街道峡谷大气颗粒物扩散对行道树覆盖的响应与调控

A multitude of studies have shown that trees can effectively absorb street particles and improve the micro-environment microclimate, with significant dust detaining and decontamination effects. However, other studies demonstrate that the dispersion of airborne particles in street canyon is blocked by the cover of tree canopy, which induces the accumulation of pollutants in the street. Aiming at the contradictions of the previous studies, the daily and seasonal variations of atmospheric airborne pollutant concentrations under different green densities (cover degree, porosity, leaf density) and street canyon characteristics were studied, associate with the ENVI-met model Simulation, to explore the spatial distribution of atmospheric particles in street canyon under different road tree coverage. This study discusses the response and regulation mechanism of the spatial distribution of atmospheric particulates on account of the coverage of trees in street canyons and establishes the green capacity index of road trees that are conducive to the dispersion of particulate pollutants in different canyon-type streets. It also provides evidences for the road tree configuration based on particle dispersion control.

大量研究结果表明，街道两边的人行道旁密植行道树可以有效吸附街道颗粒物，改善微环境小气候，具有显著的滞尘减污作用。然而另一些研究显示街道峡谷内因行道树树冠覆盖影响而导致大气颗粒物扩散受阻，从而引发街道峡谷内的污染物聚集。针对以往研究出现的矛盾，本研究拟通过不同行道树绿容率（盖度、疏透度、叶密度）与街道峡谷特征下大气颗粒污染物浓度日变化和季节变化的实测，结合ENVI-met模型模拟，探讨街道峡谷内大气颗粒物的空间扩散规律对行道树覆盖的响应与调控机制，构建不同峡谷型街道中有利于颗粒污染物扩散的行道树绿容率指标，为不同街道的行道树配置及颗粒污染物扩散调控提供依据。

为使街道行道树既能发挥其生态效益，又能避免因树冠覆盖导致大气颗粒物聚集，本项目通过街道不同行道树特征与峡谷特征下及绿化隔离带前后大气颗粒污染物浓度变化的实测与ENVI-met模型模拟，探讨了街道峡谷行道树树冠郁闭度对颗粒物衰减系数、行道树生长型对颗粒物浓度与粒径的时间变化特征和绿化隔离带疏透度对非机动车道空气颗粒物浓度的影响，分析了街道峡谷狭窄度、朝向和垂直方向的颗粒物扩散规律。结果表明，街道峡谷行道树树冠郁闭度在30%-36%时，PM10、TSP浓度最低，郁闭度超过60%则街道峡谷中产生高颗粒物浓度区。随着行道树叶面积密度增加则街道中PM2.5及PM10浓度随之增大。落叶阔叶树覆盖的街道峡谷各粒径颗粒物的浓度均高于常绿阔叶树覆盖，但夏季常绿阔叶树覆盖的街道峡谷中小粒径颗粒物浓度较高，落叶阔叶树覆盖的街道峡谷中较大粒径颗粒物浓度较高。颗粒物TSP、PM10浓度的消减效率与0-2m高度区间内绿化隔离带的的疏透度呈显著负相关。街道峡谷越开敞越有利小粒径颗粒物的扩散，各粒径颗粒物浓度均呈现随垂直高度增加而明显降低的趋势，不同狭窄度街道峡谷内背风面的颗粒物浓度均高于迎风面的颗粒物浓度。低郁闭度（≤35%）和中等郁闭度（35%-70%）行道树配置有利于街道峡谷颗粒物扩散，但在高污染区域的街道峡谷中则应将行道树树冠郁闭度控制在35%左右。稠密树冠且顶篷搭接阻碍颗粒物扩散时可以通过缩冠整型与疏枝修剪降低郁闭度。街道峡谷以小型常绿阔叶乔木为主、辅以大型落叶阔叶乔木间隔种植的行道树带滞尘效果为佳，而在交通流量较大的城市干道应该多配置结构紧密的灌木植物。采用规则型(H/W≈1)街道峡谷设计，并以通风口引入垂直峡谷轴线风向的气流有利于街道峡谷颗粒物的扩散。研究结果阐明街道峡谷内大气颗粒物的空间扩散规律对行道树覆盖的响应与调控机制，提出了基于颗粒物扩散的城市街区行道树结构配置策略，可为街道空气质量改善的城市街道峡谷狭窄度规划、道路行道树及绿化隔离带的优化设计提供理论支撑。