City trees and air pollution

First speed bumps, now trees — can you explain to me why trees in city streets have been linked to higher levels of air pollution? Does this have any implications for green walls on buildings?

The controversial findings regarding speed bumps and urban trees come from the new draft guidance published on 1 December 2016 by the National Institute for Health and Care Excellence (NICE). NICE states that: “Leaves and branches slow air currents, causing pollutants to settle. They may also act as sinks for particulates and chemicals that may have direct or indirect effects in air quality. Air quality [under trees] may deteriorate at street level near vehicles.”

However, the guidance hastens to add that local authorities should not start cutting down all their avenues of trees. Another report, published in the Journal of Environmental Science and Technology, found that strategic placement of vegetation in street canyons could reduce air pollution by 30%. And a study by the US Nature Conservancy reported that the average reduction of particulate matter near a tree was between 7% and 24%. It is only under certain circumstances that trees can worsen the air quality for pedestrians: in streets with large volumes of traffic, the leaf canopy can trap pollutants, but if trees are appropriately spaced this problem can be avoided. One recommendation is to plant a smaller number of large trees rather than lots of small ones. On the majority of streets, where traffic volumes are lower, the problem of trapped pollutants does not occur.

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