Three-dimensional pollutant dispersion in tree-lined urban canyons: Combined wind-tunnel and LES analysis

Vegetation is increasingly used in urban areas to improve microclimate and reduce pollutant exposure, yet its effect on pollutant dispersion within street canyons remains complex. This study combines high-resolution wind tunnel experiments and Large-Eddy Simulations (LES) using uDALES to provide a detailed three-dimensional characterization of airflow and pollutant concentration along the canyon. Special attention is given to the consistent scaling of velocity and scalar fields, using friction velocity and canyon geometry as reference quantities, and to the role of tree drag length in aligning the aerodynamic resistance of physical and numerical vegetation. The simulations reproduce key mean-flow structures, including large-scale recirculations, but tend to underestimate turbulent kinetic energy and local scalar fluxes. By jointly analyzing high-resolution wind-tunnel experiments and LES, we (i) confirm the spanwise and longitudinal concentration patterns observed experimentally, (ii) assess their sensitivity to the modeled tree drag, (iii) provide the first detailed experimental–numerical comparison of rooftop mean and turbulent mass fluxes, showing that bulk canyon ventilation exhibits no systematic dependence on tree number or drag intensity, and (iv) identify the specific strengths and limitations of each approach. This integrated analysis offers novel insights into the interplay between trees, turbulence, and boundary-layer forcing, informing strategies for modeling urban ventilation and pollutant dispersion in tree-lined streets.