Large-Eddy Simulations of the Entrainment and Detrainment of Pollutants at the Roof Level of Two-Dimensional Urban Street Canyons

초록

Air pollution in built-up urban areas is an important environmental problem. Many studies have examined pollutant dispersion in and above urban street canyons, but its mechanism needs to be further elucidated. In this study, we examine pollutant dispersion in and above street canyons, focusing on pollutant entrainment and detrainment in street canyons. For this, the PArallelized Large-eddy simulation Model (PALM) is employed and two 2-dimensional street canyons, one with pollutant emission from the street canyon (street canyon M) and the other without it (street canyon N), are considered. At the roof level of the street canyon M, the vertical turbulent pollutant flux is much larger than the vertical mean pollutant flux. On the other hand, at the roof level of the street canyon N, the vertical mean pollutant flux is comparable to the vertical turbulent pollutant flux. The quadrant analysis reveals that pollutant ejection and sweep events occur much more frequently than pollutant outward interaction and inward interaction events at the roof level of the street canyon M while these four events occur with similar frequencies at the roof level of the street canyon N. The similar occurrence frequencies of the four events at the roof level of the street canyon N are associated with the transport of pollutant emitted from the street canyon M into the street canyon N. The increase in the surface roughness of building roof increases turbulence intensity and thus decreases pollutant concentrations in both street canyon M and street canyon N.

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