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 (𝜎u/u∗l ∼ 3.9, 𝜎w/u∗l ∼ 1.52, and 𝜎𝜃/T∗ ∼ 3.96).