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

Abstract

Transport of passive and inert pollutant has been well explored in the past decades but its chemically reactive counterpart is rare. This paper examines the dispersion of chemically reactive pollutants over hypothetical urban areas using large-eddy simulation (LES). Under isothermal conditions, the flows in the urban canopy layer (UCL) over twelve idealized two-dimensional (2D) street canyons of unity aspect ratio are calculated. As a pilot study, the simple NOx-O3 chemistry, consisting of three chemical reactions, is considered. The ground-level area source of NO in the first street canyon and the background O3 from the prevailing wind initiate the NO2 production. NO and O3 react reversibly generating NO2 until chemical equilibrium which is measured by the photostationary state (PSS) of the system. Its spatial instability is a result of the difference in time scales in turbulent mixing and chemical reactions. Using various combinations of ground-level NO and background O3 concentrations, the sensitivity of photostationary state to turbulent mixing and chemical reactions is investigated. The PSS of the first street canyon increases with increasing ground-source NO. It is most stable at the recirculation centre due to the prolonged retention for a complete mixing of NO and O3. In the scenario of small ground-level NO and small background O3 (e.g. O3 = 1 ppb and NO = 1 ppb), the average PSS in the second to the twelfth street canyons increases gently as the NO concentration is low such that all the NO is used up in the first street canyon, leading to the negligible chemistry in the rest of the downstream canyons. While the NO-to-O3 ratio is increased to 1000/30, as an example, the average PSS from the second to twelfth canyons increases faster since the higher NO level promotes the downstream chemical reactions. Further increase the NO-to-O3 ratio, say 10000/1, the trend of the average PSS is non-linear in which a trough is observed in the fifth street canyon. This non-linear behaviour is likely governed by the turbulence and chemistry time scales.