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Living WallA 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. (See a complete list of green wall benefits.)What's more, a recent modeling study shows that green walls can potentially reduce large amounts of air pollution in what's called a "street canyon," or the corridor between tall buildings.For the study, Thomas Pugh, a biogeochemist at the Karlsruhe 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 trapped or absorbed large amounts of nitrogen dioxide and particulate matter—both pollutants harmful to people, said Pugh. (Related: "Pictures: 10 Green-Tech City Solutions for Beating the Heat.")Compared with reducing emissions from cars, little attention has been focused on how to trap or take up more of the pollutants, added Pugh, whose study was published last year in the journal Environmental Science & Technology.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.—Christine Dell'Amore

Photograph by Bertrand Garbel, Hemis.fr/Getty Images