Lanzhou, a transportation hub along the Silk Road Economic Belt, was once dubbed "a city invisible from satellite images" due to its severe air pollution. For locals, blue sky was a true luxury. However, over recent years the city has experienced a dramatic change, turning itself into a city famous for "Lanzhou Blue".

　　What is the secret behind Lanzhou's dramatic shift? How did the city win the war against air pollution? Check out in the video.