A worker walks in the gas-fired boiler plant of a heat source factory in Xi'an, capital of northwest China's Shaanxi Province, Dec. 14, 2017. The factory eliminated two coal-fired boilers and upgraded three gas-fired boilers this year. Air pollution has been notably reduced in Xi'an, especially during the winter months this year. The government has taken a series of measures to fight air pollution: replacing coal with clean energy, limiting cars, reducing dust, etc.. Levels of PM10, PM2.5 and SO2 were down 33 percent, 36 percent and 17 percent respectively year-on-year from the beginning of November. (Xinhua/Shao Rui)