By analyzing the pollutant concentrations over the urban area and over the rural area of the city of Lanzhou,Gansu Province,China,the relationships between the daytime inversion intensity and the pollutant concentration in the atmospheric boundary layer (ABL) are studied with the consideration of wind speed and direction,potential temperature,specific humidity profiles,pollutant concentration in the ABL,the surface temperature,and global radiation on the ground.It was shown that the daytime inversion is a key factor in controlling air pollution concentration.A clear and positive feedback process between the daytime inversion intensity and the air pollutants over the city was found through the analysis of influences of climatic and environmental factors.The mechanisms by which the terrain and air pollutants affect the formation of the daytime inversion are discussed.The solar radiation as the essential energy source to maintain the inversion is analyzed,as are various out-forcing factors affecting the inversion and air pollutants.At last,a physical frame of relationships of air pollution with daytime inversion and the local and out-forcing factors over Lanzhou is built.