Evaluation of atmospheric dilution factors for effluents diffused from an elevated continuous point source

Abstract

Using the wind vector fluctuations measured at a height of 69 m by a tridimensional anemometer, the standard deviations σy and σz of the lateral and vertical distributions of the effluents emitted by an elevated continuous point source have been computed. Nearly 300 hourly tests covering most of the weather conditions to be encountered on the Mol site have been made, during which have also been determined the values of a stability parameter S defined as the ratio of the hourly mean of the vertical gradient of the potential temperature between 8 and 114 m to the square of the hourly mean wind speed at 69 m. The dependence functions σy or z = f(S) are of the hyperbolic type for S > 0 as well as for S < 0; they apply for hourly mean wind speeds up to 11.5 m σ1. For the sake of simplicity and quick estimations of σy and σz for current practical uses, seven atmospheric stability categories have been defined with the corresponding median σy and σz values at different downwind distances x. When ū < 11.5 m.σ1, categories E1 to E4 are differentiated by means of a parameter λ derived from S by λ = log10 [|S| · 106]; category E7 corresponds to the case of strong winds. The data obtained show that for downwind distances x up to 50 km, the standard deviations σy and σz depend on x following a power law. The Mol theoretical results compare favourably with those found by means of field experiments carried out on the Mol site as well as elsewhere by other authors.