Trees help reduce street-side air pollution: A focus on cyclist and pedestrian exposure risk

Abstract

Trees have been reported to help reduce streetside air pollution, hence reduce public exposure risk to traffic-related air pollutants such as particulate matter (PM). This study analyzes experimental data from field measurements of PM concentration in a shallow street canyon in Xi'an, China to investigate the effect of different tree layouts on pedestrians and cyclists exposure to inhalable, thoracic and alveolic particles during their urban commuting trips. Particle concentrations for human health risk assessment were measured in the traffic lane, bicycle lane and sidewalks under four different tree layouts — open space (no trees); sparse planting spacing; medium planting spacing and dense planting spacing. Results show significant benefit of trees for reducing air pollution, particularly attenuation of inhalable particles on sidewalks. Our data shows that medium spacing (approximately equal to crown diameter) offers the highest impact in reducing air pollution on bicycle lane (attenuating inhalable, thoracic and alveolic particles by 55.85%, 50.00% and 26.81%, respectively) and sidewalk (decrease by 59.05%, 52.42% and 28.10%, respectively, in inhalable, thoracic and alveolic particle concentrations) in comparison to on-street traffic-lane measurements. Moreover, the attenuation effect of particle concentration due to trees under medium planting spacing becomes more significant under heavier traffic. In contrast, the attenuation effect of particles with sparse or dense tree spacing diminishes to varying degrees as traffic becomes heavier beyond certain point. Our findings suggest that medium tree spacing can be cost-effective for mitigating PM pollution, hence reducing public exposure risk in urban commuting environment.