Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions

Abstract

This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived.

Influence of avenue-trees on air quality at the urban neighborhood scale. Part II: Traffic pollutant concentrations at pedestrian level

“…Balczo et al, 2009;Buccolieri et al, 2011Buccolieri et al, , 2009Gromke and Ruck, 2012Gromke, 2011;Gromke et al, 2008;Li et al, 2013;Moonen et al, 2013;Salim et al, 2011;Vos et al, 2013;Wania et al, 2012), qualitative and not only quantitative changes in the flow field were observed at the neighborhood scale. The qualitative changes indicate on the one side the necessity to perform studies at the urban neighborhood rather than at the street canyon scale and on the other side to account for avenue-trees, or more general all types of vegetation typical for built-up environments, in microscale urban air quality studies.…”

“…(Balczo et al, 2009;Buccolieri et al, 2009;Gromke and Ruck, 2012Gromke, 2011;Gromke et al, 2008;Li et al, 2013;Moonen et al, 2013;Salim et al, 2011). The reason for this is the flow resistance of tree crowns which slows down the wind and reduces the air exchange between the street canyon and surrounding.…”

Prediction of Wind Environment and Indoor/Outdoor Relationships for PM2.5 in Different Building–Tree Grouping Patterns

“…The grid in the target area consisted of elements with X min = Y min = Z min = 0.028H, and grid independence evaluated by the grid convergence index (GCI) showed that the GCI value was 2.53% (less than 5%), demonstrating that the used grid arrangement was fine enough [61]. Boundary conditions were set according to wind tunnel experiments [62].…”

Spatial Evaluation of Multiple Benefits to Encourage Multi-Functional Design of Sustainable Drainage in Blue-Green Cities

“…However the installation of street trees must be carefully designed and maintained to enhance, and not worsen, local air quality. Studies have shown that adding vegetation, especially trees, to hot-spots of air pollution (e.g., poorly ventilated areas along streets) can increase pollutant concentrations by further restricting air flow and exchange [38]. Conroy has shown the strategic placement and design of green infrastructure is necessary to enhance PM 10 capture and to prevent the possibility of green infrastructure inadvertently worsening air quality [39].…”

“…When trees are planted within street canyons, careful management of the crowns, including pruning, is recommended to promote air flow [40]. Recommendations include: adequate space between the tree crowns and nearby walls, tree height shouldn't exceed the height of nearby buildings, and planting fewer trees is better [38,41]. For green streets within street canyons, designers may want to limit the number of trees in bioretention cells and suspended pavement systems and focus on using stormwater measures based on herbaceous vegetation (e.g., bio-retention cells and bio-swales).…”