Measurements of natural ice nuclei with a continuous flow diffusion chamber

Measurements of natural ice nuclei were made in winter continental airmasses with a continuous flow thermal gradient diffusion chamber (described in a separate paper). Over the range of temperatures -7 degrees C to -20 degrees C, the concentration of ice nuclei was closely related to ice supersaturation (SSi) for humidities both below and above water saturation. Measurements below water saturation were interpreted as deposition nuclei with average concentrations (per liter) approximately 0.32 SSi (%)(0.81). Measurements were made up to 5% above water saturation and activated both deposition and condensation-freezing nuclei. The average concentration of condensation-freezing nuclei was 0.25 e(-0.15 T)(C-center dot). Sample residence time in the chamber was probably too small to detect contact nuclei, unless the nucleating aerosols are extremely small. There was large variability in nucleus concentrations, as much as two orders of magnitude at -15 degrees C. Comparisons are made between these ice nuclei measurements and aircraft observations of ice crystal concentrations in winter orographic clouds.