Study of dispersion of air pollutants in urban areas using computational fluid dynamics and geographic information system

Abstract

Hong Kong is one of the typical metropolitan cities facing adverse effects of air pollution. This severe environmental problem for the ambient air is due to its high population density as well as high density of tall buildings for optimal land use. Numerical simulations of wind effects and corresponding pollutant dispersions are necessary for the urban environmental planning and re-development. A detailed and complete study is an important step to obtain in-depth understanding of the wind flow structure in street canopies so as to improve planning. This study focuses on the behaviour of air flow and airborne pollutant dispersion inside realistic street canopies over selected urban areas in Hong Kong. These areas consisted of multiple buildings with complex configurations. A substantial amount of geographical information, such as the topography and the location of pollutant source, has to be involved to assess the impact of air pollutants on the urban environment. Thus, this study proposes a cost-effective approach, with the combination of a Geographical Information System (GIS) with the techniques of Computational Fluid Dynamics (CFD), for calculations with real layouts as well as real dimensions of buildings in a monitoring area. Furthermore, this study mainly determines the dispersions from line pollutant sources which represent emissions from vehicles traversing the streets under different cases with different wind velocities and directions. Just like the calibrations of equipment for laboratory experiments, validations are the primary steps that have to be made to verify the developed models before they can be applied. Simulation results from the developed models are compared with the published and proven experimental data from the previous literature. The numerical models are then applied to simulate the airflow patterns and pollutant dispersion behaviour in street canopies. Two categories consisting of four sets of simulations have been performed to enable the study of the wind field and pollutant dispersion around a single building, among four buildings, and among multiple buildings for a real urban scenario. The main findings show that there are some special characteristics such as acceleration, re-circulation and so on for the air flow in or around the street canyons. Furthermore, the corresponding dispersion characteristics, such as the spread of the pollution dispersions, have been altered for different airflow patterns. The results also show that the pollution dispersion characteristics vary for different building characteristics and for different wind parameters. Generally, natural ventilation in an urban area deteriorates as the height of buildings increases. However, under some special circumstances, building height can facilitate the promotion of airflow and pollutant dispersion inside the canopies and these dispersion characteristics will be useful for prediction of air quality impacted by vehicular emissions from the urban street canyons.