Pollutant dispersion and thermal effects in urban street canyons

A numerical model has been built to simulate the small-scale atmospheric flows within the urban canopy, based on the lower atmosphere classical assumptions and the ''standard k-epsilon'' two-equation turbulence model. This model is used here to study the flows and vertical exchanges of pollutants within the street and at the interface with the atmospheric layer above the roofs, in the asymptotic case of infinitely long street canyons. A thorough study of the influence of the street geometrical aspect ratio leads to a refinement of the flow separation in three regimes, popularized by Oke (1988, Energy Bldg 11, 103-113), taking into account not only the influence of the buildings on the flow of the surface layer just above the roofs but also the structure of the recirculating flow within the street. The time evolution of pollutant concentration within the street canyon and al the pedestrian level is analysed as a function of the geometry and pollutant doses are presented. The number and arrangement of vortex structures within the street canyon largely influence the vertical exchange rates. A preliminary study shows that the differential heating of street surfaces can largely influence the flow's capability to transport and exchange pollutants. In fact, differential heating can even shift the in-street flow structure from one regime to another, e.g., from a one-vortex flow to a flow with several contra-rotative vortices. (C) 1996 Elsevier Science Ltd