The impact of roadside trees on pollutant dispersion for a single and infinitely repeated sources

Source abstract

Although trees provide shade for pedestrians, enhancing their thermal comfort, it is known that dense vegetation can remarkably hinder the ventilation efficiency of an urban area, resulting in greater pedestrian exposure to traffic-related air pollutants. In the present paper, an infinite network of street canyons containing intersections is analyzed for four wind directions (90°, 60°, 30°, 0°) via RANS CFD simulations. The investigated street canyons are of H/W = 0.5 aspect ratio and can contain three different tree arrangements (none, sparse, dense). The flow field is considered fully periodic in the x and y-directions, while dispersion is modelled in two ways: (1) by calculating the aperiodic concentration field of a single source, and (2) by taking periodically repeated sources into account. After optimizing the sand-grain roughness of the walls (Ks/H = 0.1) and the turbulent Schmidt number (Sct = 0.2), the velocity, TKE, and aperiodic concentration distributions show excellent agreement with previous wind tunnel results. When only a single pollutant source is considered, trees worsen the pedestrian exposure to traffic-related air pollutants for all investigated scenarios. Conversely, when the impact of infinitely many repeated street canyons and emissions is taken into account, trees have a ventilation-enhancing effect for the 0° and 30° wind directions. The wind direction-averaged mass transfer coefficient (mass Stanton number) is found to be decreased only by 6% and 16% by the sparsely and densely planted trees, respectively, which may be viewed as a trade-off worth considering when the beneficial aspects of urban trees are also considered.