  We characterise the structural properties of the quasi-liquid layer (QLL) at
two low-index ice surfaces in the presence of sodium chloride (Na$^+/$Cl$^-$)
ions by molecular dynamics simulations. We find that the presence of a high
surface density of Na$^+/$Cl$^-$ pairs changes the surface melting behaviour
from step-wise to gradual melting. The ions lead to an overall increase of the
thickness and the disorder of the QLL, and to a low-temperature roughening
transition of the air-ice interface. The local molecular structure of the QLL
is similar to that of liquid water, and the differences between the basal and
primary prismatic surface are attenuated by the presence of Na$^+/$Cl$^-$
pairs. These changes modify the crystal growth rates of different facets and
the solvation environment at the surface of sea-water ice with a potential
impact on light scattering and environmental chemical reactions.
