There are two misconceptions about Lanzhou:
1) There's not much to see or do. Well, hopefully, this website can persuade you that a city does not need 'famous' sites to be worth exploring. See here for a few recommendations. In fact, the underrated nature of Lanzhou adds to the experience of the visit. And when people talk about wanting to get off the beaten track and see the 'real' China, Lanzhou (and the rest of Gansu) would be as good a place as any to spend some time.
2) It's the most polluted city in the world / in China (delete according to which outdated source you are reading). The Air Quality page on the site covers this in more detail (and gives sources for the charts below), but in summary: visit in the summer, and there is every chance you will enjoy clear, clean skies (as well as warm, 'dry' temperatures). In 2010, between April to September, 82% of recorded days had either 'good' or 'excellent' air quality.