A study of the dispersion and transport of reactive pollutants in and above street canyons—a large eddy simulation

Abstract

Abstract This study extends a large eddy simulation model used to simulate turbulent flow in an urban street canyon to the dispersion and transport of reactive pollutants (NO, NO2 and O3). The original model, based on the regional atmospheric modelling system, has previously been validated against wind tunnel experiments for wind flow and turbulence. A real scale street canyon with an aspect ratio of one was studied with a neutrally stratified atmosphere and initial wind perpendicular to the street axis. After initialising and running the model to generate a stationary turbulent flow a constant discharge source of NOx was specified to imitate steady traffic flow through the street canyon. Line and area emissions sources for low and medium emissions of NOx were considered. A background concentration of ozone was specified and the gases were allowed to mix and react. Spatial variation of pollutants within the canyon was calculated to be significant. The photostationary state defect, ( δ ps = k [ NO ] [ O 3 ] / J NO 2 [ NO 2 ] - 1 ) , was found to be a sensitive indicator of reactive mixing within the canyon, the greater the defect the greater the local state of chemical instability. Low values for δ ps were found at the centre of the street canyon, within the primary vortex, and at the ground level corner of the windward wall, where a secondary eddy was predicted. High values were found above ground level along the windward wall, where air was entrained into the canyon along the outer edge of the primary vortex, and downstream of, and close to the NOx emission. Above the canyon a region of high δ ps corresponds to the edge of the escaping canyon plume.