Characterizing temporal and vertical distribution patterns of traffic-emitted pollutants near an elevated expressway in urban residential areas

Elevated expressways have been built in many cities worldwide to improve the accessibility and capability of urban road network. Meanwhile, intensive emissions from ground- and elevated-road traffic would significantly elevate personal exposure levels to air pollutants, such as fine particles (PM1), carbon monoxide (CO) and black carbon (BC). However, their transport and dispersion mechanisms in urban residential areas have not been clearly recognized. In this study, we characterized temporal and vertical distribution patterns of traffic-emitted pollutants near an elevated expressway in near-road buildings. Vertically-resolved measurements of traffic emissions were made at different floors of a roadside building alongside the Inner Ring Viaduct in downtown Shanghai, China from January to June 2019. Results show that higher pollutant concentrations were observed during the morning (Local Time, UTC+8, LT 08-09) in winter and during the late afternoon (LT 17-18) in summer, the result of seasonal variations in vehicular activities and meteorological factors. Additionally, the vertical profiles of traffic-emitted pollutants all exhibited bimodal distribution patterns, which was more obvious for PK_ than for CO and BC. The upper peak appeared slightly above noise barriers, attributable to the extra vehicular emissions on the elevated expressway. The lower peak occurred at ground level mainly due to on-road congested traffic flow, and the presence of the elevated road, which inhibited the dilution of ground-level vehicular exhausts. These findings reveal the general trends of the dispersion patterns of traffic emissions surrounding urban elevated expressways, and could provide implications in making control measures to mitigate air pollution exposure.