Investigating traffic-related PM exposure on and under pedestrian bridges: A case study in Xi'an, China

We measured particulate matter concentrations (PMCs), including PM10, PM2.5, and PM1.0 fractions, on and under pedestrian bridges over two different types of roads: an urban expressway and an arterial road. The objectives of the study were to 1) assess the variability of PMCs measured at pedestrian bridges, 2) identify best-fit probability distribution curves of different measures of PMCs, and 3) quantify pedestrian exposure (by respiratory deposition dose (RDD) rate). Results showed that the average on-bridge PM concentrations were significantly higher than the under-bridge values over the urban expressway, although no similar trend observed over the arterial road. More specifically, average PM1.0 concentration on the bridge was 1.29 times of that under the bridge over the urban expressway. Gaussian distribution best fit the 30s average PMCs for PM10 on and under the pedestrian bridge over the urban expressway, and LogNormal distribution best fit the PMCs for PM10 on and under the bridge for the arterial road. The average RDD rates for PM10 (143.85 mu g h(-1)), PM25 (9.41 mu g h(-1)), and PM1.0 (3.78 mu g h(-1)) on the bridge over the urban expressway (OBUE) were higher by 47%, 41%, and 11%, respectively, than those on the bridge over the arterial road (OBAR). While only minor differences were noticed in the average RDDs of PM between on and under the pedestrian bridge over the arterial road, a significant difference in the average RDD rate was found between on and under pedestrian bridges over the urban expressway.