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A brief survey is given of the present position in the specification of atmospheric dispersion parameters for use in estimating pollutant concentration from a continuous-point release.
The theorectical indications of the distribution to be expected across a time-mean plume are reviewed, with particular reference to the existence of the Gaussian form. Observational evidence, especially on vertical distribution from a surface release, is also recalled, and the practical significance of departure from an assumed Gaussian form is noted.
The use of the Taylor statistical theory in the generalized estimation of crosswind spread in quasi-ideal boundary-layer flow is briefly summarized. Recent considerations of the behavior of the crosswind component of turbulence in the surface layer and new developments from laboratory modeling of horizontal dispersion in convective mixing are noted.
A brief survey is given of the achievements of the gradient-transfer theory and the Lagrangian similarity theory in calculating vertical spread from a surface release. New tests against previous dispersion data underline inadequacies in the present approaches in very unstable conditions. Promising developments from laboratory modeling of a convectively mixed layer and from the second-order-closure modeling of the turbulent fluctuation equations are summarized.
The assimilation of theory and experience into practical systems for the specification of σy and σz is briefly reconsidered. For σy a practical procedure, based on wind direction fluctuation data, is reaffirmed. For σz a new format, which may be envisaged for future composite curves, is suggested. Finally, the inherent limitations of practical systems for estimating concentration levels are reiterated.
dispersion, eddy diffusion, Taylor statistical theory, Lagrangian similarity theory, convection, mixed layer, ozone
Winnersh, Wokingham, Berkshire RG11 5JE, England