Image two of three comparison images showing a smog haze build up over 3 days in San Jian Fang Xiang, Chao Yang, Central Beijing, 10 km to the east from Guo Mao tower (tallest building in Beijing) and Central Business District which are visible in the distance.
In winter North China burns more coal for heating, which increases air pollution. Because of the winter season the ground is colder than in summer and more stable, and inverse air pattern causes air pollution to accumulated at ground level, triggering an atmospheric chemistry reaction, which causes a much higher PM2.5 concentration.