Laboratory measurements of the homogeneous freezing of aqueous sulfuric acid and ammonium sulfate droplets: How well does the translated melting-point curve model work?

Other recent laboratory measurements for aqueous ammonium sulfate and sulfuric acid solutions show considerable variability in freezing point temperatures – much more than is predicted by the widely-used translated melting-point curve (TMPC) idea of Koop et al. (2000). This paper reports on results from laboratory measurements of homogeneous ice nucleation in aqueous droplets containing two common upper-tropospheric solutions – ammonium sulfate and sulfuric acid. Measurements were made using a droplet free-fall freezing tube technique. Our measured freezing point temperatures are in agreement with the lowest temperature results from previous laboratory experiments, but we find temperatures considerably lower than those predicted by the TMPC idea. Given our current understanding of ice cloud formation, we predict that high critical supersaturations can occur in cloud-free regions of the upper troposphere when homogeneous ice nucleation in aqueous sulfuric acid aerosol is the primary ice formation mechanism.