# Modelling Pollutant Dispersion in Urban Canyons to Enhance Air Quality and Urban Planning
## 4 Discussion
### Practical Application  
The methodology outlined in this study, along with its findings, has been applied to a real-world scenario to predict the concentration of a pollutant species in urban streets. This approach is significant as it provides valuable insights and guidance for urban designers in developing street layouts with morphologies that effectively mitigate pollutant accumulation and help to identify the existing streets that can affect inhabitants' health under some conditions. A study was conducted on five streets in the city centre of Seville. The selection of these streets was driven by two key factors: their residential nature and the presence of relatively narrow ARs. However, these streets also feature a high volume of traffic, a phenomenon attributable to their central location within the urban landscape. Additionally, a proportion of traffic is diverted to these areas due to the pedestrianisation of the surrounding avenues and main streets, further contributing to their high traffic density. Different external conditions will be studied to assess their response towards external changes. The streets and their characteristics are listed in Table 2.  
The selected streets present an AR ranging from 2 to 3, and cars are allowed to pass through them, incrementing pollution in the lower layers of the streets, which can lead to excessive pollutant accumulation under certain external air conditions. In this case, the emissions of CO derived from the traffic are going to be examined, as this is one of the most common pollutants derived from vehicles which has a high impact on human health. To verify if the contamination levels could be dangerous to inhabitants' health, the scale of risk suggested by the World Health Organization (WHO) was considered. This risk scale establishes the following limits for CO concentration in streets to prevent health issues caused by pollution exposure [42]:  
* A limit of 10 mg/m\({}^{3}\) for a time exposure inferior to 8 h;
* A limit of 35 mg/m\({}^{3}\) for a time exposure inferior to 1 h;
* A limit of 100 mg/m\({}^{3}\) for a time exposure inferior to 15 min. Two pollution scenarios were defined--a base case scenario (or low-traffic scenario) and a high-traffic scenario--since there are no official data available on that specific pollutant for the streets selected. The first scenario pretends to represent the most probable pollution situation in a low congestion residential area. For that, an external air wind velocity is set at 2 m/s, a reasonable value for low winds according to the Beaufort scale [58], and an average CO emissions rate of 10\({}^{-7}\) kg/m\({}^{3}\cdot\)s is considered in reference to the available literature [59]. On the other side, the high-traffic scenario was set considering a CO emissions rate of 10\({}^{-5}\) kg/m\({}^{3}\cdot\)s, which has been validated in other studies of outdoor urban ventilation with this pollutant species [60; 61; 62]. The equations used are detailed in Section 2.1, and the results are summarised in Table 3.  
In the low traffic case, where the initial levels of pollution are considered negligible, it is evident that all streets are below the minimum concentration limit stipulated by the World Health Organization (WHO) to ensure public health is not compromised by prolonged exposure. For the first scenario, where a low value of pollution source is considered, all the tested streets are below the first limit proposed by the WHO, which implies that there is a minimum risk for people's health. However, in the second scenario, there has been an increase in \begin{table}
\begin{tabular}{c c c c} \hline
**Street Name** & \(\overline{\mathbf{H}}\)**(m)** & \(\overline{\mathbf{W}}\)**(m)** & **AR** \\ \hline Calle Zaragoza & 8.10 & 3.20 & 2.53 \\ \hline Calle de Castelar & 11.75 & 5.60 & 2.09 \\ \hline Calle Padre Marchena & 7.50 & 3.00 & 2.50 \\ \hline Calle Gamazo & 8.50 & 3.50 & 2.43 \\ \hline Calle Doná Guiomar & 8.40 & 3.00 & 2.80 \\ \hline \end{tabular}
\end{table}