O n -H w RDFs on ice are more structured than those in water. This is only partially due to the temperature difference between the two systems, but mostly to the more ordered structure of the QLL at the air-ice interface, in which NO - 3 is fully embedded. When the system is prepared from ammonium nitrate, the solvation shell of NO - 3 in aqueous solution is very similar to that of deprotonated nitric acid. At the ice surface, two different situations should be considered: (i) paired NO - 3 /NH + 4 , and (ii) paired NO - 3 /NH 3 .