Effects of roadside green infrastructure on particle exposure: A focus on cyclists and pedestrians on pathways between urban roads and vegetative barriers

Highlights

• Field campaign and numerical simulation are used in the near-road urban scenarios.

• Effects of green infrastructure on pedestrian-cyclist particle exposure are analyzed.

• Green infrastructure exhibits limited effects on near-road air quality improvements.

• Vegetative barriers impede particle dispersion and elevate exposure on pathways.

Abstract

Roadside green infrastructure (GI) has attracted worldwide attention for its potentials to alleviate local air pollution. However, previous studies have not clearly characterized the effects of roadside GI on personal exposure levels to vehicular emissions, particularly for cyclists and pedestrians on the pathways between urban roads and vegetative barriers. In this study, field experiments were implemented to measure and compare the concentration levels of particulate matter (PM) and black carbon (BC) on pathways (e.g., bike lanes and sidewalks) and near residential buildings with and without roadside GI. Results show that the presence of GI significantly elevated the particle concentrations over bike lanes and sidewalks. Compared with the GI-free case, an increase of up to 20% and 28% was separately observed for BC and PM on the pathways before GI. This is strongly associated with the impact of GI on local wind fields that impedes the dispersion of traffic-emitted particles on the roadways. However, roadside GI brought a significant reduction of particle concentrations near residential buildings, separately decreasing by about 8% for BC and 6% for PM. Then Computational Fluid Dynamics software was adopted to simulate the diffusion process of traffic-emitted particles in the typical urban scenarios of roadways, pathways, GI, and residential buildings, and further analyze the particle distribution patterns under six roadside GI configurations. Simulation results suggest that tall vegetative barriers can lead to the accumulation of particles on the pathways and increase cyclists' and pedestrians’ exposure to local particulate pollution. The GI case of “tree canopy only” can be a potentially viable choice to reduce particle concentrations on the pathways due to the fact that its better ventilation conditions are more favorable for particle diffusion than other GI configurations. However, compared with the complex GI configurations, the scenario with no roadside GI exhibits more positive effects on decreasing particulate exposure on the pathways. These findings could provide practical insights into roadside GI design and important implications in near-road air quality improvements.

Introduction

Urban air pollution has become a global concern because of its detrimental effects on human health (Kampa and Castanas, 2008). Vehicular emissions are an important source of a rapid rise in ambient air pollutant concentrations, such as black carbon (BC), particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs), etc. (Elbir et al., 2011; Jandacka et al., 2017; Lee et al., 2006; Li et al., 2017; Megido et al., 2017; Palmgren et al., 1999; Pérez-Martínez et al., 2015; Tang et al., 2016; Wang et al., 2010). Actually, traffic-related air pollution mainly comes from the intensive emissions of tailpipe exhaust gas, which easily leads to the accumulation of ground-level pollutant concentrations and thus increases the pedestrians' and cyclists’ pollution exposure and health risks, particularly near busy urban arterial roads (Goel and Kumar, 2016; Karner et al., 2010; Schneider et al., 2015). Furthermore, long-term exposure to high pollution levels, according to epidemiological studies, could cause detrimental health hazards, such as cardiovascular diseases, pneumonia, and even death (Grahame and Schlesinger, 2010; Hoek et al., 2002; Lee et al., 2014; Pope et al., 2002). This also calls for the urgent need to improve near-road air quality and reduce personal exposure levels to traffic-related air pollution.

Motivated by the potentials for local air quality improvements, roadside green infrastructure (GI) is common practice to alleviate traffic-related air pollution in the near-road environments, such as street trees, roadside hedges, vegetative barriers, green walls, green roofs, etc. (Abhijith et al., 2017; Baldauf, 2017; Gallagher et al., 2015; Gratani and Varone, 2013; Grzędzicka, 2019; Liu et al., 2018; Tong et al., 2015). In fact, the GI is the combination of green and natural elements that are implemented over urban areas to improve the aesthetic appearance of the environment and provide ecosystem benefits to the population (Tiwari et al., 2019). In most urban areas, it is a quite typical and general built environment that the GI is erected between heavily trafficked roads and residential buildings due to the shortage of land resources (He et al., 2020; Li et al., 2016; Lusk et al., 2020; Morakinyo et al., 2016; Ottosen and Kumar, 2020; Yang et al., 2018). Furthermore, the combined scenario of urban roadways, pathways, GI, and residential buildings can better improve the comprehensive benefits of the society, for instance, to mitigate the residents’ exposure levels to near-road vehicular emissions, to provide shade, and reduce solar radiations reaching the ground, to enhance urban ecological characteristics, etc.

Many researchers have attempted to characterize the effects of roadside GI on local air quality. In most cases, related field observations revealed a significant reduction in pollutant concentrations behind near-road GI (Brantley et al., 2014; Tong et al., 2016). Furthermore, thick and tall roadside vegetative barriers can achieve a more pronounced pollutant reduction in the downwind direction (Abhijith et al., 2017; Baldauf, 2017; Deshmukh et al., 2019). However, several studies presented different findings that there were no obvious variations in traffic-related pollutant concentrations in front of and behind roadside GI and that the street trees sometimes even led to the local air quality deterioration (Morakinyo et al., 2016; Tong et al., 2015; Yli-Pelkonen et al., 2017). It is discovered that the roadside GI could exhibit different effects on local air quality, partly associated with the concomitance of several confounding factors such as vegetation characteristics and the surrounding built environments, etc. (Hagler et al., 2012). Meanwhile, related research concerning the GI and air quality impacts also suggested that the design and construction of roadside GI configurations should be problem-driven and site-specific (Brantley et al., 2014; Morakinyo et al., 2016). Hence, it is vital to select the typical scenario of roadways, pathways, GI, and residential buildings in the urban built environments for research purpose, analyze the effects of roadside GI on air quality and thus avoid inappropriate GI design that may increase the local pollution level.

In summary, previous studies have not adequately revealed the effects of roadside vegetation on traffic-emitted particle distribution patterns in the typical urban built environment of roadways, pathways, green infrastructure, and residential buildings (Brantley et al., 2014; Nowak et al., 2000). Furthermore, our understandings of roadside GI-related effects on personal exposure levels to local air pollution could be further improved, particularly for pedestrians and cyclists. In this study, portable monitors were used to characterize the variations in particle concentrations (e.g., BC, PM₁, PM_2.5, PM₄, PM₁₀, and total suspended particulates) on bike lanes and sidewalks with and without the presence of GI. Then the potential respiratory deposited doses (RDDs) were also calculated to quantify the effects of roadside GI on the cyclist-pedestrian exposure levels to vehicular emissions. Finally, numerical simulations were performed to investigate the dispersion and distribution patterns of traffic-emitted particles under six GI configurations and further explore the optimal roadside GI design to minimize air pollution exposure.