Ramp-Like PM2.5 Accumulation Process and Z-Less Similarity in the Stable Boundary Layer

Similar ramp-like structures exist in the time variations of surface PM2.5 accumulation during the four different heavy haze pollution processes in Beijing, from 3 November 2017 to 15 January 2018. Based on the ultrasonic anemometer observations at seven different altitudes on a 325 m tower, it is shown that turbulence momentum flux exchanges between different altitudes were very weak during the ramp period of PM2.5 concentration. Turbulence at the higher altitude associated with strong wind shear, occasionally mixed downward toward the surface, showing upside-down transportation of turbulent kinetic energy (TKE), especially during the rapid removal stage of the pollution. This was different from the traditional upward TKE transportation above the surface. Vertical distribution of nondimensionalized standard deviation of the horizontal velocity, vertical velocity, and potential temperature in the stable boundary layer within the ramp-like structure show obvious z-less similarity, independent of z, and almost equal to constants (sigma(u)/u(*integral) similar to 3.9, sigma(w)/u(*integral) similar to 1.52, and sigma(theta)/T-* similar to 3.96).