The nitrogen oxide gases are subject to fast chemical reactions. The time scales characterizing these reactions are of the order of tens of seconds and thus, comparable with residence time of pollutants in street canyons. The objective of the present study is the performance of small-scale computations for an urban 3D by using a CFD (computational fluid dynamics) code (ANSYS-CFX) to provide steady state wind and pollutant concentration fields. A fast chemistry module simulating chemical reactions taking place within street canyons right after traffic pollutants are emitted is implemented to the model in order to assess the concentrations of NOx and ozone in space. Circulations created by the city itself and that affect pollutant dispersion are accounted for and hotspot concentrations that depend on street canyon scale effects are also computed. Reactive pollutant dispersion in the street canyon has been numerically investigated using CFD numerical simulation by means of k-ε turbulence model and transport equations for NO, NO2, and O3 with simple photochemistry. The area emission source is divided to sub-domains describing NOx vehicles’ emissions. The Background of O3 was specified, and the gases were allowed to mix and react.