A detailed spectroscopic study of HNO3:H2O binary amorphous mixtures, and of the adsorption of HNO3 onto ice, is reported. Using a classical optics model, the extent of intermixing and of ionic dissociation of adsorbed HNO3, which forms a strong acid with liquid water, is determined as a function of HNO3 coverage and temperature. Even at temperatures as low as 45 K, where intermixing is limited to at most a few molecular layers at the interface, ionic dissociation of adsorbed HNO3 is observed to be extensive. While some amount of molecularly adsorbed HNO3 is observed at the surface of ice at 45 K, its ionic dissociation occurs irreversibly upon heating the ice substrate to 120 K. The molecularly adsorbed state of HNO3 is not restored upon cooling, suggesting HNO3 is a metastable entity at the surface of ice. Therefore, despite ionic dissociation of HNO3 being thermodynamically favored, it appears to be kinetically inhibited at the surface of amorphous solid water at temperatures below 120 K.