Aerosol silica as a possible candidate for the heterogeneous formation of nitric acid hydrates in the stratosphere

The liquid-solid phase transitions in nanometersize HNO3/H2O solution droplets obtained on fumed silica (a counterpart of aerosol silica) have been studied with differential scanning calorimetry (DSC). "Soft" transitions, reduction in the freezing and melting temperatures, T-f and T-m, and enthalpies, Delta H-f and Delta H-m, are interpreted to be caused by very small size of droplets. The observed difference between Delta Hf and Delta H-m can serve as an evidence of temperature dependence of the enthalpy of fusion for hydrates. Freezing of droplets with stoichiometry close to nitric acid trihydrate (NAT) at temperature 4 K warmer than the ice frost point indicates that, in the stratosphere, silica particles can serve as nuclei for heterogeneous freezing of NAT.