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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 κ-ϵ” 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 Bldg11, 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 at the pedestrian level is analysed as a function of the …