Effects of Building−roof Cooling on Flow and Distribution of Reactive Pollutants in street canyons

The effects of building−roof cooling on flow and dispersion of reactive pollutants were investigated in the framework of flow dynamics and chemistry using a coupled CFD−chemistry model. For this, flow characteristics were analyzed first in street canyons in the presence of building−roof cooling. A portal vortex was generated in street canyon, producing dominant reverse and outward flows near the ground in all the cases. The building−roof cooling increased horizontal wind speeds at the building roof and strengthened the downward motion near the downwind building in the street canyon, resultantly intensifying street canyon vortex strength. The flow affected the distribution of primary and secondary pollutants. Concentrations of primary pollutants such as NOx, VOC and CO was high near the upwind building because the reverse flows were dominant at street level, making this area the downwind region of emission sources. Concentration of secondary pollutant such as O3 was lower than the background near the ground, where NOX concentrations were high. Building−roof cooling decreased the concentration of primary pollutants in contrasted to those under non−cooling conditions. In contrast, building−roof cooling increased O3 by reducing NO concentrations in urban street canyon compared to concentrations under non−cooling conditions.