Assessing Green Infrastructure Interventions to Reduce Personal Exposure of Ultrafine Particles for Individuals in Bus Shelters

Abstract

Deteriorating air quality is being increasingly recognised as a significant issue, partly due to the growing population living in urban areas. Consequently, the commuting microenvironment is a widely studied topic. However, limited research is available quantifying pedestrian exposure whilst waiting at bus shelters. Given the close proximity of bus shelters to the road, the prolonged idle waiting time, and high emission levels from bus acceleration away from the stop, the primary aim of this study was to determine whether vegetation can reduce the level of ultrafine particles (UFPs) pedestrians are exposed to while waiting at bus stops. A secondary objective was to establish bus shelter design considerations that could be utilised to minimise exposure to air pollution. The results of the study show pedestrians were exposed to the highest concentrations of pollutants (UFPs and NO₂) whilst inside the bus shelter. Higher concentrations of pollutants were also observed in close proximity to shelters compared to sites located further from bus stops, suggesting the bus shelter could be reducing pollutant dispersion patterns, resulting in reduced air quality. The effectiveness of the vegetation intervention was inconclusive. A high degree of temporal and spatial variation in pollution concentrations was observed near the sites with planter boxes installed, which, combined with differences in meteorology pre- and post-intervention, made it challenging to draw any conclusive results. It is proposed that the planter box intervention did not have a statistically significant impact on exposure levels. Data from the location with the living roof and walls intervention suggests there was a material impact in reducing UFP concentrations observed both inside and outside the intervention shelter. A strong correlation between wind speed and mean UFP concentrations may account for some of the variation observed in the data. However, the data were also significantly influenced by unknown external factors. The results of this study suggest improving bus shelter design to be more contextualised may improve personal exposure of pedestrians to air pollution. Green infrastructure could be included in future designs. Further research is required to match vegetative interventions with the intended microenvironment to avoid unintentional increases in pollutant exposure.