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ON THE DISPERSION OF LARGE PARTICLES FROM A 15-M SOURCE IN THE ATMOSPHERE

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  • 1 Department of National Defence, Canada
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Abstract

In a series of nine experiments, embracing a range of wind speeds and stability conditions, fluorescent-dyed glass microspheres of nominal diameter 100µ were emitted continuously from a point source at 15 m over gently rolling prairie. The particles were collected on flat-plate adhesive sampling surfaces at ground level along arcs between the source and a distance of 800 m. The observed crosswind-integrated deposits are compared with the predictions of Rounds' solution to the K theory of eddy diffusion and with Godson’s modification of Rounds' solution. The results indicate that Rounds' solution tends to overemphasize the effects of vertical diffusion on large particles. Godson's modified solution improves the predictions in stable cases but results in excessive dispersion of the particles in the direction of the wind under lapse conditions. Some evidence is provided to suggest that, in certain cases, measurements of vertical temperature gradient and wind-speed profiles between ground level and emission height are not sufficient to provide unique or reliable indicators of the intensity of vertical turbulence for the purpose of predicting the dispersion of particles.

Abstract

In a series of nine experiments, embracing a range of wind speeds and stability conditions, fluorescent-dyed glass microspheres of nominal diameter 100µ were emitted continuously from a point source at 15 m over gently rolling prairie. The particles were collected on flat-plate adhesive sampling surfaces at ground level along arcs between the source and a distance of 800 m. The observed crosswind-integrated deposits are compared with the predictions of Rounds' solution to the K theory of eddy diffusion and with Godson’s modification of Rounds' solution. The results indicate that Rounds' solution tends to overemphasize the effects of vertical diffusion on large particles. Godson's modified solution improves the predictions in stable cases but results in excessive dispersion of the particles in the direction of the wind under lapse conditions. Some evidence is provided to suggest that, in certain cases, measurements of vertical temperature gradient and wind-speed profiles between ground level and emission height are not sufficient to provide unique or reliable indicators of the intensity of vertical turbulence for the purpose of predicting the dispersion of particles.

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