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In this study, the dispersion of chemically reactive pollutants is calculated by large-eddy simulation (LES) in a neutrally stratified urban canopy layer (UCL) over urban areas. As a pilot attempt, idealized street canyons of unity building-height-to-street-width (aspect) ratio are used. Nitric oxide (NO) is emitted from the ground surface of the first street canyon into the domain doped with ozone (O₃). In the absence of ultraviolet radiation, this irreversible chemistry produces nitrogen dioxide (NO₂), developing a reactive plume over the rough urban surface. A range of timescales of turbulence and chemistry are utilized to examine the mechanism of turbulent mixing and chemical reactions in the UCL. The Damköhler number (<i>Da</i>) and the reaction rate (<i>r</i>) are analyzed along the vertical direction on the plane normal to the prevailing flow at 10 m after the source. The maximum reaction rate peaks at an elevation where Damköhler number <i>Da</i> is equal or close to unity. Hence, comparable timescales of turbulence and reaction could enhance the chemical reactions in the plume.