# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 1 Introduction  
In urban centers, vehicles are the main source of fine (\(d\) < 10 \(\upmu\)m) and ultrafine (\(d\) < 100 nm) particles, black carbon monoxide (CO), and nitrogen oxides (NO\({}_{\rm x}\)) [1, 2]. These air pollutants cause respiratory and cardiovascular problems, chronic diseases, and early mortality in the people exposed to these pollutants [3, 4]. Studies have shown that within urban regions, pedestrians and people living near roads (<500 m) are exposed to the highest concentration levels of these pollutants [5]. Therefore, there is an urgent need for the design of countermeasures to reduce pollutant concentration near roads. This need is an example of a more general problem, which is the design of countermeasures to reduce pollutant concentration near area sources of pollutants located on flat regions, such as, paved and unpaved roads; uncovered areas exposed to the wind action, such as desertic areas, and particulate materials stored at open atmosphere (carbon piles as an example); gaseous leaks from pipes (natural gas pipes as an example); gases evaporated from pools exposed to open atmosphere (hydrocarbon spillages and sewer water as examples); and agro-industrial open burning. Barriers have been widely used for near-road noise control. However, as shown in Table 1, it has been found that the use of natural or human-made barriers located on the roadsides has the potential of reducing those concentrations. Nevertheless, the improper design of those barriers could end up increasing those concentrations or moving the concentration peaks to some distance down downwind the road. \begin{table}
\begin{tabular}{l l c c c c c} \hline \hline
**Author(s)** & **Description** & **Year** & **CFD** & \begin{tabular}{c} **Experimental** \\ **Measurements** \\ \end{tabular}  & \begin{tabular}{c} **Geometry** \\ **Variation** \\ \end{tabular}  & \ \begin{tabular}{c} **Cost** \\ **Evaluation** \\ \end{tabular}  \\ \hline [PERSON] et al. & Tracer gas dispersion study with a & & & & & \\ \multirow{2}{*}{ \begin{tabular}{l} [PERSON] et al. \\ \end{tabular}  } & straight noise barrier & 2009 & x & ✓ & x & x \\  & near roadway & & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Impact of noise barriers on & & & & & \\  & particle pollutants size & & & & & \\  & distributions and concentrations & 2010 & x & ✓ & x & x \\  & near freeways & & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Use of tree stands and brick noise & & & & & \\  & barriers for mitigation of roadside & 2012 & x & ✓ & ✓ & x \\  & ultrafine particles & & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Vegetation barrier model to & & & & & \\  & mitigate roadside pollutant & 2012 & ✓ & ✓ & x & x \\  & dispersion in terms of LAD & & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Simulation of tracer gas & & & & & \\  & dispersion under twelve different & 2013 & ✓ & ✓ & ✓ & x \\  & roadway configurations & & & & & \\ \hline \multirow{3}{*}{[PERSON] and [PERSON]} & Evaluation of solid barriers & & & & & \\  & performance for the mitigation of & 2014 & ✓ & ✓ & ✓ & x \\  & air pollution with NOx & & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Barrier shape effect on LNG & & & & & \\  & dispersion due to pipe leaks & 2014 & ✓ & ✓ & ✓ 1  & x \\ \hline \multirow{3}{*}{[PERSON] et al.} & Semiempirical dispersion model & & & & & \\  & of roadside barriers & 2014 & x & ✓ & ✓ & x \\ \hline \multirow{3}{*}{[PERSON]} & Roadside barrier impact on the & & & & & \\  & dispersion of road air CO for & 2015 & ✓ & ✓ & x & x \\  & several road configurations & & & & & \\ \hline \multirow{3}{*}{[PERSON] and [PERSON]} & PM dispersion and removal via & 2015 & ✓ & ✓ & ✓ & x \\  & natural barrier use & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.} & Noise barriers for the mitigation & & & & & \\  & of on-road and downwind & 2016 & x & ✓ & x & x \\  & pollutants (NOx, UFP) & & & & \\ \hline \multirow{3}{*}{[PERSON] et al.}