Water Vapor Depletion in the DMT Continuous-Flow CCN Chamber: Effects on Supersaturation and Droplet Growth

The continuous-flow streamwise thermal-gradient cloud condensation nuclei counter (CFSTGC) is a commercially available instrument that is widely used for laboratory and field measurements of cloud condensation nuclei (CCN). All studies to date assume that the supersaturation profile generated in its growth chamber is not influenced by the condensation of water vapor upon the growing CCN. The validity of this assumption, however, has never been systematically explored. This work examines when water vapor depletion from CCN can have an important impact on supersaturation, measured CCN concentration, and droplet growth. A fully coupled numerical flow model of the instrument is used to simulate the water vapor supersaturation, temperature, velocity profiles, and CCN growth in the CFSTGC for a wide range of operation and CCN concentrations. Laboratory CCN activation experiments of polydisperse calibration aerosol (with a DMT CFSTGC operated in constant flow mode) are used to evaluate the simulations. The simulations and laboratory experiments are then generalized using a scaling analysis of the conditions that lead to supersaturation depletion. We find that CCN concentrations below 5000 cm(-3) (regardless of their activation kinetics or instrument operating conditions) do not decrease supersaturation and outlet droplet diameter by more than 10%. For larger CCN concentrations, a simple correction can be applied that addresses both the depression in supersaturation and droplet size.