Experimental Investigation of Transient Supersaturations in a Thermal Diffusion Chamber

Abstract

The effect of transient supersaturations on the concentration of cloud condensation nuclei (CCN) in a conventional parallel-plate type thermal diffusion chamber is experimentally investigated by controlling the relative humidity of the sample and the temperature of the top and bottom plates individually. It is found that the error in CCN counts is quite significant around 90% relative humidity and might be dominant at cloud bases where the relative humidity approaches 100%. The error is a combined result of thermal and hydrodynamical transients. It is suggested that a chamber having a continuous and laminar inflow of the sample which has the steady-state temperature profile already existing in it is but suited for reliable CCN measurements. The calculations based on the theory of Saxena, Burford and Kassner help explain the obtained results which also support the earlier theoretical predictions of Fitzgerald. The results seem to disagree with recent experimental observations reported by Radke and Hegg.