The urban plume of Vienna: comparisons between aircraft measurements and photochemical model results

A Lagrangian photochemical box model (IMPO model) developed for ozone forecasts and long-term scenario calculations was evaluated by comparing simulated ozone and nitrogen dioxide concentrations with aircraft measurements done around Vienna/Austria during episodes of high ozone concentrations in 1995. The major aim of these measurements was to record position, extent and intensity of the urban ozone plume. The Lagrangian model simulations were done on a grid covering Vienna and surroundings. The simulated concentrations were interpolated to the aircraft positions. In most cases, the horizontal position of the plume was reasonably well reproduced by the model, demonstrating that despite of all the necessary simplifications, a box model based on air-mass trajectories can be a valuable tool in describing the transport of pollutants. The ozone production in the plume was underpredicted by a factor of 2. High nitrogen dioxide concentrations measured near the city centre were reproduced by the model, but there are strong indications that nitrogen dioxide concentrations were underpredicted by the model in rural areas. A clear quantification proved to be impossible due to uncertainties in the measurement data.