A Continuous-Flow Diffusion Chamber for Airborne Measurements of Ice Nuclei

A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between ∼3- and 10-μm diameter in 3–10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than ∼0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational procedures, and performance. Limitations of the technique are presented, and examples of measurements are shown.