Investigation into pedestrian exposure to traffic PM around grade separations: a case study in Xi’an, China

Abstract

To investigate the influence of grade separations on the concentration dynamics of particulate matter (PM) and pedestrian PM exposure rate, PM mass concentrations (PMCs) at and around a typical grade separation were measured. Using a statistical fitting method, the horizontal variation and frequency distributions for PMCs at the entrance and exit of a grade separation were investigated. The pedestrian exposure around the grade separation was quantified in terms of respiratory deposition dose (RDD) rates. The results of the case study showed that the average mass concentrations of PM10, PM2.5, and PM1 at the exit of the underpass were found to be about 19, 9, and 4% higher than those at the entrance, and the concentrations of PM10, PM2.5, and PM1 in the underpass were higher compared with those on the overpass. The average mass concentrations of PM10 (136 μg m−3), PM2.5 (67 μg m−3), and PM1 (44 μg m−3) in the underpass exceeded those on the overpass by about 14, 8, and 7%, respectively. Further analysis of the underpass revealed that all three types of PMCs decayed exponentially with increasing distance from the exit. However, for the entrance, as the distance from the entrance increased, PMCs began to drop slightly, and then rose to the maximum near the ramp. Based on the frequency histograms and corresponding fitted curves at the entrance and exit of the underpass, the rate of violation of ambient PM concentrations was assessed. The total respiratory deposition dose (RDD) rates of PM10, PM2.5, and PM1 around the grade separation were roughly 3.4, 2.8, and 2.5 times higher than those of the background, respectively. The findings of this study are a step toward understanding more environmentally friendly grade-separation design strategies.