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

问题陈述和动机：城市街道峡谷中的树木已被证明可以降低气温，规划者建议将其作为缓解城市热岛、适应全球变暖的炎热时期以及减少建筑降温能源需求的战略。热压力和建筑能源需求确实受到气温的影响，但也受到辐射交换、湿度和气流的共同影响。这是这些因素中最后一个被证明是研究人员最难解决的因素，无论是通过物理还是通过数值模拟。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，通风不良也可能对空气质量造成不利影响，因为污染物，特别是汽车尾气的扩散效果较差。这种潜在对比有益和有害树木影响的两难境地促使进行全面调查，寻求在改善生活和健康条件以及减少建筑能源需求方面提供平衡的植树策略。主要目标：拟议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，行道树在交通污染物扩散、室外热舒适性和建筑能源需求方面的多重和复杂的相互关联的影响将得到全面解决。其目的是促进更深入、更科学、更全面地了解所涉及的机制及其相互作用，并在城市规划和设计实践中更好地利用行道树，以知识为基础，更有针对性。研究方法：该研究将比例尺模型建筑群和树木布置的风洞试验与城市街道小气候的数值模拟和建筑能源模拟相结合。风洞实验将评估各种树木特征(例如树冠形状、树木几何形状、种植间距和密度、街道路段位置、…)的影响。关于北欧/中欧和地中海地区典型的城市地貌中不同风向的流动和污染物扩散。风洞输出将在数值小气候和建筑能源模拟中实施，以计算生物气象指数，用于评估热环境和评估对建筑能源消耗的影响。最后，将综合和分析所获得的数据和输出，以建立基本的但整体的(数学)模型，为城市规划和设计实践提供指导，旨在提供关于改善生活和健康条件以及减少建筑能源需求的平衡植树战略。

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.