# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 1 Introduction
Recently, [PERSON] and [PERSON] [20] studied the combined effect of the barrier and vehicle shape on the pollutant concentration over the road. They found that the flow recirculation zone was enhanced, increasing the road concentration levels. Soon after, [PERSON] et al. [21] conducted a CFD study on the effect of vertical barriers on pollutant reduction under various atmospheric conditions, concluding that concentration reduction rates tend to increase from ~30 to ~60% as the atmospheric conditions become stable. [PERSON] et al. [22] developed the near road CFD (NR-CFD) model, which simulates the dispersion of solid and gas-phase pollutants, over flat terrain, within a neutrally stratified atmospheric boundary layer, and includes the effect of turbulence and deposition on particle size distribution downstream. They found that pollutant concentration downwind the road exhibits a single profile when expressed in terms of three dimensionless numbers: normalized concentration, normalized distance to the road, and emission speed ratio. Recently, [PERSON] et al. [23] advanced this work and found that those pollutant concentrations vs.  
distance to the road profiles are correlated to the ones obtained under different atmospheric stability conditions and reported the correction factor for each stability condition. In the majority of the above-described studies, the barrier height was found to be the most significant influencer on the pollutant concentration, and therefore most of the studies focused on the effects of the barrier geometry only considered variations on the barrier height (Table 1). The exceptions to this trend were: (i) [PERSON] et al. (2019), who simulated the effect of using a barrier with a tilted section (45\({}^{\circ}\) from the vertical) on the dispersion of liquified natural gas (LNG) leaks from a straight pipe, finding said geometry to be the most effective at reducing the gas cloud size; (ii) [PERSON] et al. (2019), who simulated the presence of a mesh barrier with holes of various sizes finding that the most effective results are obtained locating the barrier near to the road edge. It is important to note that in the aforementioned studies, barrier variations were evaluated independently. Based on the reviewed references, the following areas of opportunity for improving the design of barriers for the mitigation of air pollution near roads were identified: (i) Consider the potential synergetic effects of the combined single-barrier variations among geometry, size, position, vegetative cover, and photocatalytic paints, among others; (ii) Quantify the effects of the barrier under different external conditions such as emission rate, wind speed, atmospheric stability conditions, among others; Include barrier cost in the design process; Report results in a generalized format in such a way that they can be used for applications beyond the specific modeled scenarios. Aiming to address the needs stated above, the objective of this work is to quantify the effects of barrier geometry, size, and location on the dispersion of pollutants downwind an emission source, such as highway-type roads, located on flat regions in order to find the barrier that best reduces pollutant concentration at the lowest cost. In the process of pursuing this objective, the following main contributions to new knowledge were found: (i) By systematically using the calibrated NR-CFD model, the effects of the shape, size, and location of near-road barriers on the resulting ground pollutant concentration downwind a surface-emission source were quantified. Results were expressed in the way that they can be used for mitigating air pollution near any linear o area source of air pollutants; (ii) The ground pollutant concentrations downwind an emission source under any barrier geometry are highly correlated to the concentrations observed when there is not any barrier to the free wind flow.