A simulation model for assessing traffic-related air pollution in street canyons of high-density cities

Abstract

Air pollution related to traffic is a significant urban environment challenge that affects public health and quality of life in contemporary cities. The high-density urban environment stagnates ventilation and traps air pollutants, resulting in higher pollutant exposure in street canyons for pedestrians compared with measurement data from government monitoring stations from afar. Hong Kong’s planning policies and design regulations have begun to address this issue in recent years. A key gap in research is the lack of reliable, efficient assessment tools that are suitable for policies and development decisions at the scale of building clusters and neighbourhoods. The Computational Fluid Dynamics (CFD) method, which is the most commonly used tool for airflow simulation, remains computationally demanding and unsuitable for complex urban configurations. Empirical and statistical methods to predict air pollution face uncertainties when applied to forecasting purposes. Smart sensors have made significant improvements, but a dense array of air pollution sensors covering a wide urban area remains costly. This thesis follows a three-paper structure. After the introduction in the first chapter, Chapter 2 introduces the development of a new simulation model to assess airflow in urban street canyons. Chapter 3 details the development and evaluation of the calculation module of this model to assess PM pollutant dispersion in street canyons. Chapter 4 presents the evaluation of the model’s applicability in a real urban context and planning policy via a case study of an urban site in Sai Ying Pun, Hong Kong. The key contribution of this thesis is three-fold: 1) a new assessment model, as a supplement to existing tools, can offer fast and reliable air flow and pollution dispersion simulation to support early stage design; 2) the efficient algorithm of this model enabled large time or space range simulation on a street canyon scale; and 3) the physics-based model can be applied to assess future scenarios such as climate change, policy or future development. A major limitation for this research is the absence of building-related air pollution and waste heat emissions that might raise uncertainties about the proposed simulation model. The next step is to include modules of building energy as well as development of planning and design tools.