Development and Characterization of a Horizontal Thermal Gradient Cloud Condensation Nuclei Spectrometer (CCNS)

We report the design and characterization of a continuous-flow instrument for measuring cloud condensation nuclei (CCN) under a range of supersaturation conditions. The main chamber of the instrument consists of two parallel copper plates separated by a thermal conducting wall and a non-conducting wall on either side. A temperature gradient is established across the width of the chamber by heating and cooling the edges of the two plates. Wetted cotton felt is placed on the inner surface of the copper plates to provide a supersaturation field. At the exit of the CCNS chamber, a movable scanning probe is employed to sample activated droplets at various supersaturations, which are counted by a downstream laser particle counter. We use a linear actuator and a computerized motion controller to control the location of the probe. A set of experiments were performed to calibrate the supersaturation field within the CCNS chamber using monodisperse NaCl aerosols. Dry particle size was varied at fixed scanning probe locations of CCNS, each corresponding to a certain supersaturation. The sharp growth curves yielded from these experiments were used to infer the actual supersaturation field. Following the instrument calibration, a second set of experiments were conducted to validate the CCNS performance, in which NaCl particles of constant sizes were allowed to grow and activate at various supersaturations. The activation curves were obtained in a timely manner by moving the scanning probe across the width of the chamber. The measured critical supersaturations for various particle sizes compare well with the theoretical values in our experimental range. This paper presents preliminary results of calibrating and validating the performance of our newly developed CCN spectrometer. The instrument features a unique design to scan through a range of supersaturations within a short time scale, allowing for fast determination of aerosol critical supersaturation and critical diameter, which is valuable for in situ measurements.