Comparison of Parallel Plate Thermal Diffusion Chambers used for Measuring the Cloud Condensation Nuclei Concentration in the Atmosphere

Abstract

Four thermal diffusion chambers with parallel plates, having different geometric characteristics, modes for reaching supersaturation (on cooling or heating the plates) and droplet-counting techniques, have been compared in the range of super saturations between 0.15 and 1.25%. The concentrations measured by two chambers which are operated simultaneously may be correlated by a power function fitted with the characteristic law of cloud condensation nuclei N = CSk, where N is the concentration of the nuclei activated at supersaturation S(%), and C and k are parameters depending on the air mass and the whole of the device. The heating of the upper plate, a small value of the ratio between the diameter of the chamber and its height (aspect ratio), and the inner turbulence are factors which increase the value of the measured concentration. For values of N at ?3500 and 4200 cm?3 the responses of the four chambers are very close, and for lower or higher values of N their discrepancies increase.