Originally Posted by jiejie

There is a large industrial belt south of Tianjin full of belching factories. When the wind is from the south/southwest (as is often the case in the summer), that's your most likely culprit.

IMO, June-August is the worst time of year for pollution in this part of China including Beijing. The lack of big wind, heat and temperature inversions keeps the pollution trapped and intensifies it. Glad I'm not around right now.