Roadside vegetation barrier designs to mitigate near-road air pollution impacts
Graphical abstract
Introduction
Roadside vegetation barriers have been evaluated as a potential mitigation strategy for near-road air pollution in several field, wind tunnel, and numerical studies (Al-Dabbous and Kumar, 2014, Baldauf et al., 2008, Bowker et al., 2007, Brantley et al., 2014, Hagler et al., 2012, Heist et al., 2009, Finn et al., 2010, Steffens et al., 2012, Steffens et al., 2013, Steffens et al., 2014). Those studies revealed that the effects of vegetation barriers on near-road air quality are primarily governed by two physical mechanisms: dispersion and deposition (Steffens et al., 2012, Steffens et al., 2013, Steffens et al., 2014). The impact of dispersion is demonstrated by upward deflection and deceleration of the approaching air flow from the highway, as well as flow recirculation on both sides of the barrier. Deposition, on the other hand, removes particulate matter (PM) by Brownian diffusion, impaction, interception and gravitational settling. The deposition velocity, Vd, varies considerably depending on the particle size, for example, from ~ 3 cm s− 1 for 20 nm particles to ~ 0.3 cm s− 1 for particles at approximately 100 nm (Zhang et al., 2001). Steffens et al. (2012) simulated the effects of vegetation barriers on near-road particle size distributions characterized by a field experiment, and examined the knowledge gaps in capturing the impacts of dispersion and deposition, as well as proposed several recommendations on how to bridge those gaps.
The main objective of this study is to advance our understanding of the effectiveness of vegetation barriers as a potential mitigation strategy by quantitatively assessing the spatial variation of PM under various road-canopy configurations commonly present in the real world. We first incorporated the Large Eddy Simulation (LES) turbulence model, as recommended by Steffens et al. (2012), into the Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry (CTAG) model, and evaluated the model performance against the same experimental dataset adopted by Steffens et al. (2012), which employed a Reynolds Averaged Navier–Stokes (RANS) turbulence model. Next, we compared six common near-road vegetation barrier configurations in terms of their impact to on-road and near-road particle concentrations. Finally, we provided design recommendations based on the results of our analysis.
Section snippets
Numerical method
The CTAG model was designed to resolve the flow field, including turbulent reacting flows, aerosol dynamics, and gas chemistry in complex environments (Steffens et al., 2013, Tong et al., 2012, Tong and Zhang, 2015, Wang and Zhang, 2009, Wang and Zhang, 2012, Wang et al., 2011, Wang et al., 2013a, Wang et al., 2013b). In this study, Large Eddy Simulation (LES) is applied to resolve the large-scale dominant unsteady motion within the vegetation canopy and requires modeling only small-scale,
Chapel Hill field experiment
The CTAG with LES model for vegetation barriers is evaluated against experimental data collected in Chapel Hill, North Carolina, USA as reported by Hagler et al. (2012). For this study, the near-road vegetation barrier consisted of a mix of 6–9 m tall coniferous trees with full cover from the ground to the top of the canopy. A section along the same stretch of limited-access roadway contained the roadside vegetation barrier as well as an area with no barrier or other obstructions to air flow
Barrier configurations
Six common near-road configurations, as well as a series of sensitivity studies on several physical parameters, are investigated in this section (Table 1, Fig. 2). The evaluated LES model is employed to simulate near-road PSDs. Table 1 shows the physical parameters of the barrier and boundary conditions studied. The composition of the canopy structure is modeled using two plant parameters widely used in the literature, Leaf Area Density (LAD) and Leaf Area Index (LAI). LAD is a ratio of leaf
Conclusion
The primary objective of this study is to provide design recommendations to assist urban planners in evaluating different green infrastructure designs. The near-road air quality is primarily driven by two physical mechanisms, i.e., dispersion and deposition, and deposition only occurs in the presence of vegetation. Our analysis demonstrates that the impacts on roadside air quality are particle size-dependent. Two potentially viable design options with regard to roadside mitigation of near-road
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Now at Center for Green Buildings and Cities, Graduate School of Design, Harvard University, Cambridge, MA, 02138.