AN INTERCOMPARISON OF SEMIEMPIRICAL DIFFUSION-MODELS UNDER LOW WIND-SPEED, STABLE CONDITIONS

The performances of some diffusion models are analysed using concentration data measured at ground level up to 400 m from the emission point, in a series of diffusion tests conducted by U.S. National Oceanic and Atmospheric Administration (NOAA) under inversion conditions with light winds. All tested models are simple semiempirical formulae based on the Gaussian formulation, with different assumptions concerning dispersion parameters; each model utilizes a minimum set of information, i.e. vertical stability category, mean wind speed and standard deviation of the horizontal wind direction over the considered averaging time, sigma(theta). Results show that for cases with very low wind speed and large plume spread, explicit consideration of diffusion along the mean wind direction, which is neglected in the standard plume model, significantly improves model results; moreover, when sigma(theta) is very large (greater than 50-60-degrees), the analysis suggests that standard deviations of the horizontal wind speed may significantly differ from the estimates commonly found in the literature.