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How Well Does Water Activity Determine Homogeneous Ice Nucleation Temperature in Aqueous Sulfuric Acid and Ammonium Sulfate Droplets?

Brian D. Swanson Department of Earth and Space Sciences, University of Washington, Seattle, Washington

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Print Publication:
01 Mar 2009
DOI:
https://doi.org/10.1175/2008JAS2542.1
Page(s):
741–754
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Abstract

Frozen fraction measurements made using a droplet free-fall freezing tube apparatus are presented and used, along with other recent laboratory measurements, to evaluate how well both the water activity idea and the translated melting-point curve idea of Koop et al. predict homogeneous freezing-point temperatures for aqueous ammonium sulfate and sulfuric acid solution droplets. The new freezing-point temperature datasets agree with the previous lowest-temperature results for both solutes. The lowest measured freezing-point temperatures for aqueous ammonium sulfate solutions agree with a curve shaped like the translated melting-point curve. However, those for aqueous sulfuric acid solutions are significantly lower than predicted by the translated melting-point curve idea, and a single water activity freezing-point temperature curve does not represent the lowest-temperature freezing-point temperature data for both solutes. A linear extrapolation of the new aqueous sulfuric acid solution freezing data to low temperatures predicts that high critical supersaturations in cloud-free regions of the upper troposphere will occur when homogeneous ice nucleation in an aqueous sulfuric acid aerosol is the primary ice formation mechanism.

Corresponding author address: Brian D. Swanson, Laucks Foundation Inc., Bellevue, WA 98004. Email: brian@ess.washington.edu

Abstract

Frozen fraction measurements made using a droplet free-fall freezing tube apparatus are presented and used, along with other recent laboratory measurements, to evaluate how well both the water activity idea and the translated melting-point curve idea of Koop et al. predict homogeneous freezing-point temperatures for aqueous ammonium sulfate and sulfuric acid solution droplets. The new freezing-point temperature datasets agree with the previous lowest-temperature results for both solutes. The lowest measured freezing-point temperatures for aqueous ammonium sulfate solutions agree with a curve shaped like the translated melting-point curve. However, those for aqueous sulfuric acid solutions are significantly lower than predicted by the translated melting-point curve idea, and a single water activity freezing-point temperature curve does not represent the lowest-temperature freezing-point temperature data for both solutes. A linear extrapolation of the new aqueous sulfuric acid solution freezing data to low temperatures predicts that high critical supersaturations in cloud-free regions of the upper troposphere will occur when homogeneous ice nucleation in an aqueous sulfuric acid aerosol is the primary ice formation mechanism.

Corresponding author address: Brian D. Swanson, Laucks Foundation Inc., Bellevue, WA 98004. Email: brian@ess.washington.edu

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