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

The exchange of reactive pollutants (NO, NO2, and O3) 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 O3 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 O3 is included within the ambient air and inflow. A clockwise-rotating vortex appears in the street canyon and transports NO, NO2, and O3. NO and NO2 are transported along the ground and leeward wall and escape from the canyon at the roof level. O3 enters the canyon at the roof level and is transported along the windward wall. The mean O3 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 O3 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 O3 concentration enters the canyon when high-speed air parcel appears because of small-scale eddies at the roof level or high-speed streak above the canyon. The time-lagged correlation analysis reveals that NO, NO2, and O3 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.