Influence of obstacles on urban canyon ventilation and air pollutant concentration: An experimental assessment

Air pollution in cities, intensified by vehicular traffic emissions and reduced ventilation, poses a significant health risk. Obstacles, like solid or vegetation barriers, are being considered as strategies to reduce pollution exposure for pedestrians and nearby residents in street canyons. This study utilises wind tunnel experiments, to simulate a typical urban canyon with street intersections on both sides, in a 1:200 scale and an H/W = 0.5, positioned perpendicular to the free stream wind flow. A passive scalar representing a vehicular pollutant is released along the length of the street canyon. Concentration measurements inside the canyon are performed to determine the effect of parked cars, boundary walls, hedges and trees on pollutant concentration exposure for pedestrians on the sidewalk. Results show that one circulating vortex is generated within the canyon, driving the pollutant to accumulate along the leeward (upwind) wall. Tightly parked cars, boundary walls and hedges, placed along the sidewalk near the leeward wall, can reduce pedestrian pollutant exposure by 15 %, 23 % and 11 % respectively along this sidewalk. Attributes such as obstacle height, surface roughness and porosity play a key role in their performance. However, trees, when placed in the same area, increase pedestrian pollutant exposure by 51 % and 17 % under dense and sparse tree arrangements, respectively. While a broader analysis that considers the variability of vegetation attributes (e.g., porosity, stand density) is desirable, this study remains crucial for validating numerical simulations and suggesting optimal urban measures to reduce pollution exposure for citizens.