Further studies of flow and reactive pollutant dispersion in a street canyon with bottom heating

Abstract

This study numerically investigates how flow and reactive pollutant dispersion in a street canyon with a canyon aspect ratio of one vary with the street-bottom heating intensity. For this, numerical simulations are performed over a wide range of street-bottom heating intensities (Δ T=0–15 °C in 1 °C intervals) using a Reynolds-averaged Navier–Stokes equations (RANS) model with NO–NO 2–O 3 photochemistry. The pollutants NO and NO 2 are emitted from near the street bottom in the presence of background O 3. A primary vortex is formed in the street canyon in all the simulated cases, but the location of the vortex center at the cross-canyon plane becomes quasi-stationary or meanders in the street canyon, depending on the street-bottom heating intensity. The time series of the street canyon-averaged NO concentration at the cross-canyon plane exhibits a quasi-steady, oscillatory, or fluctuating pattern, depending on the street-bottom heating intensity. As the street-bottom heating intensity increases, the averaged NO and NO 2 concentrations tend to decrease and the magnitude of the roof-level area-integrated and time-averaged vertical mean (turbulent) flux between Δ T=2 and 13 °C tends to increase (decrease). Some peculiar features are found in the cases of Δ T=11, 12, and 13 °C in which there are strong downward motion near the downwind building wall, downward motion below the roof level near the upwind building wall, and strong reverse flow in the lower region of the street canyon. Moreover, the vortex center is shifted toward the upwind building wall and does not meander in the street canyon after a certain period of time. Following the downward motions below the roof level, the O 3-containing ambient air is considerably entrained into the street canyon, resulting in a large increase in the O 3 concentration in the street canyon. In the cases of Δ T=11, 12, and 13 °C, the magnitude of the vertical mean flux is larger than that of the vertical turbulent flux. The direct effect of the inhomogeneous temperature distribution on the O 3 concentration via the temperature-dependent photolysis rate and reaction rate constant increases with the street-bottom heating intensity, although the averaged fractional difference in O 3 concentrations is small.