# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 4 Conclusions  
Solving numerically, via CFD, the governing equations that describe the dispersion of air pollutant emitted by the transit of vehicles in roads in the near-ground atmosphere under neutral conditions, the effects of the presence of solid barriers on the nearby resulting concentration were systematically evaluated. The use of dimensionless numbers ensured that variations observed on the obtained downwind concentration profiles (C*) were attributed solely to variations on the barrier size, geometry, or location considered. It was found that the resulting near road CFD model (NR-CFD model) produce results that agree (\(R^{2}>0.96\)) with the experimental results obtained by the NOAA in 2008 observing the SF\({}_{6}\) concentrations at several distances downwind a line source of SF\({}_{6}\), for the cases of a 6m straight barrier and for the case of no barrier. It was observed that the experimental and modeled \(C^{*}\) obtained under different barrier geometry conditions are highly correlated (\(R^{2}>0.86\)) to the \(C^{*}\)_ref_ (C* observed under no barrier conditions). The slope obtained from the correlation analysis was named as concentration ratio (C*/C*_ref_) and \(f_{r}=1-(C^{*}/C^{*}\)_ref_) as the barrier-induced concentration reduction factors. It was observed that, regardless of the barrier geometry, the downwind concentration reduction (\(f_{r}\)) increases with the barrier size and decreases with distance to the road (\(d^{*}\)). They met their maximum performance when they are located at the road edge (\(d^{*}=0\)). Under all circumstances, the quarter ellipse oriented toward the road exhibited the best performance. However, the outstanding performance of this barrier configuration becomes negligible compared to other geometries when the barrier size is larger than \(h^{*}>0.57\) (6 m for a 10.5 m road width), and the barrier are very near to the road edge. Under these circumstances (\(d^{*}=0\) and \(h^{*}>0.57\)), all barrier configurations reach an _fr_-98%. It was assumed that the manufacturing and installation costs of the barriers increase proportionally to their size and to the complexity of its geometry. Therefore, a barrier with a quarter ellipse geometry and a 45\({}^{\circ}\) inclined straight barrier cost 1.5 and 1.2 times the cost of a vertical straight barrier, respectively. Then, it was found that the barrier with the best-dispersing capacity per unit cost has a geometry of a quarter ellipse oriented toward the road with a height of \(h^{*}\)-0.155 (\(h\)-1.62 m for a road 10.5 m wide) and located at the road edge. This barrier reduces 76% of the concentration observed near the road without any barrier. The results obtained in this study may aid in the decision-making process of industrial, research, or government institutions for the selection and design of solid barriers as pollutant mitigating infrastructure near any area-source of pollutants, located over a flat area, such as leakages from pipes, particle resuspension due to wind erosion, open burning, etc. It is important to note that even though results are valid for any non-chemically reactive gas-phase pollutant, they are valid only for the case of emission sources located on flat terrain with the barrier as the only obstacle of the wind free flow. Barrier edge effects in the crosswind direction are not considered in this study. Additional work is required to confirm that results obtained in this study, when expressed in terms of dimensionless parameters, are independent of the road width since said parameter was not systematically varied in this work. **Author Contributions:** Conceptualization, J.I.H. ; methodology, J.I.H. and O.D.L.M. ; simulation, J.E.A. and [PERSON]; validation, J.E.A. ; formal analysis, J.I.H., J.E.A. and [PERSON]; literature review, J.E.A. and C.H.L. ; data curation, J.I.H. ; writing--original draft preparation, J.I.H. and J.E.A. ; writing--review and editing, J.I.H., J.E.A. and O.D.L.M. ; supervision, J.I.H. and O.D.L.M.