Calibration of a Photometric Cloud Condensation Nucleus Counter Designed for Deployment on a Balloon Package

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Abstract
The importance of atmospheric aerosols in understanding global climate changes has renewed interest in measurements of cloud condensation nuclei (CCN). To obtain high-resolution (125 m) vertical profiles of CCN number concentration, a balloon-borne instrument was developed. The instrument deduces the CCN concentration from measurements of laser light scattered by water droplets that condense on CCN within a static thermal-gradient diffusion chamber. The amount of light scattering is linearly proportional to the number of droplets within the diffusion chamber. Correlating the number of droplets within the sample volume with the amount of light scattered by the droplets provides the calibration constant that relates scattered light to CCN concentration. The calibration was tested by comparisons between the CCN counter and a condensation nuclei counter when sampling monodisperse aerosol larger than the CCN counter’s critical activation size. The calibration constant depends on supersaturation, and depends slightly on the size of CCN that activate to form droplets. For dry NaCl aerosol between 35 and 160 nm, the calibration constant varies by less than 10% at 1% supersaturation. Calibration on ambient atmospheric aerosol is similar to calibration on laboratory-generated polydisperse NaCl aerosol, which indicates that the laboratory calibration can be applied to field measurements. During field and laboratory measurements, the time required for the activation and growth of droplets within the diffusion chamber is similar. Overall, the uncertainty of the calibration constant for the balloon-borne CCN counter is approximately 10% at 1% supersaturation.

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