# 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  
Next, the calibrated NR-CFD model was used to quantify the effects of the barrier size and geometry on the pollutant concentration upwind and downwind the barrier. As  
Figure 5: Contours of wind speed obtained by the NR-CFD model simulating the dispersion of pollutants emitted from a road located on flat terrain, under neutral atmospheric conditions, \(E=1\) g s\({}^{-1}\) m\({}^{-2}\) and \(U=7.65\) m/s. (**a**) Without any barrier. Under the presence of (**b**) a vertical straight barrier, (**c**) a quarter-ellipse barrier oriented toward the road, (**d**) a straight barrier inclined 45\({}^{\circ}\) toward the road, (**e**) a quarter-ellipse barrier oriented downward the road, (**f**), a straight barrier inclined 45\({}^{\circ}\) downward the road, and (**g**) an inverted quarter-ellipse barrier oriented downward the road. All barriers are \(h^{\star}=0.57\) in size and are located at \(d^{\star}=0.57\) from the road. Figure 6: Vertical profiles, at several distances downwind the barriers, of SF\({}_{6}\) mass fluxes developed by 6 m elliptical, vertical and inclined barriers located at 6 m downwind the road (\(d^{\star}=0.57\)), for the case of neutral atmospheric conditions, \(E=1\) g s\({}^{-1}\) m\({}^{-2}\) and \(U=7.65\) m s\({}^{-1}\). previously found by [22], studying the dispersion of pollutants near roads for the case without barriers, when the downwind concentrations are expressed as C* vs. \(x^{\star}\), the resulting profile is unique. It is independent of the parameters such as wind speed, pollutant emission rate, and even the nature of the pollutant substance. Hence, the barrier dispersion performance will be reported in terms of these dimensionless variables (Equations (3)-(6)). As stated before, the analysis begun by fixing the barrier location at a 6 m downwind the road (\(d^{\star}\) = 0.57). Figure 7a-f show the ground pollutant concentration as a function of distance to the road (C* vs. \(x^{\star}\)) obtained for the different barrier geometries considered and for different barrier size. It is important to highlight that even though the results shown in Figure 7a-f were obtained for SF\({}_{6}\), these results are valid for any gas-phase pollutant, under any condition of emission rate and wind speed, when the atmosphere is neutrally stratified. Figure 7: Pollutants ground concentration as a function of barrier geometry and size, for the case of a straight vertical barrier (**a**) in the upwind region (\(0<x^{\star}<0.57\)) and (**b**) in the downwind region (\(x^{\star}>0.68\)), a quarter-ellipse barrier oriented toward the road (**c**) in the upwind region and (**d**) in the downwind region, a 45\({}^{\circ}\) inclined straight barrier oriented toward the road (**e**) in the upwind region and (**f**) in the downwind region. Barriers are located at \(d^{\star}\) = 0.57 and its thickness covers the range 0.57 \(<x^{\star}<\) 0.67. Figure 7a shows that, for the case of the vertical barrier, the pollutant concentration in the upwind region (\(x^{*}<0.57\)) remains below the reference case independently of the barrier height. For the geometries oriented towards the road (Figure 7c,e), the presence of the barrier increases the pollutant concentrations in the upwind region with respect to the case of no barrier. This effect is attributed to the tendency of pollutants to accumulate in the upwind region for those specific barrier shapes. However, a well-defined pattern could not be identified in the increase in pollutant concentration with the barrier size. It is important to remember that the main purpose of the barrier is to protect the people living in the region downwind the barrier (\(x^{*}>0.68\)). Therefore, it is the region of primary interest in the design of road barriers. Figure 7b,d,f show that in this region, the presence of the barriers reduces the pollutant concentrations compared to the case of no-barrier (dotted lines) and that this reduction increases with the barrier size.