学进去

Air pollution is one of the world’s invisible killers. It causes seven million deaths a year, making it the largest single environmental health risk. In cities, air pollution is particularly serious. More than 80% of people live in the areas where pollution levels are beyond WHO limits.

One well-established way to reduce air pollution is to plant trees, as their leaves catch and absorb harmful particulates (微粒). But planting new trees is not always a good choice for lack of enough space. That’s why the “City Tree”, a mobile tree which removes pollutants from the air, has been becoming popular in cities around the world, including Oslo, Paris, Brussels and Hong Kong.

Each City Tree is just under 4 meters tall, nearly 3 meters wide and 2.19 meters deep, acting as a chair or a bench. A screen is included for information or advertising. Berlin-based Green City Organization explains that its invention has the environmental benefit of up to 275 actual trees.

But the City Tree isn’t, in fact, a tree at all—it’s a moss culture (青苔培植). “Moss cultures have a much larger leaf surface area than any other plant. That means we can catch more pollutants,” said Zhengliang Wu, co-founder of Green City Organization.

The huge surfaces of moss in each tree can remove dust, NO₂ and O₃ gases from the air. It requires very little care: solar panels provide electricity, while rainwater is collected into a container and then sent into the soil.

To monitor the health of the moss, the City Tree has sensors which measure soil humidity (湿度), temperature and water quality. “We also have pollution sensors inside the trees, which help monitor the local air quality and tell us how efficient the tree is,” Wu said. Its creators say that each City Tree is able to absorb around 250 grams of particulate matter a day and contributes to removing 480 tons of CO₂ a year.