Single Particle Characterization of Atmospheric Ice Nucleating Aerosol

A method to determine the chemical composition of single aerosol particles capable of nucleating ice has been developed. The Colorado State University Continuous Flow Diffusion Chamber (CFDC) was used to expose sub-micron ambient aerosol to conditions supersaturated with respect to both ice and liquid water. Particles which nucleated ice and grew to super-micron size were separated from those which did not activate using a Laboratory Counter-flow Virtual Impactor (LCVI). Condensed-phase water was removed and particle composition was ascertained using the National Oceanic and Atmospheric Administration Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. The viability of the CFDC/LCVI/PALMS method to activate a fraction of the ambient aerosol population, separate ice crystals from the remainder, and analyze their chemical composition was determined by laboratory experimentation. Specifically, a dual population of AgI (capable of nucleating ice under the experimental conditions) and NH4NO3 (incapable of activation) particles was sent through the system. Results showed that only AgI particles were capable of passing through the LCVI and on to PALMS. Subsequently the CFDC/LCVI/PALMS system was deployed at 3220 m elevation in north-central Colorado. The Desert Research Institute Storm Peak Laboratory was chosen for two primary reasons. First, a suite of gas phase and aerosol characterization instruments that include differential mobility analyzers and optical particle counters to obtain aerosol size distributions up to particle diameters of several microns 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 hours. 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 composed of sulfates 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.