Study of chemically reactive pollutant transport over urban roughness in the atmospheric boundary layer

Abstract

Elevated pollutant concentrations are commonly observed in urban areas, such as street canyons, threatening human health. While practical dispersion models commonly assume inert pollutants, most emissions from traffic exhaust are chemically reactive. Reactive flue gases, including nitrogen oxides (NOx), are important pollutants that would cause a series of public health problems. There is thus a need for improved understanding of the dynamics of chemically reactive pollutants. In this study, turbulent dispersion of reactive pollutants in the atmospheric boundary layer (ABL) over hypothetical urban areas is numerically investigated using large-eddy simulation (LES). Their transport behavior in and over idealized street canyons of unity aspect ratio in isothermal conditions is examined. 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 background ozone (O3). By looking into the pollutant concentrations and dispersion characteristics over the urban canopy layer, it is realized that, apart from pollutant removal, ABL turbulence plays an important role in the mixing of chemicals, which substantially affects the reaction rates of chemical kinetics. Six different scenarios with different background O3 concentration (1 ppb, 10 ppb, 50 ppb, 100 ppb, 500 ppb and 1,000 ppb) are studied. To contrast the effect of chemistry and dispersion, the time scales of reaction and diffusion of pollutants are compared by switching on and off the NOx-O3 chemistry component in the LES. The plume characteristics and time scales in different cases will be discussed.