Trees in urban street canyons and their impact on the dispersion of automobile exhausts

The aim of the present study is to clarify the influence of trees on the dispersion of automobile exhausts in urban street canyons. For this purpose, measurements have been performed with a small scale wind tunnel model of an idealized, isolated street canyon with model trees placed along the canyon center axis. Sulfur hexafluoride (SF6) was released from a line source embedded in the street surface, simulating vehicle exhaust emissions. The influence of various tree planting arrangements on the concentrations at the canyon walls was investigated with an approaching boundary layer flow perpendicular to the canyon axis. Increasing pollutant concentrations at the leeward wall and decreasing pollutant concentrations at the windward wall were found for increasing plant density. At the ends of the street canyon, i.e. towards the intersections, a remarkable relative increase of concentrations at both canyon walls was observed. The results indicate that due to tree planting, typical vortex structures observed in empty street canyons were either significantly weakened or no longer present.