Impact of building permeability on pollutants removal effectiveness in urban streets

Improper city planning has a tremendous impact on urban air quality. Tall buildings aligned along two sides of the street canyon affect the pollutant dispersion process. Efforts have been spent on exploring strategies to mitigate air pollution problem within street canyons. However, there is still in need of consensus on which types of road and building geometry will enhance air circulation. In this study, we aim to investigate the effects of building permeability and approaching wind directions on the pollutant removal ability for different canyon geometries. A numerical model was constructed using standard k-E model under the Computational Fluid Dynamics (CFD) flow and dispersion simulation. Pollutants reductions at the pedestrian levels were examined for different building permeability ratios and wind directions. It is found that pollutant reduction has a linear relationship with the building permeability for a canyon with an aspect ratio of 2 when the approaching wind is perpendicular to the street axis. However, such relationship is not found for canyon with an aspect ratio of 6. Increasing building permeability in deep canyons may not necessarily enhance the pollutant removal ability in street canyons. When the approaching wind direction is parallel to the street axis, wind will be channeled and washed out the pollutants towards the street end. However, spacing between building blocks interrupted this process so that pollutants cannot be removed effectively. The pollutants are accumulated at the street ends and the pollutant reduction capability is reduced. Open spaces may not always favor the removal process and the approaching wind direction should also be considered during urban planning stage to ensure effective pollutant removal from the site. These findings should be of particular significance to city planners as they help reveal the impacts of building permeability on the pollutants removal effectiveness.