Modeling reactive pollutant dispersion in an urban street canyon

Abstract

Reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–ε turbulence model and includes transport equations for NO, NO 2, and O 3 with simple photochemistry. An area emission source of NO and NO 2 is considered in the presence of background O 3 and street bottom heating (Δ T=5 °C) with an ambient wind perpendicular to the along-canyon direction. A primary vortex is formed in the street canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the canyon space. The cross-canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO 2 concentrations but out of phase with O 3 concentration. The photostationary state defect is small in the street canyon except for near the roof level and the upper downwind region of the canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO 2) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O 3 concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O 3 concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.