Urban Greening

Urban Greening

Strategically placed grass, ivy, and other greenery can significantly improve air quality in urban centers.


Greener city air. I’m Bob Hirshon and this is Science Update.

Making concrete jungles greener could reduce two major air pollutants eight times more than previously thought. This according to research at Lancaster University in England. Atmospheric scientist Thomas Pugh, now at the Karlsruhe Institute of Technology in Germany, says they looked specifically at “street canyons”—urban corridors surrounded by tall buildings.

And the way that these canyons work aerodynamically, is that they tend to trap the air inside them. So that the air inside the canyon doesn’t mix very well with the air above the canyon, in the free atmosphere, if you like.

That traps pollutants like nitrogen dioxide and airborne particles. But it also makes it easier for grass, ivy, or other green surfaces to soak them up. The study suggests that covering downtown walls and roofs with greenery could have a significant impact on pollution and public health. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

It's long been known that green plants can help offset air pollution. For example, plants' leaves absorb airborne nitrogen dioxide, a toxic chemical released by car engines and power plants. The surfaces of green plants also trap particulate matter—little bits of soot, dust, and other solids that pollute the air. 

The question is by how much. Some past studies, for example, found that trees could reduce urban air pollution levels by a few percent. According to Pugh, that's a significant effect from the perspective of cleaning the air, but not really enough to have an impact on public health problems caused by air pollution, like asthma and lung cancer. That's because pollutants tend to disperse over a wide area, including parts of the atmosphere that trees and other plants don't reach.

Pugh's work differs from past studies because it looks at a very specific type of urban environment, called a “street canyon.” If you walk downtown in any major city, you've seen streets surrounded on all sides by tall buildings. These can go on for blocks or even miles, and they generally have some of the worst air quality, because pollutants get trapped in the canyons rather than being carried off by winds to the upper atmosphere.

However, when it comes to cleaning the air, that problem can be turned into an advantage. Since pollutants get trapped in the canyons, green plants in those canyons would have a better opportunity to scrub them out. According to Pugh's model, the best strategy is to cover the walls and roofs of tall buildings with grass, ivy, or shrubs—a strategy already used in some large buildings to absorb rainwater, provide insulation that reduces heating and cooling costs, and offset the tendency of concrete, asphalt, and glass surfaces to heat up urban areas.

Pugh's calculations suggest that if the greening were done in the best possible way, nitrogen dioxide levels could be reduced by about 40 percent in street canyons, and particle pollution by up to 60 percent. That's eight times higher than estimates from past studies, which looked at the effect of plants on cities as a whole. The work suggests that by choosing the right strategies, a little green really could go a long way.

Now try and answer these questions:

  1. What are the two major air pollutants that were measured in this study?
  2. What are street canyons? Why do they trap these air pollutants?
  3. Why did the green plants in this study reduce air pollution in street canyons by such a large percentage?
You may want to check out the August 10, 2012, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: Why sex can be deadly for some animals, why you’re less like your siblings than you might think, silk-based vaccine preservatives, flushing out hidden HIV, and how urban canyons can filter air pollution.

To learn about surprising sources and consequences of air pollution, see the Science Updates Air Pollution & Pregnancy and School Bus Fumes.

Going Further

For Educators

In the Science NetLinks lesson Green Roof Design, students work collaboratively in small teams to design a heat- and water-conserving “green roof” of plant material for an urban apartment building. 

To learn about surprising sources and consequences of air pollution, see the Science Update lessons Air Pollution & Pregnancy and School Bus Fumes.

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