Trees are great for air quality. They can intercept particulates on their surfaces or even take up pollutants though respiration. They can also positively (and negatively) affect how pollution from cars and trucks is dispersed. When a plant takes up a pollutant, the removal is permanent. However, when pollutants or particulate matter is deposited on plants, they may later be dispersed in the air from winds, or be deposited on the ground after being washed off by rain, or by dropping to the ground on leaves or other debris. “Vegetation type, height and thickness can all influence the extent of mixing and pollutant deposition experienced at the site. The built environment also matters greatly—air flow and impacts of trees are substantially different for a street canyon environment than an open highway environment,” writes Rich Baldauf in Recommendations for Constructing Roadside Vegetation Barriers to Improve Near-Road Air Quality.
- Higher, thicker vegetation will provide greater reduction in downwind pollution concentrations.
- “Porosity” of the vegetation to allow air movement through is important. Thick barriers force air up and over. Air moving through the vegetation will encourage particulate matter removal.
- Vegetation barriers must be maintained: gaps increase pollution downwind.
- Use multiple rows of vegetation, mixing types of shrubs and trees to provide coverage from the ground to canopy top. More pollution will be removed as plants mature.
- Having roadside vegetation perform double duty by also including bioswales to handle stormwater runoff can further reduce pollution and mitigate flooding.
- Make the barrier longer than the target protective area if it’s being planted for a specific purpose, such as to protect a school or church. Shoot for 150 ft. longer than the target area to protect. If that’s not feasible, wrap the protected area with the barrier.
In terms of plant material, evergreen shrubs and trees that will provide year-round protection are best.
- Leaf surface area is important too, as is leaf surface coverage—with waxy and hairy leaves removing more pollution compared to smooth leaves.
- Avoid species that emit VOCs (volatile organic compounds).
- Also avoid species that produce high-allergy pollens in areas where susceptible individuals may spend time.
- Select tough species that can handle brake dust, road salt, and emissions like NOx.
- When vegetation is used in conjunction with a noise barrier, plants should grow above the barrier by 3 ft. or more to enhance downwind pollution reduction.
- Finally, plants used should be traffic safe: Not attractive to wildlife that could be a road hazard, be planted in a way to maintain sightlines, etc.
Recommendations for Constructing Roadside Vegetation Barriers to Improve Near-Road Air Quality by Rich Baldauf. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-16/072, 2016.
April 3, 2017 NewTerrain.