# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 1 Introduction
& Effects of vegetative and solid & & & & & \\  & barriers on the mitigation of & 2016 & ✓ & ✓ & ✓ & x \\  & particle matter pollutants & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Mesh barrier use to mitigate & & & & & \\  & ammonia dispersion near & 2016 & ✓ & ✓ & ✓ 2 [FOOTNOTE:2]Footnote 2: The results are presented in the tables in theExperimental studiesSeveral of the studies carried out related to barriers are experimental, and include solid and vegetative barriers [6]. One of the most well-known experimental studies is the one performed by the National Oceanic and Atmospheric Administration (NOAA) in Idaho Falls, USA [7] and reported by [PERSON] et al. [5], where sulfur hexafluoride (SF\({}_{6}\)) was dispersed into two different areas, one with a 54 m long and 6m high barrier and the other without any obstacle to the free flow. Results showed that in the presence of the barrier, the SF\({}_{6}\) concentration was reduced by 15-50%. Said study was reproduced inside a wind tunnel by [PERSON] et al. [8] and obtained a concentration reduction of up to 85%. [PERSON] et al. [9] reported that the concentration of ultrafine particles in the presence of a 6m barrier is less than 50% just after the barrier compared to the base case without the barrier in major roadways in central North Carolina, USA. [PERSON] et al. [10] found that the pollutants concentration reduction downwind the barrier can increase with the height as a result of the turbulence induced by the presence of said barrier. Their study was developed experimentally using wind tunnels. [PERSON] et al. [11] reported that the average concentration of NO\({}_{\mathrm{x}}\) and ultrafine particles near a large highway in Phoenix, Arizona, USA, was reduced from ~50% at 50 m downwind a noise barrier to about 30% at 300 m from the barrier. Recently, [PERSON] et al. [12] studied the dispersion of pollutants in the crosswind direction of vertical barriers by means of a set of wind tunnel experiments. They found that downwind pollutant concentration tends to increase near the barrier crosswind edge but that the edge effects become negligible after a distance of seven times the barrier height. Numerical studiesOther researchers have opted to study the pollutant dispersion phenomena by using numerical models based on computational fluid dynamics (CFD). [PERSON] et al. [13] reported a method to model green barriers in terms of the Leaf Area Density (LAD) which has been widely used to represent the effects of green barriers in CFD simulations. Using CFD, [PERSON] et al. [14] reproduced the 2008 NOAA experimental study finding that the presence of the barrier creates a much more complex flow downwind the road than without the barrier. They found that recirculation zones are formed in the wake of the barrier that is characterized by strong mixing regions. The effect of changing the barrier configuration also has an impact on the distance from the barrier at which the concentration is significantly reduced [10]. These authors reported that the roadside barriers affect the dispersion of the vehicle-related-emissions in three ways: (i) they increase vertical dispersion through additional turbulence generated in the barrier wake, (ii) inducing vertical mixing behind the barrier in the cavity region, and (iii) they loft the emission above the barrier. Later, [PERSON] and [PERSON] [15] and [PERSON] [16] used solid barriers and studied the effects on NO\({}_{\mathrm{x}}\) and CO concentration, finding reductions of the maximum wake concentrations of ~40 and ~70%, respectively. [PERSON] and [PERSON] [17] furthered [PERSON]'s studies by varying the distance between the vegetation barrier and the road. Then, [PERSON] et al. [18] and [PERSON] et al. [19] studied different alternatives combining both solid and green barriers and concluded that the most effective design to reduce pollutant concentration near roads is placing a solid barrier alongside a green barrier.