摘要:利用北京市环境保护监测中心和美国大使馆的细颗粒物（PM2.5）逐时监测数据，中国科学院大气物理研究所325 m气象梯度塔资料以及实况天气图和探空资料，对2015年11月27日至12月1日北京的PM2.5重污染过程的边界层特征进行了分析。研究发现：这次重污染过程持续时间长、强度大，其中PM2.5浓度超过75 μg/m3的时次共计126 h，超过150 μg/m3共计116 h，小时最高PM2.5浓度为522 μg/m3。在高低空环流场配置的影响下，近地面静风和多层逆温结构抑制了污染物在水平和垂直方向上的输送，加上边界层内的深厚湿层，使得其中气溶胶不断吸湿增长，高PM2.5浓度得以维持。在重污染期间，湍流动能较低，不利于污染物的水平和垂直扩散。垂直方向的湍流动能一直占水平方向的15%~20%左右，水平湍流动能占主要贡献。摩擦速度与湍流动能呈现出相似的变化趋势，不同高度之间的摩擦速度差别不大。超出前后时次一个数量级的湍流强度尖峰的出现是湍流场发生调整的一个信号，是PM2.5浓度发生剧烈转变的前兆，预示着污染状况更加糟糕。重污染过程中感热通量的输送方向为从地面向大气输送，感热通量和潜热通量都大幅减少，并且表现出明显的日变化特征。对湍流功率谱计算和分析表明，在重污染过程期间，时间尺度为5 min至6 h的中尺度过程对从地面到大气方向的动量和热量通量输送做出了重要贡献。
关键词:重污染/
细颗粒物(PM2.5)/
大气边界层/
大气湍流/
中尺度通量/
动量通量/
热通量
Abstract: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.
Key words:Heavy air pollution/
PM2.5/
Planetary boundary layer/
Atmospheric turbulence/
Mesoscale flux/
Momentum flux/
Heat flux
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