Vertical dynamic and thermodynamic characteristics of urban lower boundary layer and its relationship with aerosol concentration over Beijing

摘要

By utilizing observational data from a 325 m tower of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) on March 19-29, 2001 and August 11-25, 2003, a comprehensive study was conducted on the vertical dynamical and thermodynamic characteristics of the urban lower boundary layer (ULBL) and its relationship with aerosol concentration over Beijing. Firstly, a comparative analysis was made on the gradient data (wind, temperature and humidity), ultrasonic data (atmospheric turbulences) and air-quality observations at different tower heights (47, 120 and 280 m). Secondly, a diagnosis was made to reveal the major features of normalized variances of velocity and temperature, turbulence kinetic energy as well as their relationship with aerosol concentrations. Furthermore, the characteristics of the ULBL vertical structure and the TSP concentration/distribution variations during a sand/dust weather process were also analyzed. The outcome of the study showed that under unstable stratification, the normalized variances of velocity (σ_u/u*, σ_v/u*, σ_w/u*) and temperature (σ_T/T*) at 47 and 120 m heights fit the Monin-Obukhov similarity (MOS) framework and the fitting formulas were given out accordingly. According to the stratification parameter (z'/L), the stable ULBL could be divided into 2 zones. With z'/L < 0.1, it was a weakly stable zone and MOS framework was applicable. The other was a highly stable zone with z'/L > 0.1 and the normalized velocity variances tended to increase along with higher stability, but it remained constant for normalized temperature variances. At daytime, the near-surface layer includes two heights of 47 and 120 m, while 280 m has been above it. The ULBL analysis in conjunction with a sand/dust weather process in Beijing in March 2001 indicated that the maximum concentration of Total Suspended Particulates (TSP) at 320 m reached 913.3 μg/m~3 and the particles were transported from the upper to lower ULBL, which was apparently related to the development process of a low-level jet and its concomitant strong sinking motion.