Reactive Plume Dispersion Model over Idealized Urban Roughness in the Urban Atmospheric Boundary Layer

Gaussian plume dispersion model is commonly adopted to represent the pollutant concentration distribution after a continuous emission. However, most air pollutants are chemically reactive that evolve to their secondary counterparts in the atmospheric boundary layer (ABL). The conventional Gaussian plume model, which assumes inert pollutants, should be used with caution. In this study, the plume dispersion of a representative, irreversible chemical reaction, oxidation of nitric oxide (NO) to nitrogen dioxide (NO₂) by ozone (O₃) titration, is examined by large-eddy simulation (LES). The reactive plume dispersion over an idealized urban surface is employed to elucidate the coupled processes of turbulent transport and chemical reactions. A surface-depletion model, which is derived from the Gaussian plume model, is adopted to handle the chemical reactions. It reduces the emission strength as a function of downstream distance according to the chemical reactions (depletion of the source). Six different scenarios with different initial O₃ concentrations (1 ppb, 10 ppb, 50 ppb, 100 ppb, 200 ppb and 500 ppb) are studied to investigate the plume dispersion of chemically reactive pollutant. The current LES data are validated by wind tunnel measurements. The accuracy of the depletion model is tested by the LES data as well.