Green streets

Image credit: BELW Advies bv

Use green to drive the polluted air upwards

Air pollution from local sources can be reduced by diluting polluted air with cleaner air. Encourage the mixture of local high concentrations of air pollution with cleaner air to prevent local hot spots. Air filtration alone cannot provide sufficient reduction. By creating a barrier effect, the air is driven upwards where it can mix with cleaner air in higher air layers. A continuous, closed and linear barrier is most effective. The ability of the air to return to background concentrations of air pollution when it reaches ground level depends on conditions such as weather, barrier type, barrier height and the local air pollution concentration. It is important to provide enough ventilation behind the barrier.

Prevent the green tunnel effect

It is important that the green solution does not increase the concentration of pollution locally. Tree canopies which form a tunnel (along narrow streets) have a negative effect on the flow of fresh air into the street and the flow of polluted air away from the street. The tunnel effect can be prevented by eliminating the obstruction, for instance by choosing a row of trees on only one side of the street or creating green façades on adjacent buildings.

The size and spacing of the green structure are important in forming an effective barrier

A dense leaf structure, smooth regular habit and compact form create a solid barrier and keep turbulence down. The porosity should be as low as possible, preferably less than 30%, when used as a wind barrier. Create a closed (leaf) surface for optimal effect: horizontally (choose a planting distance which is smaller than the tree crown or plant width so there are not gaps between the plants) and vertically (use a combination of trees and shrubs for effect at various heights in the vegetation structure). Evergreen plants have a better effect than deciduous plants.

Low hedges or green screens between the street and pedestrian zone act as a barrier to reduce the amount of gaseous pollutants and particulate matter emitted by cars near ground level that reach the pavement.

Dilute the source concentrations of air pollution with higher layers of cleaner air.