Numerical modelling of the passive control of air pollution in asymmetrical urban street canyons using refined mesh discretization schemes

This study evaluates the potential of passive controls in asymmetrical street canyons to reduce personal exposure to air pollutants on footpaths. A passive control in the form of a low boundary wall (LBW) may act as a baffle within a street canyon, producing modified airflow patterns and increase pollutant dispersion at street level. This numerical modelling study assessed the spatial distribution of concentrations of a tracer pollutant in a street canyon. Concentrations were measured along the centre of both footpaths at breathing height to determine the percentage difference between pollutant concentrations in canyons with and without passive controls. The investigation assessed asymmetrical street canyons of different building height ratios (H1/H2 ratio ranging from 0.5 to 1.5 in 0.1 intervals) in perpendicular wind conditions. The results indicated that the H1/H2 ratio affects pollutant dispersion and the implementation of a passive control can reduce the pollutant concentration on the footpaths. The percentage difference in concentrations induced by the presence of footpath LBWs ranged from an increase of up to 19% to a reduction of 30% on the leeward footpath, with reductions between 26% and 50% on the windward footpath with varying H1/H2 ratios. Comparing the results to a central LBW configuration identified the creation of two distinct vortices in the street canyon. The results also identified the effect of wind speed on the development of primary vortices. For urban planners, passive controls offer a method of increasing dispersion by modifying normal air flow patterns and potentially improve air quality in urban street canyons.