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

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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.