People walk past two LED screens on the Tiananmen Square in a dense fog in Beijing on Dec 5, 2011 . [Photo/CFP]

This is a photo of China's capital taken from the exact same spot over four days. The combination image, taken from Dec 2 to 5, captures staggering changes in the air quality in Beijing. The photos show the visibility in the area ranging from Central Business District in Chaoyang District to Communication University of China. A photographer consecutively took photos at this same place for two weeks from Nov 22 to Dec 5 to record the "fog" in this area. The recent frequent "fog" blanketing Beijing has spurred the public to call for a revamp of the air pollution regulations in China, in particular the decision to monitor levels of ultra-fine particles known as particulate matter 2.5 (PM2.5) which is deemed by some experts as the major cause of the choking smog. [Photo/CFP]