Mass transfer velocity and momentum vertical exchange in simulated deep street canyons

A box model to simulate mass transfer inside deep street canyons and with atmospheric flow above is introduced and discussed. Two ideal deep street canyons with aspect ratios of 3 and 5 (the aspect ratio being the ratio between building height and street width H/W) are considered. This range of aspect ratios found in many densely populated historical centres in Mediterranean cities as well as in other cities around the world potentially creates high air pollutant concentration levels. Our model is based on a combination of analytical solutions and computation fluid dynamics (CFD) simulations using carbon monoxide (CO) as a tracer pollutant. The analytical part of the model is based on mass transfer velocity concepts while CFD simulations are used both for a preliminary validation of the physical hypothesis underlying the model (steady-state simulations) and to evaluate the concentration pattern with time (transient or wash-out simulations). Wash-out simulation curves were fitted by model curves and mass transfer velocities were evaluated through a best fitting procedure. Upon introducing into the model the contribution of traffic-produced turbulence the modelled CO concentration levels became comparable with those obtained in real world monitoring campaigns. Mass transfer rate between the canyon and the above atmosphere was then expressed in terms of an overall mass transfer velocity, that directly allows the evaluation of the mass transfer rate between the bottom volume of the canyon (pedestrian level) with the above atmosphere. Overall mass transfer velocities are reported as a function of the operating conditions studied (H/W = 3 – 5 and wind velocity = 2 – 8 m s-1). Finally, a simple expression is reported for determining pollutant concentrations at the pedestrian level based on the overall mass transfer velocity defined.