CFD simulation of pollutant dispersion in a street canyon with realistic car sources: The potential of green infrastructure configurations

In both experimental and numerical studies of urban air quality, idealized sources are commonly employed to reproduce the traffic emissions, and green infrastructures (GI) are widely used as an approach for pollution mitigation. However, previous studies have found that (i) idealized sources may not be adequate to reproduce the pollutant emitted from realistic car sources (RCS), and (ii) GI may not necessarily have a positive impact on air quality in street canyons. The goal of this study is to investigate the impact of different GI configurations on pollutant concentrations in an urban street canyon with RCS by computational fluid dynamics (CFD) simulations. First, CFD results obtained from the scale-adaptive simulation (SAS) in terms of mean velocity and concentration are validated with wind-tunnel (WT) data for idealized line sources with vegetation. Next, SAS simulations are performed on a full-scale street canyon with RCS to evaluate the effectiveness of different GI configurations in reducing pollutant concentration. These GI configurations include a different number and arrangement of hedgerows, also combined with trees deployed in the center of the canyon. The SAS results indicate that the number of hedgerows (from one to three) can reduce pollutant concentration by 1.8% to 13.4% compared to a street canyon without GI. Hedgerows are generally more effective in reducing the pollutant concentration at the pedestrian level of a street canyon than combinations of trees and hedges. This study can support urban planners in enhancing urban green spaces and reducing pollutant levels in urban environments.