Large-eddy simulation of pollutant plume dispersion in the urban canopy layer over 2D idealized street canyons

In the last five decades, the Gaussian pollutant plume model has been widely used for the estimate of air pollutant distribution in the atmospheric boundary layer (ABL) in an operational manner. Whereas, it was originally designed for rural areas with rather open and flat terrain. The recirculating flows below the urban canopy layer substantially modify the near-ground urban wind environment and so do the pollutant distribution. Though the plume rise and dispersion coefficients are often adjusted empirically, the accuracy of applying the Gaussian pollutant plume model in urban areas is unclear. To demystify the uncertainty of employing the Gaussian pollutant plume model, as well as to reveal the characteristics of flows and pollutant transport over unban roughness, this study was performed to examine how the Gaussian-shape pollutant plume in the urban canopy layer is modified by the idealized two-dimensional (2D) street canyons at the bottom of the ABL. Since atmosphere turbulence is a key parameter affecting pollutant removal from street canyons to the ABL, the large-eddy simulation (LES) was adopted in this study. The LES results showed that, under neutral stratification, the pollutant distribution in the urban canopy layer resembled the Gaussian plume shape in general. Below certain height (z/h = 1.10), concave-shape horizontal pollutant concentration profiles are observed over individual canyons. Decelerating air masses are observed consistently at the roof level of each canyon that is likely caused by the re-circulating flow within the canyon. For those decelerating puffs, the vertical fluctuating velocity w” is mainly upward and the fluctuating pollutant concentration c” is mostly negative that collectively explain the rapid decrease in pollutant concentration over each canyon. This finding in turn suggests that the Gaussian pollutant plume model should be applied with caution in the vicinity over urban roughness.