Article: Evaluating the role of vegetation on the ventilation performance in isolated deep street canyons Journal: International Journal of Environment and Pollution (IJEP) 2012 Vol.50 No.1/2/3/4 pp.98 - 110 Abstract: Vegetation is generally considered as an essential component for a sustainable city as it helps improve its microclimate environment and reduce greenhouse gases emissions. However, there is some evidence suggesting that adverse effects may be caused by tree-planting in shallow urban street canyons. Accordingly, we intended to investigate whether similar observations are found for different canyon geometries, i.e., canyons with aspect ratios higher than 1. Also, we attempted to investigate how the air quality varies with different tree planting densities by using CO as a representative for gas pollutant emitted from vehicle sources. The findings derived from CFD models indicate that tree planting elevates the CO levels inside canyons. Increasing the tree planting density can deteriorate the ventilation performance of canyons and to a greater extent for those with lower aspect ratios. However, the change in ventilation performance is less sensitive to a change in tree planting density at high planting densities. Accordingly, the decision in relation to tree planting in deep canyons should be weighed against other benefits like cooling effect, and energy saving. The findings should be of great value to urban planners in planning a sustainable city. Inderscience Publishers - linking academia, business and industry through research

Title: Evaluating the role of vegetation on the ventilation performance in isolated deep street canyons

Authors: Wai-Yin Ng; Chi-Kwan Chau

Addresses: Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong ' Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Abstract: Vegetation is generally considered as an essential component for a sustainable city as it helps improve its microclimate environment and reduce greenhouse gases emissions. However, there is some evidence suggesting that adverse effects may be caused by tree-planting in shallow urban street canyons. Accordingly, we intended to investigate whether similar observations are found for different canyon geometries, i.e., canyons with aspect ratios higher than 1. Also, we attempted to investigate how the air quality varies with different tree planting densities by using CO as a representative for gas pollutant emitted from vehicle sources. The findings derived from CFD models indicate that tree planting elevates the CO levels inside canyons. Increasing the tree planting density can deteriorate the ventilation performance of canyons and to a greater extent for those with lower aspect ratios. However, the change in ventilation performance is less sensitive to a change in tree planting density at high planting densities. Accordingly, the decision in relation to tree planting in deep canyons should be weighed against other benefits like cooling effect, and energy saving. The findings should be of great value to urban planners in planning a sustainable city.

Keywords: deep street canyons; tree planting; air quality; vegetation; pollutants dispersion; computational fluid dynamics; CFD; atmospheric dispersion modelling; air pollution; sustainable cities; microclimate; greenhouse gases; GHG emissions; shallow street canyons; urban areas; planting densities; vehicle emissions; ventilation performance; cooling effect; energy saving; urban planning.

DOI: 10.1504/IJEP.2012.051184

International Journal of Environment and Pollution, 2012 Vol.50 No.1/2/3/4, pp.98 - 110

Published online: 19 Nov 2014 *

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Title: Evaluating the role of vegetation on the ventilation performance in isolated deep street canyons

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