Several studies show that solid barriers such as sound walls next to highways reduce the impact of vehicular emissions on near-road air quality. These barriers decrease pollutant concentrations next to roads by enhancing vertical dispersion of emitted pollutants. To the first order, the effect of a solid barrier is to move the road upwind by a distance proportional to the height of the wall divided by the turbulent intensity. Some have suggested that vegetation higher than the wall, which is often planted to hide the wall from residents, might enhance the mitigating effects of solid barriers. This paper examines the theoretical and experimental studies that have bearing on this hypothesis. We first examine past studies conducted on the impact of barriers that are composed entirely of vegetation. Some field studies show that a vegetative barrier has a mitigating effect, but is smaller than that of a solid barrier of the same height. Other studies show that the presence of a vegetative barrier increases concentrations relative those in the absence of a barrier. We have conducted a field study to estimate the incremental effect of tall vegetation on the mitigation caused by a solid barrier. The study was conducted in the vicinity of a highway in Sacramento, California. The road has two 500 m stretches, one of which has a solid barrier, and the other has tall trees behind the barrier. This allows us to make simultaneous measurements of vehicle-related species at several locations downwind of these two barrier types. We observed that the addition of vegetation behind the solid barrier reduces turbulence levels in the entire sampling period. Our measurements also showed that vegetation behind a solid barrier can cause a reduction in concentrations in general; however, this was not the case for all of the observed data. This paper reports the results from this field study and a preliminary model that accounts for the effect of vegetation.
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