The urban canopy formed by the buildings and the surrounding space has a profound impact on the processes in the atmospheric surface layer. For example, if we consider the low level dispersion of pollutants, we have competing mechanisms that increase or decrease the concentrations in the canopy as compared with flat terrain. The decreased average flow in the canopy leads to a decreased dilution and therefore to an increase of the concentrations. On the other hand, the increased turbulence intensity tend to decrease the concentrations. Depending on the situation, as reported by several authors, the concentrations in the canopy can either be lower or greater than the flat terrain value.

To investigate this and other issues, we have performed detailed, high resolution, numerical simulations of the flow in and above an idealized urban canopy, using a E-e turbulence closure model. The idealized canopy is formed by a regular array of obstacles, either cubes or billboards, with different geometrical layout (square and staggered). Different source position relative to the obstacles and different source heights are investigated. The simulations are performed at full scale, not laboratory scale.

The tridimensional fields around the obstacles are used to compute the cross stream averaged vertical concentration profiles. These profiles are then compared for different geometry to identify the effect of the canopy. Previously we had performed similar analyses with the mean velocity and turbulence and compared them with laboratory measurements and field data.

For the concentration, when we compare then with the flat terrain profile, we find that in all cases the values just above the canopy are higher, corresponding to better mixed profiles, up to 2 or 3 canopy height. Above that, the flat terrain concentrations are systematically higher, in accordance with a simple mass balance. In the canopy itself, however, we have mixed results with sometime lower and sometime higher concentrations, which are obtained for cube obstacles in a square configuration.