Abstract: Based on the conventional synoptic analysis and using emission inventories of SO₂,PM_2.5,and PM₁₀,as well as the back trajectory model,the formation and causes of a severe air pollution event in Taiyuan,Shanxi province were analyzed.The results showed that the air pollution event has a long duration and lasted nearly 5 days.Many pollutant concentrations extremely exceed their standards,and fine particles play an important role in the pollution process.The large-scale atmosphere circulation provides Taiyuan with weak cold air and plenty of water vapor,and the persistent steady cold high pressure and weak winds near surface favor to form a large area and a long time period of stable weather condition.Air flows from the northwest and the west mainly controlled the Taiyuan area during the pollution episode,and the pollutant concentrations transported along the northwesterly flow are significantly lower than that along with other three trajectories.The height of pollutant transportation probably resulted in the difference between pollutant concentrations along different trajectories.In a combination of distribution of pollution emission,this pollution event is influenced by the local emission and long/short range transportation of air pollutants.The local emission is the primary contribution.