Conversely, preliminary FPMD simulations of ammonium nitrate at the air-ice interface show that NH + 4 and NO - 3 tend to aggregate spontaneously. For this system, we ran two independent simulations, starting with the two ions 5 Å and 10 Å apart. In the former case, the distance between the N atoms of NO - 3 and NH + 4 fluctuates between 3 and 5.5 Å over the 70 ps of the simulation (Figure ). In the latter case, the two ions rapidly approach and NH + 4 deprotonates to NH 3 . After 20 ps in which the N-N distance oscillates between 3.5 and 8 Å, the NO - 3 /NH 3 complex stabilizes at an average distance of 4.5-5 Å (Figure ). The spontaneous formation of ammonia may be surprising at first glance as NH + 4 has a pK a of 9.3 in bulk water, but there is evidence that the equilibrium of protonation/deprotonation reactions is significantly altered at ice surfaces. Ion pairing on ice