Green Walls May Cut Pollution in Cities

Green Walls May Cut Pollution in Cities

Living Wall A living wall bursts with vegetation at Paris's Musée du Quai Branly—a type of green wall that's catching on in some big cities. These vegetated surfaces don't just look pretty. They have other benefits as well, including cooling city blocks, reducing loud noises, and improving a building's energy efficiency. What's more, a recent modeling study shows that green walls can potentially reduce large amounts of air pollution in the corridor between tall buildings. For the study, Thomas Pugh, a chemist at the Institute of Technology in Germany, and his colleagues created a computer model of a green wall with generic vegetation in a Western European city. Then they recorded chemical reactions based on a variety of factors, such as wind speed and building placement. The simulation revealed a clear pattern: A green wall in a street canyon absorbed large amounts of nitrogen dioxide and particulate matter— both pollutants harmful to people, said Pugh. That's why the green-wall study is "putting forward an alternative solution that might allow [governments] to improve air quality in these problem hot spots, " he said.

Vertical Garden Living walls can be indoors or outdoors, such as a blooming one on the Citi Data Centre in Frankfurt, Germany. "Vertical gardens, " as they're sometimes called, can visually enhance an otherwise forgotten space, noted Paul Erlichman of the Toronto-based group Green Roofs for Healthy Cities. There are three types of green walls: Living walls, green facades, and retaining living walls, he said. In green facades, climbing plants grow into supporting structures, according to the Green Roofs for Healthy Cities website. Living walls are composed of pre-vegetated panels, planted blankets, or bags that are affixed to a structural wall or freestanding frame. Finally, retaining living walls are engineered living structures designed to stabilize a slope.

Going Green In Madrid, plants turn a wall in the Paseo del Prada bright green. Green walls are particularly useful in reducing pollution in street canyons, which are "a particular kind of oddity in that they tend to trap air within them, while over the city as a whole the air circulates freely, " Thomas Pugh noted. Because a green wall's vegetation is working with a much smaller volume of air in a street canyon, he added, it's more effective at absorbing the pollutants than if it was tackling a whole mass of air above the city. However, Pugh cautioned, "we need more research to understand just how effective green walls can really be in real locations. "

Colorful Tableau A green wall creates a colorful tableau in Royal Square. London and some cities in Western European countries in particular have had trouble keeping air pollution in check— making green walls a possible solution.

Against the Wall The ability of green walls, such as this one at the Musée du Quai Branly in Paris, to cool street canyons is especially key in the summertime, Thomas Pugh noted. "If you have a surface covered in plants, " he said, "when the sun's heat hits them, instead of just warming up they send a lot of energy back into the atmosphere as water vapor. Whereas a concrete wall would heat up and send [the heat] back into the canyon. "

Green Gold A vertical garden won the gold medal in the 2011 Chelsea Flower Show in London. Each year Green Roofs for Healthy Cities issues the Green Roof & Wall Awards of Excellence. The 2013 winners will be announced in October at Cities Alive: 11 th Annual Green Roof and Wall Conference in San Francisco. According to the organization's website, green walls may improve your mood as well as your health. "Studies have shown that visual access to natural settings leads to increased job satisfaction and productivity … and post-operative recovery rates in medical facilities, " the website says.

How Green Walls Work Many living wall systems also have built-in irrigation systems in which water is harvested at the bottom of the wall and recirculated back to the top, as it seen above. The types of plants on green walls differ, but can include grasses, vines, and other climbing plants—all of which must have strong root structures, Green Roofs for Healthy Cities' Paul Erlichman said.