# Modelling Pollutant Dispersion in Urban Canyons to Enhance Air Quality and Urban Planning
## 4 Discussion
### Impacts on Air Quality  
The study of the airflow pattern through a parametric study allowed for the definition of the regime of typical street morphologies in the common range of Reynolds numbers found in cities. Based on the results among all the cases studied, streets with an AR \(<\) 2.5 present a single vortex for the entire Reynolds number range studied, which implies a higher ventilation efficiency since pollutants do not accumulate in the lower part of the street, which coincides with the area occupied by pedestrians. However, as the aspect ratio increases, new vortices start to appear for different Reynolds values, reducing the purging flow rate value at the roof level. This relation between the formation of new vortices and the reduction in the pollutant dispersion can be stated in Figure 8, where it is possible to observe notorious curve slope changes. From AR = 2.5 to AR = 3.5, the multi-vortex regime causes the ventilation efficiency to decrease, since the PFR at the roof level is greatly reduced for all the Reynolds number values studied. This reduction in pollutant dispersion of the whole street in a multi-vortex regime is dangerous since it implies that the pollutants in the street are not properly removed.  
Figure 10: Purging flow rate results in the bottom subvolume when there are three recirculations.  
This situation is even more critical for street configurations with an AR > 3.5 since three or more vortices appear for the whole Reynolds number range studied. The PFR values for the case of the bottom vortex, which coincides with the pedestrian height, are significantly small, which amplifies the risk of accumulation of pollutants and harmful particles. This can result in poor air quality in urban environments and can increase the probability of citizens developing diseases due to pollution accumulation.  
These results highlight the utility of this analysis as it provides a preliminary guide for the planning of new urban environments regarding both main parameters of this phenomenon, the aspect ratio of streets and the expected Reynolds number regime. With these data, it is known which street configurations ventilate naturally and can ensure a low risk of pollutant accumulation. In addition, streets with high aspect ratios and low Reynolds are the streets where pollution issues must be addressed to enhance the quality of life of the citizens.  
This research provides several valuable contributions to the analysis of pollutant dispersion efficiency for various urban layouts and external wind conditions. The parametric study conducted in this investigation is of essential importance, as it ensures the substantial applicability of the results obtained. The extant literature on this subject is typically constrained to a specific urban canyon or a rather limited range of aspect ratios. However, this study has pioneered a novel approach by encompassing a diverse spectrum of street geometries, in addition to external conditions modelled through the Reynolds number.  
Furthermore, in this study, the relationship between the vortices formed inside an urban canyon and the pollution concentration has been investigated for different conditions for the first time. This study demonstrated that the presence of lower vortices in the streets makes ventilation of the streets difficult and provides a novel approach to determining PFR values not only at the roof street level, but also for the sub-volumes formed by the vortices.  
The results of this study can be used to proactively address excessive accumulation of health-damaging pollutants on specific streets. This accumulation arises from a combination of the AR, Reynolds number, and pollutant generation rate parameters which causes a decrease in ventilation performance in these areas. The subsequent case study has been conducted to illustrate the application of this research to a real-life case scenario considering the developed framework.