The city of Xi’an, China has built an air purifier that stands 330 feet tall to clean the air of its pollution. Xi’an is home to 8.7 million people, is one of the oldest cities in China, and it has the worst air pollution in the whole country due to its coal burning heating systems. During Autumn and Winter, the residents need to wear air masks and use air cleaners to keep from breathing in the polluted air. The purifier operates by pulling the pollution into the base of the air purifier, filtered through warm greenhouses heated by solar energy, and then the warm air filters out through the top as clean air. It seems to be working because the tests show that the air is cleaner within a 4-mile radius of the tower. Because of these findings the researchers hope to build more and taller towers in Xi’an and more cities in China. The second tower that they would like to build in Xi’an would stand 984 feet tall. The first tower was built in 2016 at a cost of $2 million and has the name of Solar-assisted Large-Scale Cleaning System.