Water accommodation and desorption kinetics on ice.

The interaction of water vapor with ice remains incompletely understood despite its importance in environmental processes. A particular concern is the probability for water accommodation on the ice surface, for which results from earlier studies vary by more than 2 orders of magnitude. Here, we apply an environmental molecular beam method to directly determine water accommodation and desorption kinetics on ice. Short D2O gas pulses collide with H2O ice between 170 and 200 K, and a fraction of the adsorbed molecules desorbs within tens of milliseconds by first order kinetics. The bulk accommodation coefficient decreases nonlinearly with increasing temperature and reaches 0.41 ± 0.18 at 200 K. The kinetics are well described by a model wherein water molecules adsorb in a surface state from which they either desorb or become incorporated into the bulk ice structure. The weakly bound surface state affects water accommodation on the ice surface with important implications for atmospheric cloud processes.