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.