Improving air quality in pedestrian zones using passive barriers with vegetation

Abstract

One way of minimising pedestrian exposure to car produced pollutants is to passively divert the latter away from the pedestrian zone. In this research, the aerodynamics of roadside barriers and their impact on improving air quality within the pedestrian zone was explored. Full scale wind measurements were performed in a puropsely built model urban street canyon and then used to validate a Computational Fluid Dynamics model in OpenFOAM. A no barrier case was then numerically compared to a simple vegetative barrier by using a tracer gas (based on the diffusive properties of NO2 since it is one of the most harmful gaseous pollutants in urban environments). The vegetative barrier was modelled as a force term using well established equations. Simulations were run in steady state and the Reynolds Averaged Navier Stokes based on the Renormalisation Group k −ε model was used for turbulence. The results were then aggregated and analysed for the complete case, as well as isolations for elevations that correspond to both average child and average adult heights. This was done in order to validate both the effects of the barrier within the street canyon and to obtain a long term average concentration field as part of the results. Changes were noted between the mean and max concentrations of the scenarios in both the windward and leeward sides of the road. For adult breathing levels, the former showed a minor increase in concentration and the latter showed a decrease. A slight increase in mean but especially in the max concentrations were noted at child breathing level. The opposite was observed for adult breathing heights. Overall this research shows that passive barriers may have the potential to be effective in reducing pollutant levels in pedestrian zones but their design needs to be further engineered and developed to avoid negative effects associated with any resultant stagnant zones.