Large-eddy simulation of reactive pollutant exchange at the top of a street canyon

Abstract

The exchange of reactive pollutants (NO, NO2, and O-3) at the top (roof level) of a street canyon are investigated using the parallelized large-eddy simulation model (PALM). The transport equations of NO, NO2, and O-3 with simple photochemical reactions are combined within the LES model for this study. NO and NO2 are emitted from an area source located near the canyon floor, and O-3 is included within the ambient air and inflow. A clockwise rotating vortex appears in the street canyon and transports NO, NO2, and O-3. NO and NO2 are transported along the ground and leeward wall and escape from the canyon at the roof level. O-3 enters the canyon at the roof level and is transported along the windward wall. The mean O-3 production rate is generally negative with large magnitudes at and near the roof level and near the windward wall. The chemical reactions reduce the mean NO and O-3 concentrations in the canyon by 31% and 84%, respectively, and increase the mean NO2 concentration in the canyon by 318%. The exchange of reactive pollutants at the roof level is significantly affected by small-scale eddies at the roof level and low- or high-speed streaks above the canyon. Air in the canyon with high NO and NO2 concentrations escapes from the canyon when low-speed air parcel appears due to small-scale eddies at the roof level or low-speed streak above the canyon. In contrast, air outside the canyon with a high O-3 concentration enters the canyon when high-speed air parcel appears because of small-scale eddies at the roof level or highspeed streak above the canyon. The time-lagged correlation analysis reveals that NO, NO2, and O-3 concentrations near the ground are affected by low- or high-speed streaks above the canyon but not significantly affected by small-scale eddies at the roof level.