Cloud Condensation Nuclei Counter

The Cloud Condensation Nuclei Counter (CCNC) is a countinuous-flow streamwise thermal-gradient CCN counter which is commercially available from Droplet Measurement Technologies (DMT), Boulder, CO, USA (Figure). The main component of the instrument is a vertical column in which a certain supersaturation of water vapor is etablished. The sampled aerosol particles are guided within a sheath flow through this column and can get activated into droplets, depending on the supersaturation and on the particles’ ability to act as CCN. An optical particle counter measures the numbers of activated droplets using side-scattering technology. The Cloud Condensation Nuclei (CCN) Counter is based on the design of external page Dr. Greg Roberts of Scripps Institute of Oceanography and external page Prof. Dr. Athanasios Nenes of École Polytechnique Féderale Lausanne (EPFL). The patent for their design is licensed exclusively to DMT, patent number 7,656,510.

The establishement of the supersaturation in the column works as follows: The walls of the chamber are wetted with water and thus the air above them is saturated. A vertical temperature gradient is applied to the walls (Figure). The physical basis to establish supersaturation in the column is that diffusion of water vapor (in air) is faster than diffusion of heat (in air): The heat flux reaching point C in the centerline of the sample flow originates from point A and the vapor diffusion from point B. The temperature at point A is colder than at point B, meaning that the partial pressure of water vapor is equal to point B but the temperature is equal to point A. This results in a supersaturation of water vapor at point C, according to the Clausius-Clapeyron equation (e.g. Rogers and Yau (1989)).

The CCNC is theroretically able to establish supersaturations from 0.07% to 2.0% which is a representative range of supersaturations experienced in the atmosphere for different cloud types.