Simulation study of dispersion and removal of particulate matter from traffic by road-side vegetation barrier.

Well-positioned and configured vegetation barriers (VBs) have been suggested as one of the green infrastructures that could improve near-road (local) air quality. This is because of their influence on the underlying mechanisms: dispersion and mass removal (by deposition). Some studies have investigated air quality improvement by near-road vegetation barrier using the dispersion-related method while few studies have done the same using the deposition-related method. However, decision making on vegetation barrier's configuration and placement for need-based maximum benefit requires a combined assessment with both methods which are not commonly found in a single study. In the present study, we employed a computational fluid dynamics model, ENVI-met, to evaluate the air quality benefit of near-road vegetation barrier using an integrated dispersion-deposition approach. A technique based on distance between source (road) and point of peak concentration before dwindling concentration downwind begins referred to as "distance to maximum concentration (DMC)" has been proposed to determine optimum position from source and thickness of vegetation barrier for improved dispersion and deposition-based benefit, respectively. Generally, a higher volume of vegetation barrier increases the overall mass removal while it weakens dispersion of pollutant within the same domain. Hence, the benefit of roadside vegetation barrier is need-based and can be expressed as either higher mass deposition or higher mass dispersion. Finally, recommendations on applications of our findings were presented.