Combination photo taken on Aug. 5, 2016 (upper) and Dec. 20, 2016 shows a sharp contrast of air quality in an ancient town of Taierzhuang, east China's Shandong Province. Heavy smog lingers in Beijing, Tianjin and provinces of Hebei, Henan and Shandong on Tuesday. Over 20 cities have issued red alerts for smog. (Xinhua/Gao Qimin)