Abstract
Pedestrian-level air quality in urban areas is largely affected by the pollutants emitted from street canyons to the atmospheric boundary layer (ABL). Most vehicular exhausts are chemically reactive that evolve to their secondary counterparts. Besides, building roughness modifies the flows, which, in turn, influences the chemical reactions in the ABL. The ABL pollutant transport is affected by advection, diffusion, and chemical reactions. The roles of individual terms and their collective effect on the overall ABL pollutant transport are not yet clear. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban area in the form of an array of idealised street canyons is investigated using large-eddy simulation. Nitric oxide (NO) is emitted from the ground level of the first street canyon into the urban ABL doped with ozone (O3). Budget analysis of the transport processes is conducted. It is found that the contributions from advection, diffusion, and chemistry vary in the streamwise direction and they couple closely with each other. 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 of pollutant species is rather uniform. In view of the elevated shear stress at the roof level, advection, diffusion, and chemistry show abrupt changes, complicating the pollutant dispersion processes.
Highlights
Air quality problems have aroused major public concern recently especially in urban areas with dense population
This paper looks into the transport mechanism of chemically reactive pollutants over idealised urban roughness using Large-eddy simulation (LES)
The wall-normal distance z is normalised by the atmospheric boundary layer (ABL) thickness H and the velocity is normalised by the prevailing wind speed U∞
Summary
Air quality problems have aroused major public concern recently especially in urban areas with dense population. Elevated pedestrian-level pollutant concentrations are commonly found in metropolises because of the heavy traffic emission in dense built environment. Vehicular exhaust has become one of the major pollutant sources in urban areas. Nitrogen oxides (NOx) are some of the major pollutants in the atmospheric boundary layer (ABL) whose high concentrations would adversely affect the health of inhabitants. According to the Environmental Protection Department (EPD) of Hong Kong Special Administrative Region (HKSAR), the total NOx emission in HKSAR in 2015 was about 91,700 tonnes. One of the major emission sources is road transport, contributing more than 18% to the total N Ox emission (EPD 2016).
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