Keywords: eddy scales, plumes, sonic anemometer measurements, stability, atmospheric dispersion models, environmental impact assessment, air pollution, environmental pollution, modelling, gas concentrations, averaging time
Comparisons of models with data: effects of averaging time as indicated by sonic anemometer measurements
For continuous sources, plots of predicted against observed gas concentrations often display considerable scatter. These differences are apparent even when the averaging times for the models and data match. Possible sources of error in the observations are shifts in direction of the mean wind or disturbance by intermediate- and large-scale eddies. Errors in predictions may arise from the selection of a specific value for a dispersion parameter, such as the standard deviation of the lateral wind fluctuations. Although usually assumed constant over the period during which gas concentrations are measured, this particular parameter may vary significantly. Especially interesting was the finding that the magnitude of this parameter is particularly sensitive to averaging time when conditions are stable, implying that errors in predicted concentrations could be significant if the model averaging time differs from that of the observations. These aspects are discussed in the light of measurements made with a sonic anemometer on an industrial site.