Effect of street trees on turbulent exchange processes and flow circulation in urban street canyon: Application to air pollutant removal

This paper investigates the role of trees in modulating the pollutant removal from a real urban street canyon. An integrated methodology is designed to incorporate the aerodynamic effect of trees in modulating the inertial and thermal forcing that drives the in-canyon circulation. Contextually, the efficacy of exchange processes in mixing momentum and heat is evaluated at different levels within and above the street canyon using time scales and their ratios. By linking the exchange process with the relative circulation, the impact on pollutant removal from the canyon is evaluated. Trees are found to sustain efficient momentum exchanges under inertial forcing as the production of turbulent momentum by the crown roughness and flow channelling in the trunk region generates fast mixing within the canyon. The faster mixing at the surface than at the canyon rooftop provides the ideal conditions for efficient momentum exchange between the canyon and the atmosphere above. Conversely, trees inhibit thermal mixing through the shading effect of their crowns, leading to inefficient exchange rates under thermal forcing. The pollutant concentrations follow inertial forcing, which favours pollutant removal from the canopy, and thermal forcing, which favours stagnation, revealing the benefits and drawbacks for air quality of a common tree deployment in the urban environment.