LAUSR

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