Comparing Critical Supersaturations: Atmospheric Cloud Condensation Nuclei vs. Known Compounds

Evans, William Barrett

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https://hdl.handle.net/2142/55198 Copy

Description

Title

Comparing Critical Supersaturations: Atmospheric Cloud Condensation Nuclei vs. Known Compounds

Author(s)

Evans, William Barrett

Issue Date

1994

Keyword(s)

• atmospheric clouds
• supersaturation
• cloud condensation nuclei

Abstract

Cloud Condensation Nuclei (CCN) are microscopic particles in the atmosphere which develop into water droplets when exposed to supersaturated conditions. The clouds formed by these droplets have a significant effect on the global climate. The purpose of this research was to experimentally determine the behavior and chemical composition of actual CCN particles. This was achieved by using captured CCN from a system including a cloud chamber and several virtual impactors. The captured CCN material was then resuspended as dry particles. This aerosol was then passed through a classifier and a cloud chamber in order to determine the critical radius and corresponding critical supersaturations of the CCN material. These data were then compared to similar data taken for suspected CCN materials. This comparison indicated that the activation characteristics of total CCN and large CCN (0.1 < dp < 0.5 µm) are controlled by the ammonium sulfate present in the particles. This was verified both graphically and numerically. The numerical method involved developing Ω, the number of ions per unit volume formed by dissolution of the CCN material. Ω was also calculated for organic compounds of unknown composition which were discovered in the actual CCN samples by chemical analysis. This value was Ωorganic s = 26,425.3. This is smaller than the Ω. for ammonium sulfate (40,095.5), indicating that the organic material is less active than ammonium sulfate. Thus, the presence of the organics acts to slightly retard droplet formation on large CCN. The organic compounds are not, however, so inactive that they could not nucleate under naturally occurring supersaturated conditions. Using Ωorganics, it was calculated that particles as small as 0.03 (µm in diameter which consisted entirely of the organic compounds could activate under natural conditions. This size is well within the observed size range for actual CCN.

Publisher

Illinois State Water Survey

Series/Report Name or Number

ISWS Miscellaneous Publication MP-152

Type of Resource

text

Permalink

http://hdl.handle.net/2142/55198

Copyright and License Information

Copyright ... University of Illinois Board of Trustees. All rights reserved. This document is a product of the Illinois State Water Survey, and has been selected and made available by the Illinois State Water Survey and the University Library, University of Illinois at Urbana-Champaign. It is intended for research and educational use, and proper attribution is requested.

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