MRI: Development of a Cylindrical Cloud Condensation Nuclei (CCN) Spectrometer

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This is a Major Research Instrumentation award to enable the design and construction of a new instrument for measuring the activity spectrum of cloud condensation nuclei (CCN). The principle of operation is that air is drawn slowly and continuously through a thermal-gradient diffusion chamber in which the supersaturation increases with increasing distance into the chamber. At the exit of the chamber, the cloud droplets that have formed are counted and sized with an optical particle counter. A mathematical inversion procedure is then used to compute the CCN spectrum from the measured droplet spectrum, assuming a relationship between the size of a droplet and the critical supersaturation at which it formed. The proposed instrument is an improvement over existing designs for two reasons: modern digital electronics will be used for particle detection and counting, and by use of concentric cylindrical walls the cloud chamber will be in the shape of an annulus instead of a simple rectangular channel. The advantage of the annular shape is that undesirable edge effects associated with side walls are eliminated. The information provided by the instrument is fundamental for cloud microphysics and also bears on the way aerosol particles and their radiative effects are treated in numerical climate models.