An Instantaneous CCN Spectrometer

Abstract

A thermal gradient diffusion cloud chamber with a supersaturation field that increases along the path of the flow of sample is used as a cloud condensation nucleus (CCN) spectrometer. The CCN spectrum is derived from the final droplet size distribution which has been determined to be related to the nucleus critical super-saturation based on routine calibration procedures using laboratory-produced monodisperse salt particles. The instrument is compared against a previously reported CCN counter on natural and laboratory-produced aerosol and against an absolute CN counter on the monodisperse aerosol. Preliminary results of atmospheric measurements are given. This device produces a CCN spectrum over the entire useful range of interest for cloud physics (0.01% to 1% supersaturation). More than 40 channels of resolution over this range are simultaneously provided. Data can be collected on a continuous basis for integration times as short as 2 seconds. The instrument has obtained data from four different airborne platforms as well as from a mobile surface platform. The number of channels allows the CCN spectrum to be displayed differentially instead of the usual cumulative display traditionally used in cloud physics.