Instantaneous versus time-average dispersion models

3 Instantaneous versus time-average dispersion models 

The Gaussian dispersion models discussed so far all predict the average concentration of odor in space over a fixed time interval. The models are derived from 

The work of Murlis and Jones (1981) illustrates the discontinuous, filamentous nature of the instantaneous odor plume. The continuous stream of ions they released arrived at samplers downwind in discrete bursts. The disjunct nature of the plume is also evident in the instantaneous (1 minute) pattern of wing-fanning onset in the data of Elkinton et al. (1984, unpublished) along a continuous cross-section of the plume 20 m downwind of the source (Fig. 3.4). 

The turbulent eddies that cause an odor plume to disperse occur simultaneously over a range of sizes in the atmosphere. Energy derived from the largest eddies is passed down into smaller and smaller eddies until it dissipates as heat. As pointed out by Slade (1968), Mason (1973), Aylor (1976) and Aylor et al. (1976) the dispersion of an instantaneous odor plume is driven by eddies that are about the same size as the plume. Eddies that are smaller than the plume redistribute the pheromone within the plume whereas the larger eddies cause the plume to meander downwind intact. At the small end of the spectrum there exists a lower eddy size limit below which molecular viscosity causes the eddies to break down and dissipates their energy as heat. Typically, the minimum eddy size is in the order of 1 mm. 

Pasquill 1974, p. 46) followed by a final phase of slower growth. In contrast to Aylor s model, the explosive growth phase in Miksad and Kittredge s model occurs many meters downwind of the source. The length of the initial filament stage (up to 40 m) rests upon the assumption that the size of the typical pheromone source is much smaller than the smallest eddies. The size of the typical pheromone source they cited (0.1 to 1 mm) is smaller than that typical for most insects. Consequently, the initial filament stage may only be a few cm as in Aylor et al. (1976). 

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