Large-eddy simulation of reactive pollutant dispersion for the spatial instability of photostationary state over idealised 2D urban street canyons

Abstract

Dispersion of chemically reactive pollutants in the urban canopy layer (UCL) over twelve idealised two-dimensional (2D) street canyons of unity aspect ratio in isothermal conditions is examined. The reversible NO x -O 3 mechanism is integrated into a large-eddy simulation (LES) model. The ground-level NO emission in the first street canyon and the background O 3 in the prevailing wind initiate the NO 2 production. The chemical equilibrium is measured by the photostationary state (PSS) which is a function of the time scales of turbulent mixing and chemical reactions. PSS of the first street canyon increases with increasing NO emission. For small amount of ground-level NO emission and background O 3 in the prevailing flows, (say NO/O 3 = 1/1), PSS increases gradually from the second to last street canyons. It increases faster if the NO-to-O 3 ratio is raised, say 1,000/30. Further increasing the NO-to-O 3 ratio, say 10,000/1, PSS exhibits a non-linear behaviour in which a trough is observed in the fifth street canyon.