Opt for green barriers

That's how you do it

Moving vehicles stir up the larger dust particles. Green barriers in the city limit the exposure of cyclists and pedestrians to these dust particles. Along highways, green barriers lower the concentrations of roadside pollutants. A vegetation screen can reduce concentrations near the road by capturing the pollutants or by changing the airflow. The influence of a green strip does depend on the wind direction, the type of vegetation, the porosity and also the height and width of the vegetation.
The results in experiments and model studies are not unequivocal. In general, it is believed that concentrations at the ground and just behind the barrier are lower due to simple obstruction and/or the increase in vertical air mixing. But vegetation can also reduce ventilation. If you erect a green barrier carelessly, the disadvantages even outweigh the advantages. Therefore, a purposeful, well-thought-out design is needed for every new situation.
Planting vegetation has many additional advantages, especially in cities. Below we give you some tips.

• Does local air pollution worry you? See whether a green barrier is suitable for tackling this pollution. What is the concentration of different pollutants according to measurements and/or model maps? Is there a local source of pollution? A green barrier only makes sense if it shields the environment from a local source of pollution, so use it thoughtfully.
• Can you build the barrier without 'locking in' the sky? The barrier must not stop the air circulation: this increases the concentrations in places where people live. Think, for example, of a narrow street with terraced houses where there are tall trees with a dense canopy of leaves.
• Use specialized tools and preferably involve experts to ensure a meaningful implementation. For example, the city of Antwerp developed a Green Tool together with VITO. The area analysis only works on the territory of the city, but the other functionalities will surely guide you through the possibilities and points of interest of greenery in an urban environment.

Green barriers offer even more benefits:

• Hedges increase traffic safety on footpaths or bicycle paths along the road.
• The vegetation has a cooling effect through evaporation. This limits the heat island effect.
• Green barriers enrich urban biodiversity.
• Vegetation filters and slows down the discharge of polluted precipitation from the highways.
• Green barriers muffle sound.

And don't forget: a green environment means less stress!

Tips

• Trees and hedges along busy arterial roads often lead to higher concentrations of harmful substances in the air. The air then circulates less and is therefore less diluted. The filtering effect of the vegetation does not outweigh this.
• It is advisable not to place trees along busy roads and in narrow streets where the air quality is around the standard. If you do, choose to plant as few trees as possible, with a large mutual distance and an open crown structure that does not cover the roadways. Trees in the median strip are better than trees on either side of the road.
• The air quality just behind hedges is hardly better than before. In general, the influence of hedges on air quality is more negative than positive. We therefore recommend low hedges in inner-city areas.
• Placing closed screens between the carriageway and the bicycle and pedestrian paths does have a positive effect on the air quality behind the screens. The closer to the roadway and the higher the screen, the greater the effect. The effect of vegetation on the screens is negligible.
• Line-shaped, continuous and dense green elements along provincial and highways are often successful in protecting cyclists, pedestrians and local residents against air pollution.
• In the vicinity of industrial sources where 'dust sensitive processes' take place (in other words where there is emission at low altitude), green elements limit the spread to the nearby environment. A completely closed green element often offers the greatest effect, for example trees with low undergrowth.
• On livestock farms, it is a good idea to place linear green elements one after the other to improve air quality.
• Vegetation improves air quality only minimally. In streets with little traffic and therefore little air pollution, decent air quality is therefore not an argument for not installing trees.
  

Knowing more

Read the experimental study in which scientists measured the concentrations of NO _x and PM ₁₀ along a highway with a green barrier and along a highway without a green barrier. The results were inconclusive for PM ₁₀ . With regard to _NOx , the researchers observed higher concentrations just behind the green barrier, and lower concentrations 50 meters behind the barrier.

More information

• Clean Air "Green and air quality in the city" (Interreg Flanders-Netherlands)
• Each tree species has a different potential capacity to improve air quality . (Environmental Science & Technology)
• Improving local air quality in cities: To tree or not to tree? (Environmental Pollution)
• The effect of vegetation on air quality (RIVM, Netherlands)