solution (white star in Fig. ). However, configurations in which nitrate and ammonium are paired (N-N distance ∼ 4 Å, red star in Fig. ) are metastable, and a low barrier of the order of the thermal energy separates the two free energy basins. The FES shows that ammonium deprotonation is more likely when the two ions are separated, whereas the nitrate-ammonia 11 complex (pink star in Fig. ) is not thermodynamically stable. However, the unbiased FPMD runs suggest that even though according to the reconstructed free energy surface they are not the most probable scenarios, both the NO - 3 /NH 3 and NO - 3 /NH + 4 complexes are long-lived at the air-ice interface over time scales of several tens of ps. The relatively favorable clustering of ammonia and ammonium with nitrate molecules at the ice surface can be explained by the fact that the hydration of ions leads to an increase in surface liquidity at the ice surface, which in turn creates a water-mediated attraction of ions, as observed previously. Figure maps the path taken by our two sampled trajectories on the free energy surface and shows that neither of our starting configurations for the unbiased runs reached the minimum of the reconstructed free energy surface. So, we carried out a 20 ps long, unbiased, molecular dynamics run of ammonium nitrate at the air-ice interface -starting from a configuration extracted from the metadynamics simulation in which the ions are separated.