Evaluation and development of tools to quantify the impacts of roadside vegetation barriers on near-road air quality

Abstract

Traffic emissions are associated with the elevation of health risks of people living close to highways. Roadside vegetation barriers have the potential of reducing these risks by decreasing near-road air pollution concentrations. However, while we understand the mechanisms that determine the mitigation caused by solid barriers, we still have questions about how vegetative barriers affect dispersion. The US EPA conducted several field experiments to understand the effects of vegetation barriers on dispersion of pollutants near roadways (e.g., 2008 North Carolina study and 2014 California study) that indicate the reduction of near-road pollutant concentrations can be up to 30% due to the barrier effects. The results of these field studies are being used to develop and evaluate dispersion models that account for the effects of near-road vegetative barriers. These models can be used for evaluating the effectiveness of vegetation barriers as a potential mitigation strategy to reduce exposure to traffic-related pollutants and their associated adverse health effects. This paper presents the results of the analysis of the field studies and discusses dispersion models being used to describe the data in order to simulate the effects of near-road barriers and to develop recommendations for model improvements.

Urban Vegetation in Air Quality Management: A Review and Policy Framework

“…Computational fluid dynamics (CFD) simulations with established, previously validated models or models validated against data from previous studies or existing databases [80,97,98,[101][102][103][104][105][106][107][108][109] CFD simulations and on-site air quality measurements, or measurements of pollution deposition on plant samples using laboratory techniques or wind tunnel experiments [110][111][112] CFD simulations and drift flux model simulations [113] CFD simulations and drift flux model simulations and on-site air quality measurements [114,115] Mesoscale Weather Research and Forecasting (WRF) model simulations and CFD simulations [116] Mesoscale WRF model coupled with chemistry (WRF-Chem) and Building Effect Parameterization (BEP) simulations [117] At the local scale street canyons, including street canyon trees [101,104,105,109,113,114] and green walls and roofs [115], near-road vegetation [102,108,110,111] and urban green spaces [98,106,112] were investigated. In 3D simulations idealised models [101,105,106,[109][110][111][112]…”

“…In 3D simulations idealised models [101,105,106,[109][110][111][112][113][114][115] or more complex 3D models based on existing geometries [104] were used with vegetation modelled as a porous medium with simplified geometry. For roadside barrier arrangements, their different configurations and characteristics, such as porosity, gaps, or thickness, were considered [102,108]. In the studies, a neighbourhood scale of up to several square kilometres was applied.…”

“…Both hedge and tree barriers were found to be effective in locally reducing near-road air pollution concentration levels in open road conditions, but trees are more crucial in this process [111]. Wide vegetation barriers with high LAD and vegetation-solid barrier combinations were most effective in traffic-related pollution mitigation [102,108]. Apart from vegetation barriers, near-road uniform-type green spaces were found to be less effective in PM removal, while more complex forms were more effective.…”

“…Similarly to studies accounting only for the aerodynamic effect of vegetation, general guidelines cannot be given. The model-based design recommendations, made based on isolated, generic configurations, may not account for the site-specific conditions [102]. For example, the results obtained for a road with a vegetation barrier on one side cannot be extrapolated for a street or avenue with vegetation on both sides because different design scenarios and conditions require individual case studies [111].…”