Impacts of noise barriers on near-viaduct air quality in a city: A case study in Xi'an

Viaducts built in cities all over the world can increase their traffic capacity. Yet whether specific dispersion and reduction strategies for traffic-related pollutants, such as ultrafine particles (UFP), fine particles (PM2.5), and black carbon (BC), are needed in urban residents? activity areas is uncertain. Here we investigated the distribution patterns of traffic-related pollutants near a viaduct having various infrastructures. Measurements were taken using mobile devices in three sampling sections alongside and extending away from the viaduct in Xi?an, China. Difference-in-differences analysis was used to explore the mitigation effect of noise barriers. The results showed that the pollutant distribution profiles behind both noise barriers and combination barriers (i.e., solid noise barrier plus tree vegetation) are characterized by a novel, unimodal mode whereby the UFP and PM2.5 concentrations first increased and then decreased. Lower traffic-related air pollutant concentrations were observed on the lee side of the noise barrier than at the non-barrier site section. Evidently, compared with a noise barrier alone, the mitigation effect upon UFP, PM2.5, and BC near a viaduct is greater when barrier types are combined, enabling concentration reductions of 5% (UFP), 21% (BC), and 50% (PM2.5). In addition, this study also observed lesser reductions of the three pollutants at greater wind speeds. However, conditions did occur when pollutant concentrations were higher behind a barrier than when no barrier was present, namely when the wind speed exceeded 0.8 m?s? 1. Overall, our findings suggest beneficial effects of noise barriers and combination barriers for mitigating the air pollution near viaducts.

Automated summary

The study investigated the distribution patterns of traffic-related pollutants near a viaduct in Xi'an, China, and found that noise barriers and combination barriers have beneficial effects in mitigating air pollution near viaducts. The research revealed that the combination of different barrier types led to greater reductions in UFP, PM2.5, and BC concentrations compared to noise barriers alone.