Particulate matter inside and around elevated walkways

A number of cities have developed an elevated walkway network within street canyons in order to alleviate pedestrian congestion. Naively one may expect lower pollutant concentrations inside elevated walkways, on account of the increased distance from traffic-related emissions, but this hypothesis has received little critical attention. The primary objective of this study is to determine whether pedestrians on the elevated walkway experience lower exposure than those utilizing the sidewalk below. Measurements of particulate matter (PM) inside and around elevated walkways in Hong Kong indicate that this is not always true. Nevertheless the variation in the ratio of sidewalk to walkway concentrations is relatively narrow (typically lying between 1 and 1.5). The robustness of the walkway-sidewalk ratio for PM_2.5, i.e. PM with a diameter of less than 2.5 μm, is verified by examining the horizontal dependence, influence of traffic volume and diurnal cycle. This work highlights the importance of urban design, as well as the complex factors influencing PM concentrations.