Impact of roadside conifers vegetation growth on air pollution mitigation

Abstract

As a Nature-Based Solution, roadside green infrastructure (also known as roadside barriers) can potentially mitigate traffic-related air pollution by increasing dispersion and promoting pollutant deposition. For new and existing roadside barriers, the vegetation’s physical and ecological attributes (dimensions and density) are dynamic in nature, and thus affect the barriers’ pollution reduction capabilities. In this study, we first synthesized the results from existing field measurements characterizing the properties of coniferous vegetation, which show that its growth over time was characterized by an increase in height and a decrease in density. Motivated by this finding, a total of 75 simulations was conducted using a coupled aerodynamics and deposition model to investigate how the growth patterns of roadside vegetation barriers (e.g., heights from 2 to 10 m, and leaf area index (LAI) from 4 to 11) affects air pollutant reduction under different urban conditions (wind speeds 1–5 ms−1). The results indicated that the ideal stage of maturity for the vegetation barrier to achieve the most pollutant reduction is from heights of 4–6 m. In this scenario, the vegetation barrier enhances pollutant deposition, has a moderate wake region, and generates a high level of turbulence that promotes downwind pollutant dispersion. It is imperative to account for growth patterns when selecting vegetation as roadside barriers to ensure that it can be maintained through pruning to achieve an ideal barrier height and optimal air pollutant reduction. Based on our findings, we provide five recommendations for implementing roadside vegetation barriers in a landscape and urban planning context.