Transport and Surface Accumulation of Hydroniums and Chlorides in an Ice Film. A High Temperature (140-180 K) Study

We studied the transport of hydronium, chloride, and water in an ice film at 140-180 K, a temperature range that encompasses the onset of ice sublimation, crystallization, and roughening transition of the ice film. The ice film was grown in vacuum to have the structure of a bottom H2O layer, an upper D2O layer, and hydronium and chloride ions embedded at the H2O/D2O interface, with a total sample thickness >= 100 BL. The transport of hydronium and chloride ions from the interface layer to the ice surface was monitored as a function of the sample temperature using a low energy sputtering (LES) method. The transport of water molecules and their HID exchange reactions were monitored using a Cs+ reactive ion scattering (RIS) method. Temperature-programmed desorption (TPD) experiments measured the desorption of water and hydrogen chloride from the surface. The study revealed that hydroniums and chlorides accumulate at the ice surface at an elevated temperature due to their thermodynamic affinity for the surface. The ion transports may occur via isotropic diffusion in the interior region of the ice. Near the ice surface, thermodynamic forces accelerate the directional migration of ions toward the surface. The accumulation of hydroniums and chlorides at the surface facilitates the recombinative thermal desorption of hydrogen chloride gas. These observations support the idea that the heterogeneous reactions of hydroniums and chlorides in ice particles in the atmosphere, such as the chlorine activation reactions in polar stratospheric clouds, occur preferentially at the surface of ice rather than in the bulk phase.