Ice nucleation in sulfuric acid and ammonium sulfate particles

Cirrus clouds are composed of ice particles and are expected to form in the upper troposphere when highly dilute sulfate aerosols cool and become supersaturated with respect to ice. In the laboratory we have used Fourier transform infrared spectroscopy to monitor ice nucleation from sulfate particles for relevant compositions of sulfuric acid/water and ammonium sulfate/water aerosols. Measured freezing temperatures are presented as a function of aerosol composition, and results are compared to existing aerosol data. We find that sulfuric acid solution aerosol exhibits greater supercooling than ammonium sulfate solution aerosol of similar weight percent. Ice saturation ratios based on these measurements are also reported. We find that ammonium sulfate solution aerosol exhibits a relatively constant ice saturation of S similar to1.48 for ice nucleation from 232 to 222 K, while sulfuric acid solution aerosol shows an increase in ice saturation from S similar to1.53 to S similar to1.6 as temperature decreases from 220 K to 200 K. These high-saturation ratios imply selective nucleation of ice from sulfate aerosols.