Our system collects air quality data every hour 
from 2,296 stations in 302 Chinese cities. Each air quality instance consists of the concentration 
of six air pollutants: NO2, SO2, O3, CO, PM2.5 and PM10. We 
convert these concentrations into corresponding (individual) AQIs (Air 
Quality Indexes) for each air pollutant according to Chinese AQI standards (but 
without doing a 24-hour moving average over the AQIs). In total, 
over 12 million air quality instances have been collected from 
August 2012 to May 2015. As the cities are added into our system 
at different stages, the specific time spans of the AQI data in 
particular cities are different. 