Predicting Air Quality at Street Level

A State-of-Science Review

April 8, 2008
Prepared for the Environmental Commissioner of Ontario
by RWDI Consulting Engineers and Scientists


Poor air quality can cause a number of adverse health effects in humans. As summarized by Toronto Public Health (TPH, 2004), short-term exposure to pollutants commonly found in urban air can cause increases in respiratory symptoms, infections, emergency room and hospital admissions, and even premature death in some cases. Long-term exposure to air pollution can cause chronic health diseases, reduce life expectancy, and increase the risk of lung cancer (TPH, 2004; Pope, 2004; and Halton Region, 2007).

For many years, both the Canadian Federal and Ontario Provincial governments have had programs in place to advise the public of air pollution episodes. These programs make use of both air quality measurements and computer-generated forecasts. For cost reasons, air quality measurement networks tend to be limited to a small number of monitoring stations in any urban area, all located away from major local emission sources, in places such as city parks. Measurements from these sites give information on the regional average exposure to air pollutants. Similarly, computer-generated forecasts are typically resolved at horizontal scales of several kilometres, because of limitations in computing power, and only provide information on regional average exposure.

These measurement and forecast systems have proven useful to advise the public about large scale smog events, but do not deal with public exposure to pollutants at a local or street level. Large numbers of people in urban centres are exposed, at least for parts of their day, to air pollution at street level where vehicle emissions may be trapped in the canyon created by large buildings on either side. As such, the exposure to pollutants at street level is typically very different from the regional average exposure measured at monitoring stations and predicted by computer models.

Due to the human, environmental and economic costs associated with poor air quality, the Environment Commissioner of Ontario (ECO) is interested in knowing how the current air quality forecasting programs in Ontario and the rest of Canada compare to those of other jurisdictions and to the state-of-the-science, in terms of representing the true air pollution exposure of urban populations. This information will allow the ECO to assess whether these programs are adequate for informing the public concerning exposure to poor air quality in Ontario.

To assist with this initiative, the ECO retained RWDI AIR Inc. to perform the following tasks.

  • Review and summarize current air quality forecasting and ambient monitoring initiatives being implemented by the Ontario Ministry of the Environment and Environment Canada. Also, research and briefly summarize operational air quality forecasting and ambient monitoring initiatives used in other national and international jurisdictions.
  • Review and summarize the current state-of-the-science for systems designed to predict air pollutant concentrations at street level, to gain an appreciation for what is currently possible and where the science is headed in the future.
  • Prepare a “Lay Language” report (this report). Although the report contains some technical content, the focus is on presenting the information in brief and general terms.
  • At a workshop with the ECO, present the findings from this study and explore the approaches being used in Ontario (and Canada as a whole) in light of operational modelling and monitoring initiatives in other jurisdictions and the current state-of-the- science. Incorporate into a final report the key issues discussed during the workshop and a list of recommendations for “next steps”.
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