The liquid-solid phase transitions in nanometer-size 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, Tf and Tm, and enthalpies, ΔHf and ΔHm, are interpreted to be caused by very small size of droplets. The observed difference between ΔHf and ΔHm 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. ¿ 1999 American Geophysical Union