Abstract
Hygroscopic materials in the air are collected on a membrane filter together with ice nuclei. They behave as a strong vapour sink and interfere with the detection of ice nuclei in the diffusion chamber. To remove hygroscopic materials from the specimen filter, a simple method was developed that was a kind of dialysis to float the filter on the surface of distilled water. The method was applied to the volcanic ash samples collected on the filter. Dialyzed and controlled specimen filters were subjected to the morphological test of particles by means of the electron microscope and to the test of ice nucleation by the use of the diffusion chamber. Supersaturation relative to water being achieved in the diffusion chamber was confirmed by Bergeron-Findeisen effect that appeared in the processing of specimen filters.
No difference was observed in the morphology of particles between controlled and dialyzed specimens. However, water soluble components were detected in the water used for dialysis. Thus volcanic ash particles were deduced to be mixed nuclei which consist of solid particles and water soluble components.
The ice nucleating activity of volcanic ash increased with decreasing temperature and with increasing supersaturation relative to ice. The dialyzed volcanic ash showed 2 or 3 times higher activity of ice nucleation than the controlled within the temperature range from -12 to -25°C and the humidity range of 13 to 35% supersaturation relative to ice.
On the basis of the nature of volcanic ash as mixed nuclei and the temperature and humidity dependence of ice nucleating activity, the mode of action of volcanic ash was presumed to be primarily a condensation freezing mechanism in the atmosphere.
