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Some Size Distribution Measurements of AgI Nuclei with an Aerosol Spectrometer

Hermann E. GerberAtmospheric Science Laboratory, U.S. Army, Ft. Monmouth, N.J.

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Paul A. AlleeAtmospheric Physics and Chemistry Laboratory, ESSA, Boulder, Colo.

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Ulprich KatzDept. of Atmospheric Science, Colorado State University, Ft. Collins.

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Charles I. DavisDept. of Atmospheric Science, Colorado State University, Ft. Collins.

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Lewis O. GrantDept. of Atmospheric Science, Colorado State University, Ft. Collins.

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Abstract

The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000Å In diameter.

Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuclei.

Also, field measurements were conducted on 12,000-ft Chalk Mountain (Climax, Colo.) for the purpose of measuring the sizes of active AgI-NaI nuclei emanating from acetone ground generators located at least 6 mi upwind. The size distribution of the nuclei on seeding days proved similar to what might he expected from this generator type. On non-seeding days, the number of active nuclei decreased sharply while the peak of the size distributions shifted to larger sizes.

Abstract

The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000Å In diameter.

Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuclei.

Also, field measurements were conducted on 12,000-ft Chalk Mountain (Climax, Colo.) for the purpose of measuring the sizes of active AgI-NaI nuclei emanating from acetone ground generators located at least 6 mi upwind. The size distribution of the nuclei on seeding days proved similar to what might he expected from this generator type. On non-seeding days, the number of active nuclei decreased sharply while the peak of the size distributions shifted to larger sizes.

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