Numerical study of reactive pollutants diffusion in urban street canyons with a viaduct

In this paper, the influences of the ambient wind speed and the height and width of a viaduct in a 2-dimensional street canyon on the diffusion of reactive pollutants emitted by motor vehicles were investigated using computational fluid dynamics (CFD) method. Pollutants were treated as reactive by including a NO-NO2-O3 photochemical reaction mechanism in the simulation. The Reynolds-averaged Navier-Stokes (RANS) k–e turbulence model and the discrete phase model were used to simulate the airflow movement and the concentration distribution, respectively, of the reactive pollutants in the street canyon. Three indices, i.e., the chemical reaction contribution of NO (CRCNO), the chemical reaction contribution of NO2 $$\left({{\rm{CR}}{{\rm{C}}_{{\rm{N}}{{\rm{O}}_2}}}} \right)$$ , and the O3 depletion rate, were used to evaluate the relative importance of the photochemical reactions. It was found that the presence of a viaduct changed the flow field structure in the street canyon. The CRCNO and $${\rm{CR}}{{\rm{C}}_{{\rm{N}}{{\rm{O}}_2}}}$$ decreased from the windward side to the leeward side of the canyon. The maximum values of the CRCNO and $${\rm{CR}}{{\rm{C}}_{{\rm{N}}{{\rm{O}}_2}}}$$ were observed at the pollution source (x = 245 m) due to the influence of a clockwise vortex in the street canyon. As the height and width of the viaduct increased, concentration of the ground pollutants and the O3 depletion rate increased. The O3 depletion rate was much higher on the leeward side (90%) than on the windward side. The pollutant concentrations after the reaction were twice as high with the viaduct as without the viaduct when the viaduct height was the same as the building height. The viaduct had a significantly larger influence on the concentration of the reactive pollutants than the chemical reactions. The O3 depletion rate in the canyon and the pollutant concentrations decreased as the ambient wind speed increased, whereas the $${\rm{CR}}{{\rm{C}}_{{\rm{N}}{{\rm{O}}_2}}}$$ increased.