Supersaturation in the Wyoming CCN Instrument

Two thermal diffusion cloud condensation nuclei (CCN) instruments were intercompared using a nearly monodisperse test aerosol composed of sodium chloride and ammonium sulfate. The main objective of this work is the determination of the maximum steady-state supersaturation within a thermal diffusion chamber. This required analysis of the uncertainties associated with the calibration of the instrument’s plate temperature and light scattering measurement systems. The fraction of test particles activating to form cloud droplets, the activated fraction, was evaluated over a range of dry particle diameters at four supersaturations. Values of both the particle size, corresponding to an activated fraction equal to 0.5, and the width of a fitted activation function were derived. The former was used to evaluate the maximum steady-state chamber supersaturation. These studies reveal three significant findings. First, the derived chamber supersaturation is ∼40% smaller than values based on temperature measured at the top and bottom of the diffusion chamber. Second, using deliquesced test particles, mobility selected at a prescribed relative humidity, it is shown that the 40% discrepancy is not the result of a test particle shape effect. Third, the width of the activation function is substantially larger than the width of the test particle size spectra. Plausible explanations for the 40% bias are considered. Contributors are the unintentional overestimation of the temperature difference imposed across the CCN chamber and the implicit assumption that water at the top and bottom of the CCN chamber has a vapor pressure equal to that over pure water.