The Pollutant Removal Capacity of an Urban Street Canyon and its Link to the Breathability and Exchange Velocity

The rate of removal of pollutants within simplified urban street canyons is investigated using air flow fields obtained from Particle Image Velocimetry (PIV) experiments and Computational Fluid Dynamics (CFD) simulations of the resulting pollutant dispersion. In particular, the link between the pollutant removal capacity and the air-flow exchange velocity, a characteristic velocity widely used to characterize this pollutant removal capacity, is examined. First, velocity fields through a series of homogeneous urban street canyons with flat roofs are obtained using ensemble-averaged PIV measurements as obtained within water channel experiments. The experimental results for the fully-developed street canyon air velocity field are used to drive numerical simulations of turbulent pollutant dispersion from a pollutant release source located within the street canyon. The rate of removal of pollutant from the canyon is deduced from the numerical simulations and analyzed in terms of mean convective and turbulent exchange mechanisms. It is found that rate of pollutant removal as expressed through a pollutant-exchange velocity is dominated by the turbulent pollutant diffusion in the street canyon case. It is found that the air-exchange velocity cannot be used as a representative measure for pollutant removal, since it does not take into account all physics involved in the removal process. It is only a measure for the convective pollutant flux when the pollutants are uniformly distributed in the control volume.