Dispersion mechanism and parameterizations of chemically reactive pollutant in the atmospheric boundary layer

Abstract

Air quality is a key public concern nowadays because of its impact especially on cities with dense population. Dispersion of pollutants 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 urban inhabitants. Most vehicular exhausts are chemically reactive that evolve to their secondary counterparts in the atmospheric boundary layer (ABL). Whereas, rather limited studies have attempted to parameterize the transport processes and explore the mechanism of reactive pollutant dispersion in the ABL. In this thesis, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban areas in the form of idealised street canyons is investigated using large-eddy simulation (LES). Isothermal boundary conditions are applied to the LES model to create neutral stratification. The worst scenario is considered in which the prevailing flows are perpendicular to the street axes. In pseudo-steady-state, fully developed turbulent flows, nitric oxide (NO) is emitted from the ground surface in the first street canyon into the urban ABL doped with ozone (O3). The prevailing wind enters the spatial domain from the upstream inlet with background O3 concentrations in the range of 1 ppb to 500 ppb. An area source of NO of concentration 1,000 ppb is continuously emitted from the ground surface of the first street canyon to simulate vehicular emission. The surface roughness is controlled by the aspect ratio (AR) of the building height to the street width in the range of 1 to 1/11. The dimensionless tracer concentrations over different rough surfaces collapse regardless of the aerodynamic resistance, providing strong evidence that Gaussian plume model can be used to well estimate the scalar dispersion over different roughness surface as long as the accurate dispersion coefficient is obtained. LES output also shows that for the chemically reactive pollutant, the conventional Gaussian model cannot fully parameterize the pollutant plume anymore. The conventional approach of modified dispersion coefficients in terms of the timescales of pollution physics and chemistry is inapplicable due to the source depletion analogy which, however, estimates well the NO concentrations only above the mean plume rise. Inaccuracy is caused by the dominated NO oxidation in the near-wall region. Finally, regression to the LES output shows that the vertical dimensionless NO profiles exhibit the Gamma γ-distribution for a range of background O3 concentrations, unveiling a new parameterization of reactive plume dispersion over urban areas. Budget analysis of the transport processes is conducted to further elucidate the dispersion mechanism of chemically reactive pollutant. It is found that the contributions from advection, diffusion and chemistry, who couple closely with each other, vary in the streamwise direction. For inert pollutants, streamwise advection and vertical diffusion mainly counterbalance each other. For chemically reactive pollutants, on the other hand, chemistry plays a key role in the far field where the mixing is rather uniform. In view of the elevated roof-level shear stress, advection, diffusion and chemistry show abrupt changes, complicating the pollutant dispersion processes.