兰州日间稳定边界层生消机制及其环境效应的数值模拟研究

Studies have demonstrated that air pollution in valley city is mostly related to structure of atmospheric boundary layer as well as pollutant emission. Lanzhou is a typical valley city. Though its pollutant emission is in the middle rank of all large cities in China, the city is famous for its heavy air pollution in China or even in the word. For the reason, day-time stable boundary layer observed in Lanzhou is a key factor to cause the heavy pollution. It limits vertical mixture development in the day time. It traps air particle delays the time in pollution condition from the night. Thus it is a special atmospheric boundary layer that greatly contributes to the heavy air pollution. However, so far it is just an observed fact. The physical mechanism of the boundary layer formation and disappearance is not studied as well as its environmental effect on vertical distribution and accumulation of air particles. By using numerical simulation combined with observational method, the project is set up to analysis structure of the day-time stable boundary layer and influence factors, probe mechanism of the formation and disappearance, study its effect on air particles vertical distribution and temporal accumulation. The research has not only significant value to understand atmospheric boundary layer and its diffusion over complex terrain, but also provide theory basis for high-resolution air pollution model development and air pollution control.

现有研究表明，山谷城市严重的空气污染除受污染源排放影响外，主要与大气边界层结构有密切的关系。兰州是一个典型的山谷城市，虽然其空气污染物排放量在全国各大城市中仅居中等，但却早已成为我国乃至全世界空气污染最严重的城市之一。究其原因，兰州市区的日间稳定边界层是主要罪魁祸首，它制约了日间垂直混合层的发展，延长污染物在谷中的积累时间，是一种特殊且易造成严重空气污染的大气边界层结构。但是迄今为止，对日间稳定边界层的研究仅限于观察事实，其物理过程及生消机制、及其对颗粒物的垂直分布和累积变化作用的研究尚未深入开展。本项目以观测数据为依据，以数值模拟为手段，将两者有机地结合起来研究兰州日间稳定边界层结构特征、影响因素及其生消机制，探究其对颗粒物垂直分布和累积变化的作用。这不仅可以深化对复杂地形上大气边界层和大气扩散问题的认识，也可为城市空气污染精细化预报模式的发展及空气污染的合理防治提供科学理论依据。

兰州是一个典型的山谷城市，是我国乃至全世界空气污染最严重的城市之一。除污染源排放外，兰州空气污染与大气边界层结构密切相关，与其他城市一样，夜间存在的稳定边界层，是导致兰州严重污染的重要原因，但仅存在的夜间稳定边界层，不足以使兰州成为全国名列前茅的污染城市，兰州的污染排放源仅位于全国的中等行列，观测发现兰州存在特有的日间稳定边界层，它制约了日间垂直混合层的发展，延长污染物在谷中的积累时间，是一种特殊且易造成严重空气污染的大气边界层结构。本项目采用观测数据分析与数值模拟研究相结合，分析兰州空气污染物时空变化特征、影响空气污染的主要气象条件和因子，探析导致兰州严重污染事件的日间稳定边界层结构特征、影响因素及其生消变化机制。观测数据分析表明，兰州空气污染近年来有下降趋势，严重污染事件与日间稳定边界层有关，数值模拟结果分析表明，中层尺度数值模拟WRF能模拟出日间存在的逆温层，其生消变化受河谷地形影响，其强度决定了空气中颗粒物的扩散能力和持续时间。与一般认识不同，冷空气过程不仅没有打破存在的日间稳定边界层，还加强逆温在河谷上空逆温层结构，形成“冷层逆温”，导致兰州严重的污染事件。相关研究不仅可以深化对复杂地形上大气边界层和大气扩散问题的认识，也可为城市空气污染预报及合理防治提供科学理论依据。