Many city dwellers have a special appreciation for a tree-lined street. The trees provide much-needed summertime shade to sidewalks and houses. Chirping birds make their homes in the branches. Dappled green light filters through the trees’ leaves. A new paper reports that properly arranged greenery can also cut air pollution on city streets by up to eight times more than was previously believed.
Thomas Pugh and colleagues explain that “urban canyons” formed by city buildings concentrate harmful pollutants. Burning fossil fuels in car engines and power plants produces pollutants — like nitrogen dioxide (NO2) and particulate matter (PM) – which may harm our lungs. The World Health Organization estimates that outdoor air pollution kills 1.3 million people each year. The authors report that levels of these two pollutants routinely reach unsafe levels on city streets and that the situation is only getting worse in many places.
Their Environmental Science & Technology paper describes three basic ways to reduce pollutant levels: curbing their emission, increasing their dispersion or getting them out of the air and onto something solid. The first two are tough in a city, where constant traffic is a given and tall buildings and right-angle intersections conspire to trap polluted air.
Both NO2 and PM will stick to hard surfaces like concrete and asphalt, but the authors say that plants do a better job of trapping pollutants, pointing to the “stickiness” of plant leaves, the way air moves around them and their large surface areas. That’s the key to their assertion that well-placed plants can reduce street-level concentrations of NO2 by as much as 40 percent and PM by 60 percent. The researchers say that more judicious placement of grasses, shrubs and trees could go much further than previous research suggested in making the air at street level cleaner.
The authors suggest that climbing plants like ivy, ground-covering grasses and shrubs and vertical gardens on the sides of buildings — which they liken to “green billboards” — might be the most effective way to combat pollution. They provide a lot of surface area for pollution deposition without blocking air movement.
On streets with light traffic, trees can still be effective because pollution is comparatively low. But where cabs, trucks, buses and cars are spewing out a lot of NO2 and PM, they caution against using trees, which can trap polluted air underneath their canopies. The authors even say that plants in urban spaces could make city air cleaner than in the surrounding areas.
Greening city streets has side benefits, too; the authors point out that more plants can lower temperatures, dampen loud noises and make a neighborhood more attractive.
What creative ways can you think of to bring more plants onto city streets?