# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 3 Results and Discussion
### Effect of the Barrier Geometry and Size on Ground Pollutant Concentration
This qualitative result agrees with previous experimental and analytical results reported in several authors (Table 1). Aiming to quantify the reduction effects, we notice that the concentration profiles, near to the road seem to be correlated to the concentrations obtained without any barrier (Figure 7a,b,f). Aiming to confirm these hypotheses, results for the downwind concentrations obtained for the case of no-barrier were selected as the base case of comparison, and correlation analyses with the results obtained under the other scenarios were carried out. Results confirmed this hypothesis. Figure 8a shows that near the barrier, in the downwind region (\(0.68<x^{*}<5\), region with the largest concentrations), pollutant concentration obtained with different barrier geometries are highly correlated (\(R^{2}>0.86\)) with the one obtained without any barrier. For the case of the vertical barrier, a slope of \(\sim\)0.087 was obtained in the correlation analysis, which means that the presence of the 6 m straight vertical barrier leads to downwind concentrations \(\sim\)91% lower than the observed without any barrier. It also means that the presence of the barrier does not generate peaks of pollutant concentration downwind the barrier. The concentration always tends to reduce, following the same trend that for the case without the barrier. The slopes of each correlation analysis were named as the ground concentration ratio (\(C^{*}/C^{*}_{n\!f}\)) and \(f_{r}\) (Equation (8)) as the barrier-induced concentration reduction factors. \[f_{r}\ =\ 1\ -\ \frac{C^{*}}{C^{*}_{n\!f}} \tag{8}\]  
Figure 8: Quantification of the effects of the barrier geometry and size on the ground-level pollutant concentration observed in the region downwind the barrier (\(x^{*}>0.68\)). (**a**) comparison of the concentrations observed with the barrier geometries with respect to the reference concentrations (case without any barrier). (**b**) Concentration reductions as a function of the barrier normalized size (\(t^{*}\)) for some of the barrier geometries considered. Barriers were located at \(d^{*}=0.57\). Figure 8b shows that when a 6 m quarter-ellipse and a 6 m inclined barrier oriented toward the road were used, the pollutant concentrations exhibited \(f_{r}\)\(\sim\)97% and \(f_{r}\)\(\sim\)96%, respectively. Reductions obtained with the same barriers, but oriented against the road, were \(f_{r}\)\(\sim\)88%, \(f_{r}\)\(\sim\)92%, and \(f_{r}\)\(\sim\)84%, for the quarter ellipse, inverted quarter ellipse, and inclined barrier, respectively. Previous results demonstrate that for 6 m barrier sizes, located at 6 m from the road (\(d^{\star}\) = 0.57) the quarter ellipse oriented toward the road exhibits the best performance dispersing pollutants near the road. Then, the effect of the barrier size was studied. Figure 8b confirmed that, for any barrier geometry, \(f_{r}\) increases with barrier size. However, reductions in pollutant concentration (\(f_{r}\)) do not vary linearly with the barrier size. This figure also confirmed that the quarter ellipse barrier exhibits the best performance dispersing pollutants for all barrier sizes. Nonetheless, this superior performance becomes negligible among barrier geometries for sizes greater than 0.6 (\(h^{\star}\) > 0.6).