The impact of sandwich foehn on air pollution in Urumqi, a gap town located on the northern leeside of the Tianshan Mountains of China, is analyzed.The results show that during days with high pollution, the boundary layer over the city and the down valley area can be divided into a three-layer structure, with the southeasterly foehn sandwiched between the northwesterly winds on top and the cold-air surface pool beneath.The southeasterly foehn at heights between 480and 2,100 m results in a very stable boundary layer structure.In combination with the decoupling between the foehn flow and cold air pool, such boundary layer structure prevents vertical mixing of atmospheric pollutants.In the up-valley area from the northern leeside flank to the southern urban area, the ground-based foehn confronts the thermally driven valley breeze and forms a "mini-front," which moves northwards in the morning and retreats southwards in the afternoon.Although the mini-front disappears in the early evening, the wind shear of the mountain breeze between the southern suburb and downtown areas is still remarkable, which is favorable for a convergence line to persist around the city all day long.In this case, air pollutants emitted from the up valley and down valley areas are transported toward the urban area.Therefore, the air pollutants accumulate daily.leading to the frequent occurrence of heavy pollution events in Ummqi.This indicates that the sandwich foehn plays a critical role in the formation of heavy air pollution events in Urumqi.