FREEZING OF HNO3 H2SO4/H2O SOLUTIONS AT STRATOSPHERIC TEMPERATURES - NUCLEATION STATISTICS AND EXPERIMENTS/

Calorimetric freezing experiments with aqueous sulfuric and nitric aci d solutions are presented and applied to the formation of polar strato spheric clouds (PSCs). We show that the nucleation of hydrates from th ese solutions is a stochastic process and that nucleation rates and th eir uncertainties can be determined using Poisson statistics. Under th ermodynamic equilibrium conditions above the ice frost point, the homo geneous nucleation rates of stratospheric aerosols are exceedingly low , ruling out homogeneous freezing as a pathway for PSC formation. Seve ral stratospherically important substrates were tested concerning thei r 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 nucl eation rates of the hydrates, rather than their crystal growth rates, thus ruling out the possibility of a glassy state of stratospheric aer osol droplets. Also, we argue why a glacial state of the aerosols seem s to be unlikely. The only processes leading to freezing of the hydrat es appear to be the heterogeneous nucleation on water ice crystals for ming below the frost point and the homogeneous freezing of almost bina ry HNO3/H2O droplets with H2SO4 concentrations below approximately 0.0 1 wt %.