Street Canyon Effect — a useful model?

I mentioned briefly about the Street Canyon Effect in my previous posts, and I thought that I should be elaborating on what it is about in more detail.

In summary, the street canyon model is a model of air pollution on the streets, flanked by columns of high rise buildings (what we increasingly see in urban areas). Important geometrical information for this model includes the aspect ratio (aspect ratio = height of buildings/width of street), which is used to classify the street canyons into different categories.

Because the street canyon affects the temperature, wind speed, and wind direction within the canyon, it consequently affects the air quality within the street canyon. For instance, through fluid mechanics, researchers found that street level pollution through contributions by vehicle exhaust is higher at the leeward side as compared to the windward side. Neighbouring street canyons may also affect the type of wind flow and hence air quality.

Why is this important?

  1. Implications of Traffic-related Air Pollution (TRAP) — small particulate pollutants such as PM2.5 and other traffic pollutants are harmful to health. In fact, the Global Burden of Disease 2010 estimated that 3.1 million deaths are due to exposure to ambient particulate matter.
  2. Increasing urbanisation trend in many regions around the world meant that more people are exposed to heavy traffic associated with the urban environment.

Researchers have applied this model to many different cities. One example is the research by Zhou and Levy (2008) on the impact of street canyon effects on population exposure to traffic pollutants in mid-town Manhattan, New York City, using the Operational Street Pollution Model (mentioned 2 posts previously!)  which is based on the Street Canyon Model. Their research found that there is a pressing need to control mobile emission sources on the streets to reduce population exposure to traffic air pollutants.

Also, Hong Kong is notorious for its street canyons; perhaps the air purifier (that I posted about yesterday) will definitely be a step to targeting the city’s pollution issues.

All in all, there are practical usage of such models, especially since it is hard to obtain data from every single region. However, we must always be aware of any potential limitations — for instance, the exclusion of certain air pollutants, simplification of reality to models (because that will happen to make models work!).


Cheung, C. and Kao, E. (2015). Scientists examine the health risks of Hong Kong’s notorious “Street Canyons”. South China Morning Post. [Online] Retrieved from: [last accessed: 9 April 2015]

Wang, A. and Ho, B. (2013). Characterising Urban Street Canyons in Downtown Vancouver. [Online] Retrieved from: [last accessed: 9 April 2015]

Zhou, Y. and Levy, J. I. (2008). The Impact of Urban Street Canyons on population exposure to traffic-related primary pollutants. Atmospheric Environment 42(13): 3087-3098.


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