The Integrated Impacts of Street Trees on Air Quality, Human Thermal Comfort and Building Energy Consumption in the Urban Environment

问题说明和动机：城市街道峡谷中的树木已被证明可以降低空气温度，规划者建议将其作为缓解城市热岛的一种策略，作为适应与全球变暖相结合的高温事件的一种策略，并作为减少建筑制冷能源需求的一种策略。热应力和建筑能源需求确实受到空气温度的影响，但也受到辐射交换、湿度和气流的综合影响。这是最后一个因素，对于研究人员来说，无论是通过物理还是通过数值模拟，都是最难解决的。然而，如果街道峡谷的通风严重受损，树木对气流的影响可能会抵消它们对气温的影响。此外，通风受损也可能对空气质量产生不利影响，因为污染物的扩散效果较差，尤其是汽车排放的污染物。这种潜在的有利和有害的树木效应对比的困境促使了一项全面的调查，旨在提供平衡的植树战略，以改善生活和健康条件，减少建筑能源需求。主要目标：本研究的主要目标是了解、描述和预测树木在城市街道峡谷中的各种影响。特别是，行道树对交通污染物扩散、室外热舒适和建筑能源需求的多重和复杂关联的影响将得到全面解决。其目的是促进对所涉及的机制及其相互作用的更深入的科学和更全面的理解，以及在城市规划和设计实践中更以知识为基础和更有针对性地使用行道树。研究方法：该研究结合了对建筑集群和树木排列的比例模型的风洞试验，以及城市街道小气候和建筑能源模拟的数值模拟。风洞实验将评估不同树木特征（如树冠形式、树木几何形状、种植间距和密度、街道位置等）对北欧/中欧和地中海地区典型城市形态中不同风向的流量和污染物扩散的影响。将风洞模拟结果应用于微气候数值模拟和建筑能耗模拟，计算生物气象指标，评价热环境，评价对建筑能耗的影响。最后，将对获得的数据和产出进行综合和分析，以建立基本但整体的（数学）模型，为城市规划和设计实践提供指导，旨在提供平衡的植树战略，改善生活和健康条件，减少建筑能源需求。

Problem Statement and Motivation: Trees in urban street canyons have been shown to reduce air temperature, and planners have recommended them as a strategy for mitigation of the urban heat island, as adaptation to heat episodes in conjunction with global warming, and for reduction of building energy demand for cooling. Heat stress and building energy demand are indeed affected by air temperature - but also by radiant exchange, humidity and airflow, in combination. It is the last of these factors which has proven most difficult for researchers to tackle, either by physical or by numerical modelling. Yet the effect of trees on airflow may offset their effect on air temperature, if ventilation of street canyons is severely impaired. Furthermore, impaired ventilation may also have detrimental effects on air quality, as the result of less effective dispersion of pollutants, especially of automobile emissions. This dilemma of potentially contrasting beneficial and detrimental tree effects motivates a holistic investigation which seeks to provide balanced tree-planting strategies with respect to improved living and health conditions and reduced building energy demand. Key Objectives: The key objectives of the proposed study are to understand, describe and predict the various effects of trees in urban street canyons. In particular, the multiple and complexly interrelated effects of street trees on dispersion of traffic pollutants, outdoor thermal comfort and building energy demand will be addressed in a comprehensive way. The aim is to contribute to a deeper scientific and more holistic understanding of the mechanisms involved and their interactions as well as for a stronger knowledge-based and more targeted employment of street trees in urban planning and design practice. Methodology: The study combines wind tunnel examinations of scale-model building clusters and tree arrangements with numerical simulations of the microclimate of city streets and building energy simulations. The wind tunnel experiments will assess the effect of various tree characteristics (e.g. crown form, tree geometry, planting spacing and density, location in the street section, …) on flow and pollutant dispersion for different wind directions in urban morphologies typical for Northern / Central Europe and the Mediterranean area. The wind tunnel outputs will be implemented in the numerical microclimate and building energy simulations in order to calculate bio-meteorological indices for the assessment of the thermal environment and to assess effects on building energy consumption. Finally, the acquired data and outputs will be synthesized and analyzed to establish basic but holistic (mathematical) models providing guidance for urban planning and design practice targeted to provide balanced tree-planting strategies with respect to improved living and health conditions and reduced building energy demand.