Abstract: Lanzhou is a typical valley city in Northwest China. Based on the ground conventional meteorological observation data, air quality index at Lanzhou and Baiyin stations and NCEP-DOE reanalysis Ⅱ data, the weather situations during the severe and extreme air pollution processes in Lanzhou from 2015 to 2017 were classified, and the similarities and differences of air pollution between Lanzhou and its satellite city Baiyin under different weather situations were analyzed. And on this basis the influence of meteorological elements on air pollution under different weather situations were discussed by numerical simulation of 21 typical cases. The results are as follows: (1) The weather situations during the severe and extreme air pollution in Lanzhou were mainly trough-ridge pattern and westerly wind pattern from 2015 to 2017. (2) The valley wind circulation was weaker under trough-ridge pattern weather situation. When the degree of air pollution was the same in two cities, the near ground wind field converged over Lanzhou, while the convergence and divergence of wind field wasn’t obvious over Baiyin, and the downward movement over Baiyin was more significant than that over Lanzhou in the afternoon. When the degree of air pollution was different in two cities, the wind field appeared convergence near ground of Lanzhou, and there was a weak downward movement at night, while the wind field appeared divergence near ground of Baiyin, the downward movement wasn’t obvious. (3) The valley wind circulation was stronger under westerly wind pattern weather situation. When the degree of air pollution was the same in two cities, the convergence and divergence of wind field wasn’t obvious over Lanzhou and Baiyin, but there was downward motion near ground of Lanzhou at night. When the degree of air pollution was different, the wind field appeared convergence near ground of Lanzhou, and the downward airflow was dominant in the daytime, while the convergence and divergence wasn’t obvious over Baiyin, and the updraft was dominant near ground during the daytime and nighttime. (4) Under two patterns weather conditions, the wind speed near ground of Lanzhou was usually smaller than that over Baiyin on polluted day, the inversion gradient over Lanzhou was usually larger than that over Baiyin, and the height of boundary layer over Lanzhou was smaller than or close to that over Baiyin.

Key words: air pollution, similarities and differences characteristics, valley city, satellite city, meteorological influence elements

摘要： 兰州是中国西北地区典型的河谷城市。利用兰州和白银地面常规气象观测资料、空气质量指数以及NCEP-DOE再分析Ⅱ资料，统计2015—2017年兰州发生重度和严重空气污染的天气形势，并分析不同天气形势下河谷城市兰州及其卫星城市白银空气污染的异同。在此基础上，通过21个典型个例的数值模拟探讨了不同天气形势下空气污染的气象影响要素。结果表明：（1）兰州发生重度及以上空气污染的天气形势主要为槽脊型和西风型。（2）槽脊型，山谷风环流较弱，两地污染程度相同时，兰州近地面风场辐合，而白银无明显的风场辐合辐散，且午后白银较兰州下沉运动显著；两地污染程度不同时，兰州近地面风场辐合，夜间有弱的下沉运动，而白银近地面风场辐散，夜间无明显垂直运动。（3）西风型，山谷风环流较强，两地污染程度相同时，兰州和白银均无明显的风场辐合辐散，但夜间兰州有下沉运动，白银无明显垂直运动；两地污染程度存在差异时，兰州近地面风场辐合，白天近地面以下沉气流为主，而白银无明显风场辐合辐散，白天和夜间近地面均以上升气流为主。（4）两类天气形势下，污染日兰州近地面风速小于白银，逆温梯度大于白银，边界层高度小于或接近于白银。

关键词: 空气污染, 异同特征, 河谷城市, 卫星城市, 气象影响要素