A field experiment was conducted based on a 255 m meteorological tower in Tianjin from Dec 29 2016 to Jan 8 2017 to study microphysics characteristics of urban canopy top and its effect on a long-lasting severe haze and fog (HF) episode. The results show that the gradients of PM2.5 concentration in the vertical direction varied greatly during the clean days but smaller during HF days, which was consistent with the variation of PBL height and turbulence activity. During HF1-1, PM2.5 concentrations at 120 m was obviously higher than those at surface and 200 m. Wind shear was one of the important factor due to accumulation of pollutants at 120 m. During HF1-2 period, PM2.5 concentration at three levels declined with a mode of "cliff". The vertical gradient of concentration between 120 and 200 m was small, but larger between surface and 120 m. The wet scavenging effect of fog, the damaged inversion layer and strengthened turbulence were closely related to "cliff" declined of concentration. While, when fog transformed to haze during HF1-3 period, turbulence energy rapidly decreased, and the atmospheric layer again began stable with the height of PBL at nearly 120 m.