Modelling horizontal and vertical concentration profiles of ozone and oxides of nitrogen within high-latitude urban areas

Urban ozone concentrations are determined by the balance between ozone destruction, chemical production and supply through advection and turbulent down-mixing from higher levels. At high latitudes, low levels of solar insolation and high horizontal advection speeds reduce the photochemical production and the spatial ozone concentration patterns are largely determined by the reaction of ozone with nitric oxide and dry deposition to the surface. A Lagrangian column model has been developed to simulate the mean (monthly and annual) three-dimensional structure in ozone and nitrogen oxides (NO x ) concentrations in the boundary-layer within and immediately around an urban areas. The short-time-scale photochemical processes of ozone and NO x , as well as emissions and deposition to the ground, are simulated. The model has a horizontal resolution of 1×1 km and high resolution in the vertical. It has been applied over a 100×100 km domain containing the city of Edinburgh (at latitude 56°N) to simulate the city-scale processes of pollutants. Results are presented, using averaged wind-flow frequencies and appropriate stability conditions, to show the extent of the depletion of ozone by city emmisions. The long-term average spatial patterns in the surface ozone and NO x concentrations over the model domain are reproduced quantitatively. The model shows the average surface ozone concentrations in the urban area to be lower than the surrounding rural areas by typically 50% and that the areas experiencing a 20% ozone depletion are generally restricted to within the urban area. The depletion of the ozone concentration to less than 50% of the rural surface values extends only 20 m vertically above the urban area. A series of monitoring sites for ozone, nitric oxide and nitrogen dioxide on a north–south transect through the city – from an urban, through a semi-rural, to a remote rural location – allows the comparison of modelled with observed data for the mean diurnal cycle of ozone concentrations. In the city centre, the cycle is well reproduced, but the ozone concentration is consistently underestimated.