行道树冠形对街谷内流场及PM_(10)浓度场的影响

Impacts of tree crown profile on flow and PM_(10) concentration fields inside street canyon

城市道路绿化通常被作为应对机动车污染物排放和改善空气质量的主要措施。但近年来研究表明,城市街谷中行道树的种植易导致内部空气流动受阻并引发污染物堆积。为进一步揭示行道树冠形特征对街谷颗粒污染物分布的影响,采用城市微气候数值模拟软件ENVI-met,模拟分析了平行、斜交、垂直3种来流风向下4种典型行道树冠形对街谷内流场及PM_(10)浓度分布的影响。结果表明,无论在何种来流风向下,4种行道树树冠对街谷空气流动均存在阻塞效应;树木冠层的叶面积密度分布是影响阻塞效应的重要因素,通常在叶面积密度最大的冠层高度风速降幅最为显著;行道树冠形在不同来流风向下对两侧行人高度PM_(10)扩散的影响存在一定差异,其中平行、斜交风向下对PM_(10)积聚程度的强弱顺序为:圆锥形树冠>球形树冠>卵形树冠>倒卵形树冠,而垂直风向下为:倒卵形树冠>球形树冠>卵形树冠>圆锥形树冠;未来在街谷行道树种植管理过程中,应当结合街谷来流风向选择适宜的行道树或对行道树树冠进行修剪整形,从而降低街谷两侧行人的污染暴露风险。研究结果可为行道树种植与管理提供参考依据。

Urban street greening is usually used as the main measure to mitigate vehicle emissions and improve air quality.However,recent studies have shown that the planting of trees in street canyons may easily lead to the obstruction of internal air flow and the accumulation of pollutants.In order to further reveal the influence of tree crown characteristics on the distribution of the particulate pollutants in street canyon,the urban microclimate numerical simulation software ENVI-met was used to simulate and analyze the influence of the four tree crown profiles on the flow field and PM_(10) concentration distribution in street canyon under three wind directions of parallel,oblique and vertical inlet winds downward.The results showed that there was a blocking effect on the air flow in the street canyon of four street tree crowns regardless of the inlet wind downward.The leaf area density(LAD)distribution of tree canopy was an important factor influencing the blocking effect of tree crown.In general,the canopy height with the highest LAD had the most significant decrease on wind speed.There was a certain difference in the influence of tree crown on the PM_(10) dispersion of pedestrian height on both sides under different inlet winds.The order of PM_(10) accumulation under parallel and oblique wind direction was:conical crown,spherical crown,oval crown,obovate crown,while under vertical wind direction was:obovate crown,spherical crown,oval crown,conical crown.In the future,in the process of tree planting and management in street canyons,it should be combined with the wind direction to select appropriate trees or prune tree crowns to reduce the risk of pollution exposure for pedestrians.The results of the study provide a reference for the planting and management of street trees.