Operation of a Thermal Diffusion Chamber for Measurements on Cloud Condensation Nuclei

Abstract

In recent years the thermal diffusion chamber has found wide application in studying condensation nuclei (CCN) which are effective in natural cloud and fog formation. Explicit solution of the equations governing the transient behavior of the chamber suggests the necessity for precise control of the temperature or the relative humidity of the incoming sample if meaningful measurements are to be obtained. A recent conclusion of Fitzgerald, that transient supersaturations exceeding the steady-state peak value may arise if the incoming sample is saturated at a temperature less than that of the (hot) top plate, is also verified. Further, it is pointed out that as long as turbulence occurs while introducing the sample, the transient behavior of the chamber remains indeterminate. This defect may be eliminated by giving special consideration to the method of sample introduction. The measurements of CCN concentration as a function of time at Rolla taken with a thermal diffusion chamber are also presented. The CCN concentration, in general, is found to follow local meteorological conditions. The trend of our data shows a qualitative agreement with that available in the literature. It is suggested that an intercomparison of the data taken on a carefully fabricated thermal diffusion chamber with those taken on the other types of cloud chamber would help to decide the potential value of the former as a CCN counter.