Canadian Smog Science Assessment Highlights and Key Messages: chapter 6

Factors Influencing Levels of Smog Across Canada

• Meteorology
• Coastal and Urban Effects
• Local Chemical Mixture
• Natural Emissions of Precursors
• Transboundary Transport

One of the difficulties in evaluating the impact of emission reductions on air quality is the complexity of the numerous chemical and physical processes that lead to the observed ambient levels. Aside from local emissions of precursors, smog levels are influenced by meteorology, coastal and urban effects, the local chemical mixture, natural emissions of precursors, and transboundary transport. Such factors, which vary regionally across the country, explain much of the variation seen in ambient levels of fine particulate matter (PM_2.5) and ozone (O₃). These are important to understand in order to determine the causes of and the main emission sources contributing to elevated pollutant concentrations in specific areas. Improvements in air quality models also depend upon gaining a better understanding of the factors influencing concentrations.

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Meteorology

Once air pollutants have been introduced into the atmosphere their transport, transformation and deposition are largely controlled by meteorology. In the summer, there is a greater frequency of air stagnation periods along with more intense sunlight. This meteorological situation leads to the build up of local emissions and greater formation of secondary pollutants such as O₃ and secondary organic aerosol, the latter of which can include more toxic particulate species. Warmer temperatures and higher humidity can also increase the demand on electricity generation (e.g., for cooling) and enhance natural emissions of volatile organic compounds (VOC). Summer smog episodes in urban locations, especially in Ontario and Quebec, are characterized by these conditions and O₃ is most enhanced downwind of the high emission areas. The occurrence of winter stagnation episodes which are associated with even less vertical mixing and high pollution (especially PM_2.5) levels have increasingly become of interest. Several of the highest PM_2.5 levels over Ontario and Quebec have occurred during these winter stagnation periods, including the highest event during the past 10 years. 

In the Prairies, the wide open spaces and relatively windy conditions keep concentrations of pollutants low despite areas of high emissions. In the winter time, higher PM_2.5 concentration events are generally linked to periods of temperature inversions (colder air below and warmer air above) that trap and concentrate pollutants close to the ground. The latter is often the case in the interior valleys of British Columbia, the Rocky Mountains, and in the Quebec valleys. Furthermore, during cold and calm periods in the winter in the Prairies, urban and industrial areas experience some of the highest ground-level concentrations of primary pollutants in Canada.

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Coastal and Urban Effects

Along coastal areas, the land-water temperature contrasts have an important influence on ambient levels of PM_2.5 and O₃. In southern Ontario, the Great Lakes influence local pollution levels by restricting dispersion and deposition of primary and secondary pollutants over the water. Lake breezes then move the accumulated pollutants onshore, even to more distant coastal locations. In Atlantic Canada, the cool ocean waters can lead to air temperature inversions, particularly in spring and summer. This slows down dispersion of onshore air masses, in turn increasing the impact of local emissions on ambient smog levels in coastal cities. In this region the high frequency of coastal fog has an added influence on the formation of PM and the deposition rates as fog moves inland. Along the coast of British Columbia, land-sea wind circulation patterns, along with restricted air mass movement because of the mountainous topography, can lead to air stagnation in the summer and elevated PM_2.5 and O₃ levels. The levels can increase as the air propagates inland up the Lower Fraser Valley due to the valley becoming narrower and the increased time for photochemical formation of secondary pollutants. Pollutant transport in urban environments is subject to the density, orientation and geometry of buildings in addition to wind speed and direction and atmospheric stability.

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Local Chemical Mixture

The local atmospheric chemical mixture plays a role in the formation of O₃ and can contribute to secondary PM_2.5 and thus is critical to understand in order to explain spatial and temporal variations. In urban centres, where there are high nitrogen oxides (NO_X) emissions, O₃ levels are suppressed because O₃ reacts with nitric oxide (NO) (referred to as NO titration). On a temporal scale, fewer people go to work on weekends and the number of heavy duty diesel vehicles is substantially less, therefore O₃ concentrations exhibit differences between weekends and weekdays. In many instances, O₃ formation in urban areas is VOC-limited, thus the rate of O₃ production is dependent primarily on ambient VOC concentrations. In these areas, reducing NO_X levels alone will not be as effective at reducing local ambient O₃ concentrations. Furthermore, reductions in NO_X emissions that have been occurring in Canadian cities have resulted in increases in O₃ concentrations since less of the regional O₃ that is transported into the city is removed by titration. Weighing this downside with the benefit of the lower NO_X concentrations and the other pollutants that might be affected in the urban air pollutant mixture represents an ongoing challenge to air quality management.

Interactions between ammonia (NH₃) and the reaction products of primary NO_X or sulphur dioxide (SO₂) emissions (i.e., nitrate and sulphate) affect the formation and/or composition of secondary PM_2.5. PM_2.5 levels can respond non-linearly or negatively (PM_2.5 increase with sulphate decrease) to changes in sulphate and/or NH₃ particularly in winter and depending on the local chemical regime. The potential for this PM_2.5 non-linear or negative response was estimated to be highest in winter over southern Ontario. Examination of the monitoring data suggests this possible drawback to SO₂ emissions reductions, but more research and data are needed. For example, in rural and remote sites in southern Quebec and Ontario, SO₂ and NO_X emissions remained fairly stable from 1995 to 1999 following a decrease in the early 1990s. However, ambient particle nitrate increased from 1995 peaking between 1998 and 2001 while particle sulphate saw a continual decrease over that same time period.

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Natural Emissions of Precursors

Natural sources of gaseous precursors, including wild fires, biomass burning and sea salt are important in many regions. In the summer, the sea salt (NaCl) component of PM_2.5 is significantly higher at sites in Atlantic Canada (Halifax) and the Lower Fraser Valley (Abbotsford and Burnaby) than at other sites shown on Figure 2 due to their proximity to the coast. Forest fires can be a significant contributor to ambient PM_2.5 levels. For example, it was estimated that the 2002 Quebec fires were responsible for releasing an amount equivalent to 50% of the total annual Canadian anthropogenic PM_2.5 emissions, and in British Columbia, many PM_2.5 episodes can be traced to high emissions from forest fires. Natural emissions of VOCs are important in the formation of secondary pollutants in the warm months. The overall contribution of these emissions to secondary PM_2.5 is just beginning to be characterized.

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Transboundary Transport

Transboundary transport is an important source of pollution in many regions of Canada, especially Atlantic Canada, Quebec and southern Ontario. Along with high local emissions, this results in the higher-on-average PM_2.5 and O₃ levels and peak episodes seen in these areas. Weather systems typically move from west to east, carrying pollutants from the Midwest U.S., southern Ontario and the U.S. eastern seaboard to Quebec and Atlantic Canada. In the Prairies and on the west coast, while air from the south typically brings some higher levels of PM_2.5 and O₃ into the region, transboundary transport is less of an important contributor in comparison to eastern Canada. High elevation sites (above 2000 metres in altitude) have often been associated with long-range flows of air pollutants. The transport of particles and gases from other continents also influences smog levels in Canada, as is discussed in more detail in the “Emerging Issues” section of this report. 

¹⁴.  Program of Energy Research and Development (PERD) (http://www.nrcan.gc.ca/eneene/science/perdprde-eng.phphttp://www.nrcan.gc.ca/eneene/science/perdprde-eng.php), Spring 2009 Cycle Update, PERD Clean Transportation Systems Portfolio Program 2.1.1.