Adding greenery to city sidewalks can eliminate about half of at least two types of pollution from the air, especially on streets that snake through rows of tall buildings.
Just by strategically planting the right kinds of shrubbery in the right places, the new discovery suggests, we may be able to clean urban air and reduce related threats to human health.
“There’s a potential application here for simply reducing air pollution concentrations that hasn’t really been considered before,” said Thomas Pugh, an atmospheric scientist now at the Karlsruhe Institute of Technology in Germany. “It’s been considered that you could use vegetation, but not to this level of effectiveness.”
For at least 20 years, there has been great interest in understanding how trees might absorb air pollution in urban areas, Pugh said. But most studies have considered cities as a whole, from end to end and bottom to top, including air up to hundreds of meters high, where human health effects become less relevant.
Pugh, who conducted the new study while at Lancaster University in the United Kingdom, decided to zero in on street canyons – city streets that are bounded on both sides by continuous rows of tall buildings.
Previous studies have shown that street canyons can trap air, allowing pollutants to accumulate when traffic is heavy and the street’s geometry and weather conditions are just right.
To get a street-level sense of how air pollution varies from one avenue to another, Pugh and colleagues created a computer model of a basic street canyon that could take into account wind speed, vegetation coverage and other simple variables.
The study focused on two types of air pollution: nitrogen dioxide and particulate matter measuring less than 10 micrometers (0.0004 inches) in diameter. Both pollutants are prevalent in cities around the world. And both have been linked to diseases of the heart and lungs.
At low wind speeds, the simple addition of shrubbery to the walls of city streets reduced street-level concentrations of nitrogen dioxide by as much as 40 percent and of particulate matter by as much as 60 percent, the researchers reported in the journal Environmental Science & Technology.
Trees are less useful, the model suggested, because they can actually trap pollution beneath canopies.
“What we think is particularly interesting about this work is that we can use it to target particular streets,” Pugh said. “So, if you have a particular canyon where you have very high pollution levels, maybe you can target that canyon on its own and get it below pollution standards.”
While still theoretical and preliminary, the new findings challenge traditional thinking, said Jonathan Foley, director of the Institute on the Environment at the University of Minnesota, Twin Cities. Scientists have long believed that vegetation could only remove small amounts of pollutants from city air. But the potential for using greenery to clean city air may be unexpectedly large.
This “gives us another tool to fight urban air pollution -- a major cause of illness and premature death around the world,” Foley said. “That's great news, especially since urban plantings can also provide many other benefits -- by reducing the urban heat island effect, reducing noise pollution, providing habitats for birds and wildlife, plus providing aesthetic benefits. Win-win-win.”