RANS simulation of near-field dispersion of reactive air pollutants

In conventional modeling of air pollution dispersion, pollutants are treated as passive scalars or inert species even though most of them are chemically reactive [1]. Chemical reactions contribute to pollutant dispersion via the generation and depletion of pollutants, in addition to other two mechanisms: advection and turbulent diffusion. This study investigated how chemical reactions affect near-field pollution dispersion by integrating the simple NO_x-O₃ chemistry into RANS-based computational fluid dynamics (CFD) simulation. CFD simulation was used to model a mixed emission of NO and NO₂ from a short stack attached to a building into ambient O₃, prompting chemical reactions between the NO, NO₂, and O₃. Various degrees of chemical reactivity were modeled by varying the Damkhöler number (Da) between 0.073 and 4.363. The results showed significant chemical reactivity for cases where Da [NO] > 1, while cases with Da [NO] < 1 had pollutant dispersion patterns similar to inert species. Noticeable modifications in concentrations were detected at ground level, where the NO concentration was depleted and NO₂ concentration increased significantly. A budget analysis revealed major contributions of chemistry and turbulent diffusion to plume dispersion in the surroundings, while advection mainly carried the pollutants downstream from the source.