As for the AQI, its gravity center did not have an obvious trend in different months ( Figure 7 ). 4.
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The Spatial-Temporal Characteristics of Air Pollution in China from 2001-2014.
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The API values had a decreasing trend: the high values had a clustering trend in some northern cities, and the low values had a clustering trend in some southern cities.
The API values of some cities had an increasing trend in 2012, compared to 2011, such as Hohhot, Urumqi, Chengdu and Lhasa. 3.2.