Calculate Peak to Mean RatioThe peaktomean scalingis important for understanding the response to windborne odours. Here Cm is the measure of mean concentration over an averaging time scale tm for which meteorological conditions are persistent, while Cp is the expected peak concentration, averaged over time tp, during the time tm. Cm is typically a tenminute or onehour average and is routinely and reliably modelled. Peak concentrations Cp occur in relatively rare patches of poorly diluted odorous source material sensed by a ‘nose’ downwind of the source. The value of tp can vary from molecular diffusion timescales (subseconds) to seconds and minutes. We have been able to relate these peak concentrations, the scaling properties, and the physical structure of the odour plumes to a recent prediction for the probability density function for an odour concentration (in fact, any passive tracer), which is characterised by a power law for the largeconcentration rareevent tail. This socalled inertialrange behaviour extends from concentrations near the ensemble mean value, which is nominally small at any significant distance from the source, up to very large concentrations compared to the mean, limited only by the maximal initial source concentration and by molecular diffusion. We find that for a pointsource plume the exponent p is given by p = 6/17 ~0.35, and for a linesource plume, p = 3/14 ~0.21. Both these values are already established empirical results, but we have shown that they are a consequence of modern turbulence theory and are universal properties independent of material and dispersion parameters. See:Borgas, M.S. (2000). The mathematics of whiffs and pongs. Presented at Enviro2000 Odour Conference, Sydney, 913 April 2000: Clean Air Society of Australia and New Zealand (see a slide presentation). Hibberd, M. F. (1998). Peaktomean ratios for isolated tall stacks (for averaging times from minutes to hours). In: Proceedings of the 14th International Clean Air and Environment Conference , Melbourne. Mitcham, Vic.: Clean Air Society of Australia and New Zealand. p. 255260. The code for the calculator can be viewed here. For more information, email: michael.borgas@csiro.au


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