Combination photo taken on Feb. 20, 2014 (upper) and Feb. 19, 2014 (lower) shows a residential community in the Xicheng District of Beijing, capital of China. The first-ever yellow alert for air pollution in the next three days has been issued on Thursday in Beijing. The city has a four-tier alert system, using blue, yellow, orange and red to indicate the air pollution level in order of increasing severity. [Xinhua]