Originally Posted by jiejie

Nobody can give you any guarantees on the pollution conditions or trends for a specific month anymore and particularly not for Beijing but if you had to gamble on a month, September is as good as any and the weather is usually very pleasant. September in Shanghai is going to be roughly comparable to Jacksonville in avg temperature and humidity though cooler in the last half of the month. Assuming a trip of 2-3 weeks, consider starting in Beijing with arrival around September 10 (after the mid-Autumn holiday) and working your way south perhaps ending Shanghai or Hong Kong for flight home. You definitely want to exit China by around September 28 when the run up to a big national holiday week kicks in. If you are OK with cooler weather and particularly in Beijing/Xi'an, then postpone arrival until October 10 or so. Though Beijing and some other cities have many iffy days of pollution, there are some parts of China and places to visit that have relatively better air that shouldn't cause you issues, and maybe putting some of those in your trip would help.