China’s latest attempt to mitigate some of its air pollution is an experimental air purification tower standing 100 meters high in the city of Xian. In preliminary tests, researchers found that the giant tower can improve air quality over an area some 10 square kilometres in size. The smog tower isn’t a power hog, either, thanks to the design that uses large greenhouses at the base of it to heat incoming air with solar energy so that it rises passively through the tower’s multiple filters before exiting the top much cleaner than before.