McMurry, P., M. Shepherd, J. Vickery (Eds.) Particulate Matter Science for Policy Makers: A NARSTO Assessment, Cambridge: Cambridge University Press. p. 147.