Accuracy Limitations on CCN Measurements with Thermal Gradient Diffusion Chambers

Abstract

Limitations on the accuracy of CCN counts obtained with thermal gradient diffusion cloud chambers (TGDCC) are discussed. Evidence presented here regarding 1) the absence of a well-defined plateau when the number of CCN in the TGDCC are plotted as a function of time, 2) the discrepancy between CCN counts obtained by instantaneous photography and the time-exposure method, and 3) the difficulty in reaching a sufficiently small minimum detectable size throughout the range of supersaturations, points to the possibility of considerable error in instantaneous CCN counts. The time-exposure method is demonstrated as a means of eliminating the problems associated with the instantaneous method and of extending the measurements to lower supersaturations.