Walls blanketed in moss (苔鲜) are popping up in major cities, along with promises that they can reduce air pollution 一 but can a few square metres of plant matter really deal with the smog?
A Berlin-based firm, Green City Solutions, believes so. Its moss walls, called the CityTree, are roughly 4 square metres in size. Armed with Wi-Fi sensors to monitor the health of moss, a City Tree functions autonomously and requires very little maintenance(维护) . The wall collects rainwater, which is pumped through a built-in irrigation system to the plants, powered by solar energy. As a result, the fi 皿 says each CityTree is able to "eat" around 250 grams of particulate (颗粒) matter a day (nearly 90 kgs a year) and removes about 240 metric tons of CO2 annually. It also cools the surrounding air.
Aware that getting the surrounding air in contact with the moss wall is crucial for the CityTree to be effective, the inventors ensure that the location of each installation (安装) is chosen carefully s pots where pollution is heavy due to traffic and where air flow is limited are picked. The importance of this step is explained by the fact that the waste gas from a car generally goes vertically a few kilometres into the air.
But this doesn't mean moss walls will necessarily protect people from pollution. In the Netherlands, researchers found that eight walls installed in Amsterdam failed to reduce the concentration of particulate matter (PM) and nitrogen dioxide (N02). Their report concluded that even doubling the number of moss walls would do little to improve their effectiveness.
The CityTree is not meant for parks or to substitute for street trees, but to add greenery to concrete-heavy spaces where planting is not an option. It's important to remember that street trees provide a whole host of other benefits, including shelter and habitat for urban wildlife, shade and cooling for people on the street, and reduction of urban heat islands.
