Budget Analysis of Pollutant Transport over Urban Roughness

Abstract

Dispersion of pollutant emitted from vehicles over urban areas largely affects pedestrian-level air quality. Poor ventilation inside street canyons often results in pollutant accumulation which is harmful to human health. Most vehicular exhausts are chemically reactive that evolve to their secondary counterparts in the atmospheric boundary layer (ABL). Therefore, the conventional Gaussian plume model, which assumes inert pollutants, should be used with caution. Transport process is partitioned into advection, diffusion and chemistry processes. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban area of an array of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL doped with ozone (O3). Instead of the conventional Gaussian plume model, the source depletion analogy is used to estimate the plume shape which shows self-similar vertical NO profiles. Inaccuracy is caused by the dominated NO oxidation in the near-wall region where the Gaussian plume model fails. Regression to the LES output shows that the dimensionless vertical NO profiles exhibit Gamma γ-distribution for a range of background O3 concentrations, unveiling a new parameterization of reactive plume dispersion over urban areas. Apart from the plume characteristic, budget analysis of the pollutant transport is conducted. Different levels of background O3 are used to contrast the importance of individual chemistry terms to the pollutant transport. It is found that the three terms (advection, diffusion and chemistry) vary in the streamwise location and couple with each other. More details of budget analysis for both the mean properties and fluctuating components will be reported in the conference.