Results from Elevated-Source Urban Area Dispersion Experiments Compared to Model Calculations

Atmospheric dispersion experiments are carried out in Copenhagen under neutral and unstable conditions in order to study the atmospheric dispersion process in a built-up area. The tracer sulphurhexafluoride is released without buoyancy from a meterological instrumented tower at a height of 115 m, and then collected at ground-level positions in up to three crosswind series of tracer-sampling units, positioned 2–6 km from the point of release. In addition to standard meteorological profile measurements along the tower, the three-dimensional wind fluctuations at the height of release are measured. Characteristic dispersion parameters are estimated from the measured tracer concentrations (averaging time 1 hour), and compared to the dispersion parameters that can be calculated from the atmospheric parameters by various methods. Some of these methods (based on a stability-index) have been in common use for a long time, other (based on wind-fluctuation measurements) reflect recent research. The wind-fluctuation based methods turned out to compare most favourably with the results from the experiments. Based on these experiments a half-empirical model is devised for the prediction of the lateral and vertical dispersion parameters, σ_y and σ_Z, for elevated point sources in an urban area under neutral to unstable conditions.