Comparative Analysis on API Changes of Five Cities in Northwest of China

From the views of monthly mean and air pollution level, the variation characteristics and trend of API and pollution proportion of five cities(Xining, Lanzhou, Yinchuan, Xi′an and Urumqi)are researched based on the daily air pollution index(API) during 2001 to 2012. The results show as follows:(1)Air quality levels in five cities were rated level Ⅱ. The primary pollutants is inhalable particles. API show a decrease trend in inter-annual, seasonal, heating period and non-heating period, the days of pollution also show obvious decline trend. Air pollution presentregionalizationcharacteristics.(2)API index have obvious difference in temporal and spatial scale. Air quality in Xi′an, Lanzhou and Urumqi is higher than air quality in Yinchuan and Xining from annual average of API analysis. Air quality is stable in Xi′an and Yinchuan but fluctuating in Urumqi, Lanzhou and Xining. Annual API appear different characteristic in five cities but air quality is better in summer than winter and spring.(3)Peak identification of API reveal that API skewed to low concentration and the number of days increased. High concentrations of API events reduce gradually. The year that API changed to low concentration is not same in each city.