Impact of urban viaducts on the vertical distribution of fine particles in street canyons

In the urbanization process, the viaduct has been regarded as an effective way to relieve metropolis traffic pressures in the urban area. However, due to the double emission source from elevated- and ground-road vehicles, the viaduct-built environment inevitably deteriorates the street canyon's air quality. Besides, the emitted air pollutants are trapped in such street canyons for extended periods. With this consideration, we used measured data to explore the impact of the viaduct on the vertical distribution of PM_2.5 in street canyons. First, the experiments were conducted in a residential building near the inner ring of a viaduct in Shanghai, China. Next, the vertical distribution of PM_2.5 and its diurnal variation were revealed based on these data. The results indicated the bimodal distribution patterns in the vertical profiles of PM_2.5 mass concentrations, with the lower peak at height of 5 m under the viaduct and the upper peak at height of 10 m above the noise barrier on the viaduct. Further, the impact of the viaduct on the vertical distribution of PM_2.5 was investigated. Results from both observation and simulation are consistent, revealing that 21–23% of PM_2.5 was trapped beneath the viaduct, and the noise barrier causes the PM_2.5 to decrease by 4–5%. The traffic source on the viaduct contributes 20–40% of the PM_2.5. This study revealed the dispersion patterns of PM_2.5, and the impact of the viaduct on it, which could be beneficial to optimizing the viaduct and the noise barrier to release traffic-related air pollutants in street canyons.