Morakinyo Tobi Eniolu, Lam Yun Fat
School of Energy and Environment, City University of Hong Kong, Hong Kong, SAR, China.
Guy Carpenter Asia-Pacific Climate Impact Centre, City University of Hong Kong, Hong Kong, SAR, China.
Environ Sci Pollut Res Int. 2016 Nov;23(21):21652-21668. doi: 10.1007/s11356-016-7322-9. Epub 2016 Aug 13.
Numerical experiments involving street canyons of varying aspect ratio with traffic-induced pollutants (PM) and implanted trees of varying aspect ratio, leaf area index, leaf area density distribution, trunk height, tree-covered area, and tree planting pattern under different wind conditions were conducted using a computational fluid dynamics (CFD) model, ENVI-met. Various aspects of dispersion and deposition were investigated, which include the influence of various tree configurations and wind condition on dispersion within the street canyon, pollutant mass at the free stream layer and street canyon, and comparison between mass removal by surface (leaf) deposition and mass enhancement due to the presence of trees. Results revealed that concentration level was enhanced especially within pedestrian level in street canyons with trees relative to their tree-free counterparts. Additionally, we found a dependence of the magnitude of concentration increase (within pedestrian level) and decrease (above pedestrian level) due to tree configuration and wind condition. Furthermore, we realized that only ∼0.1-3 % of PM was dispersed to the free stream layer while a larger percentage (∼97 %) remained in the canyon, regardless of its aspect ratio, prevailing wind condition, and either tree-free or with tree (of various configuration). Lastly, results indicate that pollutant removal due to deposition on leaf surfaces is potentially sufficient to counterbalance the enhancement of PM by such trees under some tree planting scenarios and wind conditions.
利用计算流体动力学（CFD）模型ENVI-met进行了数值实验，实验涉及不同宽高比的街道峡谷，其中含有交通排放污染物（颗粒物），以及不同宽高比、叶面积指数、叶面积密度分布、树干高度、树木覆盖面积和种植模式的人工种植树木，实验在不同风况下进行。研究了扩散和沉降的各个方面，包括各种树木配置和风况对街道峡谷内扩散的影响、自由流层和街道峡谷处的污染物质量，以及表面（树叶）沉降去除的质量与因树木存在而增加的质量之间的比较。结果表明，与没有树木的街道峡谷相比，有树木的街道峡谷中尤其是行人高度范围内的污染物浓度有所增加。此外，我们发现因树木配置和风况导致的浓度增加幅度（行人高度范围内）和降低幅度（行人高度以上）存在相关性。此外，我们还认识到，无论街道峡谷的宽高比、主导风况如何，也无论有无树木（各种配置），只有约0.1%-3%的颗粒物扩散到自由流层，而更大比例（约97%）的颗粒物仍留在峡谷中。最后，结果表明，在某些种植方案和风况下，树叶表面沉降导致的污染物去除量有可能足以抵消这些树木造成的颗粒物增加量。
