Boundary Layer Characteristics during a Heavy PM2.5 Pollution Process in Beijing

Based on hourly PM2.5 monitoring data from Beijing Environmental Protection Monitoring Center and the U.S. Embassy, the 325 m gradient tower data from the Institute of Atmospheric Physics, Chinese Academy of Sciences, the synoptic charts and sounding data, the atmospheric boundary layer characteristics during the heavy PM2.5pollution period from 27 Nov to 1 Dec 2015 are analyzed. The results show that this heavy pollution process was persistent and severe, since the duration of ρ(PM2.5) exceeding 75 μg/m³ was 126 hours in total and the duration of ρ(PM2.5) exceeding 150 μg/m³ was 116 hours in total. The maximum hourly ρ(PM2.5) was up to 522 μg/m³. Under the influence of the weather situation, light winds prevailed in the near-surface layer with multi-layer inverse temperature structure, which inhibited both the horizontal and vertical transport and dispersion of pollutants. In addition, a thick wet layer developed in the boundary layer, in which the aerosols kept absorbing moisture and grew up. As a result, high ρ(PM2.5) concentration maintained during the pollution process. The turbulent kinetic energy was relatively small during the heavy pollution process, which was not conducive to the dispersion of pollutants. Note that horizontal turbulent kinetic energy accounted for the major part of the total turbulent kinetic energy, and the turbulent kinetic energy in the vertical direction was only about 15%-20% of that in the horizontal direction. Friction velocities at different heights exhibited the same characteristics as turbulent kinetic energy. The occurrence of two turbulence intensity spikes was a sign of turbulent flow adjustment and a precursor to the sharp shift in PM2.5 concentration, and the air quality would become worse. During the process of heavy pollution, the sensible heat flux was transported from the ground to the atmosphere; both sensible and latent heat fluxes significantly reduced compared with that in the non-polluting moment and exhibited distinct diurnal changes. Power spectral analysis and calculations show that during the heavy pollution process, mesoscale processes on time scales from 5 min to 6 h made important contributions to the transfer of momentum and heat fluxes from the surface to the atmosphere.