Experimental investigation of the homogeneous freezing of aqueous ammonium sulfate and sulfuric acid droplets

We have measured the light scattering intensity and homogeneous ice nucleation temperatures from water droplets containing 0 to 33 wt% ammonium sulfate and 0 to 8 wt% sulfuric acid. In these laboratory experiments, we use a free-fall freezing tube technique to determine the fraction of frozen droplets at a particular droplet temperature by measuring the depolarized light scattering intensity from the droplets in free-fall.

Previously reported freezing temperatures for ammonium sulfate solution concentrations greater than 5 wt% display a larger spread than can be accounted for by the reported experimental errors. We find ammonium sulfate-water solution freezing temperatures in good agreement with the lowest temperature freezing results reported by previous experiments. Our ammonium sulfate freezing temperature dataset with water activity less than 0.98 is consistent with a curve that deviates in activity shift by about 5% from the best-fit ice nucleation temperature versus water activity curve found by Koop et al. 2000.

Results for sulfuric acid-water solution freezing temperatures also exhibit lower temperature freezing than is predicted by the Koop 2000 curve, but, as with the above ammonium sulfate solution results, the significance of this deviation will only be known with further high-precision ice nucleation temperature measurements for other aqueous solutions.