Freezing of binary and ternary solutions of H2SO4, HNO3 and H2O under stratospheric conditions: Nucleation statistics and experiments

Abstract

New calorimetric freezing experiments of binary and ternary H2SO4/HNO3/H2O solutions are presented with applications to the formation of polar stratospheric clouds (PSCs). We show that the nucleation of hydrates from these solutions is a stochastic process and that nucleation rates and their uncertainties can be determined using Poisson statistics. The experiments reveal that under stratospheric conditions the freezing process of aerosol droplets is limited by tile nucleation rates of the hydrates, rather than their crystal growth rates. Under thermodynamic equilibrium conditions above the ice frost point the homogeneous nucleation rates of stratospheric aerosols are exceedingly low, ruling out homogeneous freezing as a pathway for PSC formation.