Abstract
Numerical experiments involving street canyons of varying aspect ratio with traffic-induced pollutants (PM2.5) and implanted trees of varying aspect ratio, leaf area index, leaf area density distribution, trunk height, tree-covered area, and tree planting pattern under different wind conditions were conducted using a computational fluid dynamics (CFD) model, ENVI-met. Various aspects of dispersion and deposition were investigated, which include the influence of various tree configurations and wind condition on dispersion within the street canyon, pollutant mass at the free stream layer and street canyon, and comparison between mass removal by surface (leaf) deposition and mass enhancement due to the presence of trees. Results revealed that concentration level was enhanced especially within pedestrian level in street canyons with trees relative to their tree-free counterparts. Additionally, we found a dependence of the magnitude of concentration increase (within pedestrian level) and decrease (above pedestrian level) due to tree configuration and wind condition. Furthermore, we realized that only ∼0.1–3 % of PM2.5 was dispersed to the free stream layer while a larger percentage (∼97 %) remained in the canyon, regardless of its aspect ratio, prevailing wind condition, and either tree-free or with tree (of various configuration). Lastly, results indicate that pollutant removal due to deposition on leaf surfaces is potentially sufficient to counterbalance the enhancement of PM2.5 by such trees under some tree planting scenarios and wind conditions
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Notes
LAI is the ratio of total leaf surface area to total ground area for an area with vegetation cover. It is a function of plant configuration such as shape, height, leaf density, and height of the crown
LAD defined as the “total one-sided leaf area (m2) per unit layer volume (m3) in each horizontal layer of the tree crown” in parameterization of vegetation
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Acknowledgments
The work described in this paper was jointly supported by the Guy Carpenter Asia-Pacific Climate Impact Centre, City University of Hong Kong (Project No. 9360126) and the Research Studentship of City University of Hong Kong. The authors also gratefully acknowledge the Laboratory of Building and Environmental Aerodynamics at the University of Karlsruhe/Germany for the provision of CODASC data used in this publication.
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Morakinyo, T.E., Lam, Y.F. Study of traffic-related pollutant removal from street canyon with trees: dispersion and deposition perspective. Environ Sci Pollut Res 23, 21652–21668 (2016). https://doi.org/10.1007/s11356-016-7322-9
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DOI: https://doi.org/10.1007/s11356-016-7322-9