Influence of trees on the dispersion of pollutants in an urban street canyon—Experimental investigation of the flow and concentration field

Abstract

Flow field and concentration measurements have been performed in an idealized model of an urban street canyon with one row of trees arranged along the center axis. The model was set up in an atmospheric boundary layer wind tunnel and the approach flow was directed perpendicular to the street axis. A line source embedded in the bottom of the street was used to release tracer gas for the simulation of traffic exhaust emissions. Trees with spherical crowns were modeled and positioned inside the street canyon, varying crown diameter, crown permeability, trunk height and tree spacing. Trafficinduced turbulence was simulated by rotating belts with thin plates. Concentrations were measured at the facades of the street canyon. For small tree crowns, only little changes in concentration were measured, however, increasing crown diameters led to increasing concentrations at the leeward street canyon wall associated with a reduction of local concentrations at the windward wall. For some cases, a variation of trunk height led to a modification of the concentration pattern on the walls. Increasing the tree spacing resulted in a noticeable concentration decrease. When compared to the situation with standing (but emitting) traffic, the traffic-induced turbulence by two-way car movements always contributed to a more homogenous concentration field inside the street canyon yielding to reduced mean concentration levels. r

Experimental simulation of air quality in street canyon under changes of building orientation and aspect ratio

“…Simoens et al 19,20 measured scalar dispersion of smoke released from a two-dimensional slot in a wall perpendicular to boundary layer flow and situated parallel to and midway between two square obstacles placed on the wall. Gromke and Ruck 21,22 carried out wind tunnel studies of the impact of avenuelike tree planting on flow fields and dispersion of traffic exhaust within urban street canyons.…”

Numerical Investigation on the Effect of Avenue Trees on PM2.5 Dispersion in Urban Street Canyons

“…Simulations have shown that wide streets and low building heights favored air ventilation and the removal of air pollutants within an isolated street canyon [23]. Furthermore, expanding tree crown diameters increased the particle concentration along the leeward wall of a street canyon and decreased local concentrations at the windward wall [24]. A dispersion numerical study revealed that the larger street canyon aspect ratio (H/W) corresponded to the smaller effect of trees on pollutant reduction regardless of tree morphology and arrangement, and lower air velocity resulted in more traffic-generated pollutants [25].…”

The Impact of Planting Trees on NOx Concentrations: The Case of the Plaza de la Cruz Neighborhood in Pamplona (Spain)

“…Our validation results have been presented in Krayenhoff et al [37] and Santiago et al [28]. In addition, the current modelling of urban vegetation was evaluated by using CODASC wind-tunnel dataset (COncentration DAta of Street Canyons -www.windforschung.de/CODASC.htm) [49,50] by simulating a street canyon with and without vegetation. Two different tree porosities were used with a pressure loss coefficient (λ) of 80 and 200 m −1 (0.53 and 1.33 m −1 at full scale).…”