Analysis of local scale tree–atmosphere interaction on pollutant concentration in idealized street canyons and application to a real urban junction

• Published in: Atmospheric environment (1994) 2011-03, Vol.45 (9), p.1702-1713
• Main Authors: Buccolieri, Riccardo, Salim, Salim Mohamed, Leo, Laura Sandra, Di Sabatino, Silvana, Chan, Andrew, Ielpo, Pierina, de Gennaro, Gianluigi, Gromke, Christof
• Format: Article
• Language: English
• Subjects: Applied sciences; Aspect ratio; atmospheric chemistry; Atmospheric pollution; buildings; CFD simulation; City planning; Computational fluid dynamics; Dispersions; Exact sciences and technology; fluid mechanics; laboratory experimentation; Mathematical models; Pollutants; Pollution; porosity; prediction; spring; stand density; Street canyon; Street canyons; tree crown; Trees; Urban vegetation; Vegetation; Wind direction; Wind tunnel measurement; winter
• ISSN: 1352-2310; 1873-2844
• DOI: 10.1016/j.atmosenv.2010.12.058

This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant concentration in idealized street canyon configurations by means of laboratory experiments and Computational Fluid Dynamics (CFD). These analyses are then used as a reference modelling study for the extension a the neighbourhood scale by investigating a real urban junction of a medium size city in southern Italy. A comparison with previous investigations shows that street-level concentrations crucially depend on the wind direction and street canyon aspect ratio W/H (with W and H the width and the height of buildings, respectively) rather than on tree crown porosity and stand density. It is usually assumed in the literature that larger concentrations are associated with perpendicular approaching wind. In this study, we demonstrate that while for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration, in presence of trees the expected reduction of street-level concentration with aspect ratio is less pronounced. Observations made for the idealized street canyons are re-interpreted in real case scenario focusing on the neighbourhood scale in proximity of a complex urban junction formed by street canyons of similar aspect ratios as those investigated in the laboratory. The aim is to show the combined influence of building morphology and vegetation on flow and dispersion and to assess the effect of vegetation on local concentration levels. To this aim, CFD simulations for two typical winter/spring days show that trees contribute to alter the local flow and act to trap pollutants. This preliminary study indicates that failing to account for the presence of vegetation, as typically practiced in most operational dispersion models, would result in non-negligible errors in the predictions. ► Trees influence pollutant concentration distribution in urban street canyons. ► Failing to account for the presence of trees may result in critical errors in model predictions. ► Building morphology and vegetation have to be analysed together. ► Analyses of data from various sources (CFD modelling, wind tunnel and monitoring stations) are necessary to produce a comprehensive picture of complex flow and dispersion phenomena within urban areas.