Effects of trees on the dilution of vehicle exhaust emissions in urban street canyons

In order to investigate the natural ventilation and air quality of urban street canyons with trees, boundary layer wind tunnel studies at a small-scale model have been performed. Concentrations in street canyons with a tracer gas emitting line source at the ground level and one row of trees arranged along the canyon centre have been measured for several equidistant tree spacings. In the case of flow approaching perpendicular to the street axis, increases in pollutant concentration at the leeward canyon wall and decreases at the windward canyon wall were found when compared with the tree-free canyon.