城市机动车道颗粒污染物扩散对绿化隔离带空间结构的响应

通过设置绿化隔离带以减少道路污染物的扩散是目前采用的道路生态防控措施之一。如何减少道路污染对机动车道行人的影响等问题则少有关注。基于对城市非机动车道行人生态安全的考虑,采取现场模拟方法研究道路绿化隔离带对道路中心颗粒污染物向非机动车道扩散的影响。 试验地点选择太原市城区,具体方法是在机动车道和非机动车道之间设置遮阳网。通过增减遮阳网的层数调整模拟隔离带的宽度和疏透度,每50 cm间隔1层遮阳网。为了更接近实际的绿化隔离带和更利于调整疏透度,近机动车道一侧的遮阳网均匀捆扎新鲜的离体植物枝条。实验采用三因素、四水平正交设计方法,通过测定绿带两侧污染物含量的变化评价绿带对非机动车道的净化效应。结果表明,道路绿化隔离带对于减少道路颗粒污染物向非机动车道的扩散具有明显的作用。道路绿化隔离带的诸因子净化非机动车道环境的作用有明显的差异。其中疏透度和高度所起的作用较为显著。本试验中净化PM₁₀过程中三因子的重要性排序依次为高度、疏透度、宽度。净化TSP过程中的重要性依次为疏透度、高度、宽度。三因子净化非机动车道颗粒污染物的最佳水平分别是:疏透度20%,高度350 cm,宽度250 cm,对TSP和PM₁₀的净化百分率分别达到46.10%和42.94%。各因子不同水平的净化效应排序分别为:疏透度20%、30%、40%、10% ,高度350 cm、250 cm、150 cm、50 cm,宽度250 cm、350 cm、150 cm、50 cm。

Influence of green belt structure on the dispersion of particle pollutants in street canyons

Motorized vehicles are one of the main anthropogenic sources of aerosols in modern cities across the world. Road green belts can purify air and improve air quality. Urban vegetation is considered to potentially filter fine and ultra-fine dust particles (particulate matter, PM₁₀ and smaller) from the air through a deposition process that takes place on leaf surfaces. Because of concern regarding pollution in urban streets, there is increasing interest in the ability of vegetative treatment systems to mitigate the consequences of pollution. Performance of these systems can depend on the characteristics of the green belt, for example, the type of vegetation, width, canopy density and porosity, among others. Generally, green road belts have been used to promote the spread of air pollutants outside of the road, while non-motorized vehicles roads have been disregarded. The focus of this study was on the efficiency of trees and hedges that are parallel to roads to reduce air pollution along a non-motorized vehicles road. We investigated the effect of the green belt structure on the pollutant concentration above roads for non-motorized vehicles with the aim of providing a basis for the design of a road green belt pattern. Concentrations of total suspended particulate (TSP) and PM₁₀ were measured along both sides of an experimental green belt in Yingze Street, Taiyuan city. Field measurements were taken using a horizontally ground-mounted tripping device of 10 m in length, which was covered with shade cloth, branches and leaves, to generate a boundary layer for the experimental vegetation canopy. Porosity, height and width were manipulated by installing different layers of shade cloth. The results clearly showed an effect of the green belt on pollutant dispersion in a street side. The most important factor affecting TSP was porosity, followed by height, and width. The most important factor affecting PM₁₀ concentrations was height, followed by porosity and width. The concentration of PM₁₀ was significantly correlated with the porosity and height of vegetation canopy. The optimum porosity, height and width were 20%, 350 cm and 250 cm, respectively. The percentage removals of TSP and PM₁₀ were 46.58% and 42.58% under the optimum green belt design. The effects of different combinations of each green belt factor on pollutant removal (TSP and PM₁₀) were 20%, 30%, 40% and 10% for porosity, 350 cm, 250 cm, 150 cm and 50 cm for height, and 250 cm, 350 cm, 150 cm and 50 cm for width. By applying an appropriate structure for urban road green belts, we can improve air quality over roads for non-motorized vehicles. For intra-urban arterial roads, green belts of low planting density and tree species of high crown porosity and large canopy should be used to reduce the concentrations of traffic-released pollutants in non-motorized roads.