Yang Xu, Cai Zi-Ying, Han Su-Qin, Shi Jing, Tang Ying-Xiao, Jiang Ming, Qiu Xiao-Bin
Tianjin Environmental Meteorological Center, Tianjin 300074, China.
CMA-NKU Cooperative Laboratory for Atmospheric Environment-Health Research, Tianjin 300074, China.
Huan Jing Ke Xue. 2021 Jan 8;42(1):9-18. doi: 10.13227/j.hjkx.202007068.
Pollution occurs in the boundary layer, and the thermal and dynamic vertical structure of the boundary layer has a significant influence on the formation of heavy pollution episodes. Based on unmanned aerial vehicle (UAV) sounding, ground-based remote sensing and numerical modeling, this paper analyzes the vertical structure of the boundary layer and the causes of pollution during the heavy pollution episode in Tianjin from January 10 to 15, 2019, with a view to strengthening the understanding of the influence law of boundary layer processes on heavy pollution in northern coastal cities and improving the accuracy of weather forecasts and heavy pollution warnings. The results show that atmospheric temperature stratification had a significant influence on the formation, persistence, and dissipation of heavy pollution episodes. During an episode, accompanied by the development and dissipation of the inversion layer, a high PM concentration area developed to the upper atmosphere with a height of over 300 m in the daytime and compressed to the ground at night with a height about 100 m. When fog appeared and continued in the daytime, the vertical structure characteristics of the boundary layer changed. A temperature inversion above the fog restrained the diffusion of pollutants to the upper air and made the contribution of turbulence vertical mixing process decrease significantly in the daytime, leading to the persistence and development of heavy pollution near the surface. Regional pollution transport accounted for 66.6% during the episode, which was closely related to regional pollution transport. Regional pollution transport mainly appeared at the top of the boundary layer and above the fog inversion layer where high wind speeds occurred. Pollutants were transported to the ground by a sinking motion as the boundary layer and fog height changed. This is how regional pollution transport occurred when Tianjin was controlled by a weak high pressure field in the north. The vertical structure of the boundary layer also affected the improvement of air quality by cold air. The strong temperature inversion at the top of the fog resulted in the failure of the cold air to transmit to the ground through turbulent shear stress in the S3 stage. There was an obvious difference in wind speed between the upper and lower air. The influence of cold air on the ground was delayed, and the effect of it was weakened. Thus, the heavy pollution episode could not be alleviated completely.
污染发生在边界层，边界层的热力和动力垂直结构对重污染事件的形成有重大影响。基于无人机探测、地基遥感和数值模拟，本文分析了2019年1月10日至15日天津重污染事件期间边界层的垂直结构及污染成因，旨在加强对北方沿海城市边界层过程对重污染影响规律的认识，提高天气预报和重污染预警的准确性。结果表明，大气温度层结对重污染事件的形成、持续和消散有显著影响。在一次事件中，伴随着逆温层的发展和消散，高PM浓度区在白天向上发展至300米以上高空，夜间压缩至地面，高度约100米。当白天出现并持续有雾时，边界层的垂直结构特征发生变化。雾层上方的逆温抑制了污染物向上扩散，使白天湍流垂直混合过程的贡献显著降低，导致近地面重污染持续和发展。该事件期间区域污染传输占66.6%，这与区域污染传输密切相关。区域污染传输主要出现在边界层顶部和雾逆温层上方风速较大的区域。随着边界层和雾层高度变化，污染物通过下沉运动传输至地面。这就是天津受北方弱高压场控制时区域污染传输发生的过程。边界层的垂直结构也影响了冷空气对空气质量的改善。雾顶强烈的温度逆温导致冷空气在S3阶段无法通过湍流切应力传输至地面。上下层空气风速存在明显差异。冷空气对地面的影响延迟，效果减弱。因此，重污染事件无法得到彻底缓解。
