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The vertical distribution of aerosols was directly observed under various atmospheric conditions in the free troposphere using surface micro-pulse lidar(MPLA)at the Zhangye Station(39.08°N,100.27°E)in west China in the spring of 2008.The study shows that the aerosol distribution over Zhangye can be vertically classified into upper,middle and lower layers with altitudes of 4.5 to 9 km,2.5 to 4.5 km,and less than 2.5 km,respectively.The aerosol in the upper layer originated from the extal sources at higher altitude regions,from far desert regions upwind of Zhangye or transported from higher atmospheric layers by free convection,and the altitude of this aerosol layer decreased with time; the aerosols in the middle and lower layers originated from both extal and local sources.The aerosol extinction coefficients in the upper and lower layers decreased with altitude,whereas the coefficient in the middle layer changed only slightly,which suggests that aerosol mixing occurs in the middle layer.The distribution of aerosols with altitude has three features:a single peak that forms under stable atmospheric conditions,an exponential decrease with altitude that occurs under unstable atmospheric conditions,and slight change in the mixed layer.Due to the impact of the top of the atmospheric boundary layer,the dial variation in the aerosol extinction coefficient has a single peak,which is higher in the aftoon and lower in the ming.