The impact of solar radiation and street layout on pollutant dispersion in street canyon

The investigation was carried out to reveal the impact of solar radiation on pollutant dispersion in different urban street layouts using computational fluid dynamics (CFD) technique. It is indicated that the street geometry has a strong influence on the wind field and pollution dispersion patterns. For example, when a stepdown configuration is applied, there are two counterrotating vortices in the canyon, a higher concentration appears on the windward side, and the pollution levels within the canyon area increase. Furthermore, it is found that the heating from surfaces leads to a strong buoyancy force close to the surfaces that receive direct solar radiation. This thermally induced flow is combined with mechanically induced flow, and affects the flow field and pollutants dispersion. The upward buoyancy flux divides the flow structure into two counterrotating cells at symmetrical configuration or a small vortex at the lower windward building corner is generated at stepup notch when the windward side is heated. This effect causes the pollutant concentration in the street canyon to increase. In the stepdown notch, when the sun shines on the windward side of the building and the ground level, the intensity of the lower vortex is increased. Pollutant concentrations in the street canyon decrease. If the sun shines on the leeward side of the building, the upper vortex is enlarged downward and pollutant concentrations in street canyon increase. The relative influence of thermal radiation can be estimated by the parameter Gr/Re2.