Weather situation classification and its feature in severe air pollution days in winter in Urumqi

Based on the daily ground synoptic chart and the upper synoptic chart at 500 hPa at 08:00 in winter from 2000 to 2008 as well as the daily air pollution index (API) in Urumqi from the Internet, the seasonal and monthly atmospheric pollution characters were analyzed. The effect of circulation situation on air quality was discussed. The results indicate that circulation situation influencing Urumqi could be divided into 7 types in upper level and 5 types on the ground. The analysis of correlation between the different circulation situations and the serious pollution days in winter shows that circulation situations that causes easily the heavy pollution are high pressure ridge type and low vortex type, followed by a high pressure ridge front type and ground cold high pressure type.