A new research study by Professor Thomas Pugh at Lancaster University and other scientists in the UK has found that adding trees, bushes, innovative systems like green walls, or even ivy or other creeping vines, can cut street-level nitrogen dioxide (NO2) and microscopic particulate matter (PM), two of the worst forms of pollution, by eight times more than previously thought. Many urban streets have high levels of these types of pollution, far exceeding healthy amounts for humans.
According to Science Daily, previous research has said trees and other greenery can only improve urban air quality by around 5 percent. ASLA’s recent animation on the positive impact of urban forests on air pollution used U.S. Environmental Protection Agency and National Park Service-cited results, which pointed to around 10-13 percent improvement in air quality from major increases in urban greenery.
The new study focused on better understanding the effects of adding greenery in the stagnant corridors of cities, what the authors termed “urban street canyons.” Science Daily says the team concluded that “judicious placement of grass, climbing ivy, and other plants in urban canyons can reduce the concentration at street level of NO2 by as much as 40 percent and PM by 60 percent, much more than previously believed.”
Green walls in particular could be used to further increase the amount of pollutant-absorbing foliage available in these spaces. Co-author Rob MacKenzie from the University of Birmingham told BBC News: “The benefit of green walls is that they clean up the air coming into and staying in the street canyon. Planting more [green walls] in a strategic way could be a relatively easy way to take control of our local pollution problems.”
According to the team, researchers found trees were also effective, but “only if care is taken to avoid trapping pollutants beneath their crowns.”
To ensure these systems work, Pugh also mentioned to the BBC that “more care needs to be taken as to how and where we plant vegetation.” Street trees, which have up to a 30 percent mortality rate in big cities like New York City, need better treatment if they are going to work effectively as filters. In the same way, green walls need to be protected from heat stress and a lack of water.
Beyond these issues, others in the UK debated the value of green walls for addressing air pollution on a cost basis. Given the higher maintenance costs associated with elaborate installed green wall systems, the Trees and Design Action Group wondered whether ivy or other creeping vines weren’t easier and cheaper to use. We’d just add that any comparison between green walls and ivy should also examine the stormwater management, urban heat island effect, and noise reduction benefits of both approaches, too.
Image credit: ASLA 2011 Professional General Design Award. Adding Green to Urban Design / City of Chicago and Hitchcock Design Group