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Single Particle Mass Spectrometry of Ice Nuclei

Daniel J. Cziczo
N.O.A.A. Aeronomy Laboratory
Boulder, Colorado

A method to determine the chemical composition of particles capable of nucleating ice has been developed by researchers from CIRES, NOAA, and CSU. A continuous flow diffusion chamber was used to expose sub-micron ambient aerosol to conditions supersaturated with respect to both ice and water. Particles which nucleated ice and grew to super-micron size were separated from those which did not activate using a counterflow virtual impactor. Particle composition was ascertained using the NOAA PALMS instrument. This system was deployed at 3220 m elevation in north-central Colorado. The location was chosen for two primary reasons. First, a suite of gas phase and aerosol characterization instruments are continuously run on-site. Second, the elevation of the lab is such that it sits in free tropospheric air during extended periods, often during the nighttime hrs. Several hundred mass spectra of aerosol particles which nucleated ice were collected during a three week period in November, 2001. Results indicate that the dominant background aerosol did not act as ice nuclei. A subset, enriched in silicon and many of possible anthropogenic origin, was observed to induce ice formation. These same particles therefore make likely candidates for the formation of cirrus clouds.