The Role Of Urban Street Canyon Geometry In The Pollutant Removal Capacity

The ever-increasing urbanization is leading to an unprecedented increase in the use of various means of transport and overall energy consumption which unavoidably results into high levels of greenhouse gases and pollution emissions. Persistent exposure to urban air pollution has been shown to be linked to a significant risk of health. In particular, pedestrians in urban street canyons are exposed to a considerable risk, since a significant part of the emission sources are due to traffic and are primarily found at the near-ground or street level. Dispersion of pollution within an urban street canyon and thereby exposure of civilians is determined substantially by the air flow field within the street canyon and its capacity to remove pollutants from atop the rooftop levels and to replenish with fresh clean air. The pollutant removal capacity was recently investigated by Kubilay et al. (2016) for an idealized square street-canyon cavity, where different metrics were assessed. In this work, we investigate the pollutant removal capacity for a range of different urban street-canyon geometries frequently encountered in urban settings. The aim of this investigation is to determine the role that main geometric design of an urban street can play in the pollutant removal and thereby to what degree it can affect urban air quality and pollution exposure of pedestrians. The geometries we examine are: flat-roof canyons of different aspect ratios, a pitched-roof canyon, and a canyon with pilotis. First we present results from PIV experiments conducted inside a water channel facility in order to obtain the velocity flow fields in the different urban configurations; the results for the pitched-roof and pilotis are new in literature. Then, the experimental measurements for the air velocity field are used to drive numerical simulations of turbulent pollutant dispersion from a pollutant release source located within the street canyon; in particular, we use realistic source emission release scenarios and source locations derived from typical traffic patterns in the literature. The rate of removal of pollutant from the canyon is finally deduced from the numerical simulations and analyzed in terms of mean convective and turbulent exchange mechanisms for all the urban street canyon configurations.

Reference:

Kubilay, Neophytou, Matsentides, Loizou, Carmeliet (2016). The pollutant removal capacity of urban street canyons and its link to the exchange velocity. Submitted to the Science of The Total Environment (under review).