Based on the Reynolds-averaged Navier-Stokes (RANS) method, a modified standard k-ϵ turbulence model with a source term rooted on Monin-Obukhov similarity theory was used to investigate the effects of temperature stratifications on the flow field and pollutant dispersion in 3D street canyons. The results showed that airflow and pollutant transport was highly dependent on temperature stratifications. The vortex velocity in the canyon varied with temperature stratifications. With the more unstable conditions, the centre of the vortex was closer to the ground, and the intensity and turbulent kinetic energy of the eddy increased. Because the source of pollution was located on the ground, the pollutants migrated to the leeward side of the building with the eddy and then moved upstream or downstream with the airflow. Under neutral conditions, the pollutants migrated to the leeward side first with the eddy in the canyon and then mainly migrated downstream with the airflow but rarely migrated to the upstream street canyons. However, under unstable conditions, the pollutant concentration in the upstream street canyons also increased, mainly because the intensity of the vortex increased, which made the pollutant easier to transport upstream. According to the results of the pollutant flux analysis, the turbulent fluxes on the top plane of the buildings were positive and increased significantly with the more unstable conditions, indicating that turbulent fluxes play a positive leading role in the dispersion of pollutants. The transverse transport of the pollutants was mainly dominated by convective motion, and turbulent transport was relatively minor.
Keywords: Numerical simulation; pollutant dispersion; unstable temperature stratification; urban street canyons; Monin–Obukhov similarity theory.