At this month's Beijing Winter Olympics, the air is clear enough for athletes to see from downtown the mountains surrounding the city, the Associated Press (AP) has reported.

China launched an ambitious plan to improve its air quality in 2013 and said it would fight pollution "with an iron fist."

Tougher emissions standards were imposed on coal-fired plants and the number of cars on the road was curbed to cut vehicle emissions.

Guojun He, a researcher at the University of Hong Kong and co-author of a recent report from the Chicago-based Energy Policy Institute, said China has made significant progress in curbing emissions and is rapidly developing clean energy from sources like wind and solar.