Photo taken on Jan. 22, 2013 shows the frost on the trees in north China's Tianjin Municipality. Dense fog hit Tianjin Tuesday. (Xinhua/Zhang Chenlin)

A man goes out with his pet dog in the fog in north China's Tianjin Municipality, Jan. 22, 2013. Dense fog hit Tianjin Tuesday. (Xinhua/Zhang Jindao)

Cars run in the fog in Tianjin, north China, Jan. 22, 2013. Dense fog hit Tianjin Tuesday. (Xinhua/Liu Dongyue)

Photo taken on Jan. 22, 2013 shows a fog-shrouded building in north China's Tianjin Municipality. Dense fog hit Tianjin Tuesday. (Xinhua/Zhang Chenlin)

Cars run on a road in fog in Tianjin, north China, Jan. 22, 2013. Dense fog hit Tianjin Tuesday. (Xinhua/Zhang Chenlin)

Citizens walk in fog in Tianjin, north China, Jan. 22, 2013. Dense fog hit Tianjin Tuesday. (Xinhua/Zhang Chenlin)

Cars run in the fog in Tianjin, north China, Jan. 22, 2013. Dense fog hit Tianjin Tuesday. (Xinhua/Liu Dongyue)