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.
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.
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.
“An integrated policy which separates people spatially from major pollution sources (especially traffic) as far as possible and in which vegetation is used between the sources and the urban population maximises its beneficial effects.” Defra report: Effects of vegetation on urban air quality.
In new development:
Lay new streets and neighbourhoods out based on the wind direction and possible sources of air pollution.
In existing development:
Adjust existing green structures to avoid the green tunnel effect and encourage the flow of polluted air away from people and buildings.
Green barriers along highways are less effective for the reduction of noise than artificial barriers. However, when used in combination, they can help reduce air pollution and noise and prevent graffiti.
Sketch: Luchtgroen Genk: BELW Advies bv
Flow of air behind a barrier
Source: BELW Advies bv
A vegetation barrier works best if the wind hits it at a 90˚angle (taking into account the prevailing wind direction). On the top of the barrier, the turbulence in the air mixes the polluted air with the passing cleaner air. The polluted air is forced upwards, where the concentrations will lower because of mixing with cleaner air. The air stream will flow back down to ground level after approximately 10-15x the height of the barrier.