Freezing of HNO3/H2SO4/H2O solutions at stratospheric temperatures: Nucleation statistics and experiments

Calorimetric freezing experiments with aqueous sulfuric and nitric acid solutions are presented and applied 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. 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. Several stratospherically important substrates were tested concerning their ability to induce heterogeneous nucleation. None of the experiments indicated a relevant enhancement of the freezing probability of liquid aerosols. Moreover, the experiments reveal that the freezing process of the solutions under stratospheric conditions is limited by the nucleation rates of the hydrates, rather than their crystal growth rates, thus ruling out the possibility of a glassy state of stratospheric aerosol droplets. Also, we argue why a glacial state of the aerosols seems to be unlikely. The only processes leading to freezing of the hydrates appear to be the heterogeneous nucleation on water ice crystals forming below the frost point and the homogeneous freezing of almost binary HNO3/H2O droplets with H2SO4 concentrations below approximately 0.01 wt %.