You can check air quality around China courtesy of Air Quality in China (.org). Below, you can see how Lanzhou compares right now to some other cities that visitors to China typically spend time in.
The Redefining Lanzhou site has a (out of date, it must be said) page on air quality, which attempts to persuade visitors that seasonal variations in air quality makes visiting Lanzhou in the summer a pleasant option. 
 
 
There are some new interactive features on the Air Quality page, which lets you see the daily Air Pollution Index results of provincial cities and municipalities measured by the Ministry of Environmental Protection. Below are some screen shots:
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Motion chart: major cities ranked in order of air quality, 10 April 2011
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Motion chart: Lanzhou's Air Quality in 2011 (1 Jan - 10 April)
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Data table, which can be filtered to highlight the information as required
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Data table, filtered to show Lanzhou's results in April
 
 
Guidebooks to the area really do Lanzhou a disservice, and the aim of this site is to redress the balance. 

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2) It's the most polluted city in the world / in China (delete according to which outdated source you are reading).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. 
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Out of 359 recorded days, 63% were either 'good' or 'excellent'
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Out of 177 recorded days, 40% were either 'good' or 'excellent' (first and last quarters)
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Out of 182 recorded days, 82% were either 'good' or 'excellent' (second and third quarters)
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As ever, there are plenty of well-known cities below Lanzhou on today's air quality rankings (Dec 14), published by the Ministry of Environmental Protection.

Out of 86 locations, Lanzhou came 54th. Here is a selection of those places with worse air quality. The ranking is in brackets:

Lanzhou (54)
Xiamen (55)
Qingdao (58)
Guangzhou (60)
Tianjin (61)
Beijing (66)
Urumqi (73)
Chongqing (76)
Xi'an (79)
Harbin (81)
Chengdu (82)
Zhangjiajie (85)

That's not to say the air is pristeen here at the moment, just another reminder that Lanzhou is a pretty average city these days, when it comes to pollution. Here is a photo of Anning District, taken today from the hills:

 
 

The Ministry of Environmental Protection placed Lanzhou 50th out of 86 locations around China (stats for October 22). Lanzhou is on page 3, but as the results are not ranked I copied and pasted into Excel and reordered them.

The cities ranked 25-80 all got a 'good' rating. However, given that Lanzhou is often criticized for its poor air quality, I thought it would be interesting to see which places came below it. Here is a selection of the most well-known (with their ranking in brackets):

Lanzhou (50)
Changsha (54)
Chongqing (55)
Xining (60)
Hohot (61)
Chengdu (67)
Hangzhou (70)
Urumqi (74)
Xi'an (78)
Beijing (81) Light pollution
Harbin (86) Light pollution

This is what Lanzhou looks like today: