How effective are acoustic screening or tree planting alongside busy roads in reducing pollution at adjacent residential areas?

2010-03-01

 

The Highways Agency has previously undertaken research into the effectiveness of tree planting on reducing pollution levels. The work primarily addressed the effects of trees, whilst additionally looking at the physical effects of a barrier. The work concluded that shelterbelt trees have an influence on pollution levels through altering the dispersion of emissions by changing air-flow patterns, windspeed and surface roughness. Any obstacle that results in an increase in surface roughness will also enhance turbulence and so encourage atmospheric mixing of pollutants. The study concluded that these elements were more important in influencing the levels of pollution than that of general uptake of pollution through absorption.
There are considered to be two main effects of a barrier on pollutant concentrations:

The Highways Agency has previously undertaken research into the effectiveness of tree planting on reducing pollution levels. The work primarily addressed the effects of trees, whilst additionally looking at the physical effects of a barrier. The work concluded that shelterbelt trees have an influence on pollution levels through altering the dispersion of emissions by changing air-flow patterns, windspeed and surface roughness. Any obstacle that results in an increase in surface roughness will also enhance turbulence and so encourage atmospheric mixing of pollutants. The study concluded that these elements were more important in influencing the levels of pollution than that of general uptake of pollution through absorption.

There are considered to be two main effects of a barrier on pollutant concentrations:

  1. The barrier can increase the residence time of the pollution above the road and this effect is more noticeable if the road is already in a cutting. This containment effect allows vertical mixing to occur within the 'plume' diluting the pollutant with clean air from above the road.
  2. There is also a vortex effect downwind of the barrier. If the wind is blowing from the road towards the barrier there will be an area downwind of the barrier where concentrations are lower as the polluted air from the road will tend not to reach this area. If the wind is blowing from the barrier towards the road, the vortex will form over the road and the polluted air will be partially contained over the road, which will aid vertical mixing. The horizontal length of the vortex will depend on the wind speed but it will normally extend 1.5 times the barrier height downwind (normally 5 – 10 m). As well as this more marked decrease very close to the barrier a slight decrease in concentrations in the 20 – 50 m range was also shown.

The precise benefits of a barrier are therefore dependent upon the orientation of the barrier to the prevailing wind direction and the proximity of residential properties to the barrier in the downwind location. It is unlikely that barriers as stand-alone measure will lead to the achievement of air quality objectives within itself, although this is entirely dependent upon the extent of exceedence.

 

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