Large eddy simulation of NOx-O3 dispersion in a street canyon

This study extends a large eddy simulation (LES) model used to simulate turbulent flow in an urban street canyon to the dispersion and transfer of reactive pollutants (NO, NO₂, and O₃). The original model, based on the Regional Atmospheric Modelling System (RAMS), has previously been validated against wind tunnel experiments for wind dynamics and passive pollutant concentrations emitted as line sources along the street. A real scale street canyon with Reynolds number of the order of 106 and aspect ratio (width/height) of one was studied with a neutrally stratified atmosphere and an initial wind perpendicular to the street canyon axis. After initialising and running the model to generate a stationary turbulent flow, a constant discharge line sources of nitric oxide and nitrogen dioxide is set at the centre of the canyon floor to imitate a steady traffic flow through the street. A background concentration of ozone is specified for the street canyon and the area above and the gases are allowed to mix and react. The temporal and spatial distribution of pollutants and the variation in the effective rate of reaction between NO and O3 are analysed. It is found that average concentrations within the canyon stabilise over a short period of time but that significant spatial variations exist that have implications for assessing human exposure. The effective photochemical state ratio, [NO][O₃ ]/[NO₂], was found to be a sensitive indicator of mixing and reaction within the canyon.