Influence of shrub hedge on CO diffusion in elevated bridge street canyons

To investigate the influence of shrub hedge layout on the flow field and pollutant dispersion in elevated bridge street canyons, the shrub hedges were assumed to be porous media, and the standard k-ε model was used in combination with the species transport equation in the simulation. The diffusion process of CO emitted from motor vehicles on the bottom and bridge deck of the elevated bridge street canyons with central two-side shrub hedges was simulated separately. The simulation results were compared and validated with wind tunnel test ones. The results indicate that this numerical simulation method is reliable, and unlike standard street canyons, the shrub hedges in elevated bridge street canyons not only do not hinder the movement of the flow field, but also enhance the convection between the blocked wind field at the bottom and top. This is beneficial for the dispersion of pollutants emitted from the bottom of the road, and can effectively reduce the overall pollutant concentration in the street canyon and improve the issues of pollutant accumulation on the leeward side, above the bridge deck, and at the bottom. When hedges are set on both sides, the convection between the upper and lower wind fields is stronger, and the CO concentration at the leeward wall is reduced by 60%. Therefore, adding shrub hedges can be an effective measure to reduce the level of air pollution in elevated bridge street canyons.