The Residence Time of Pollutants Emitted within the Urban Canopy Influenced by Street Canyon Geometry and Emission Conditions

The complex urban geometry and conditions of emission sources play an important role in affecting the dispersion of airborne pollutants emitted within the urban canopy layer. In particular, specific urban geometry may reduce the ventilation of pollutants and prolong the residence time, imposing threats to indoor and outdoor air quality and human health. In this study, we investigate the trapping effect of urban street canyons on the dispersion of passive airborne pollutants. The transport trajectories of pollutants are simulated using a coupled large-eddy simulation–Lagrangian stochastic model. The coupled model has been evaluated with water channel and wind tunnel experiments. We estimate the residence time of pollutants in scenarios with different domain forms (2-dimensional versus 3-dimensional street canyons), canyon aspect ratios, and emission conditions (source position and strength). Results show that the 3-dimensional street canyons with finite length along the lateral direction can significantly improve the ventilation effect and reduce the residence time as compared to the 2-dimensional case. On the other hand, the increasing depth of street canyons intensified the trapping effect, as suggested by the prolonged residence time. In addition, the residence time is also dependent on the position and strength of emission sources. The study shows the sensitivity of pollutant residence time and indicates the potential time that pollutants can participate in chemical reactions, highlighting the importance of urban geometry design in improving the air quality of street canyons.