Influence of surface defects on the adsorption of HCl on ice

The reaction between HCl and ClONO2 to produce Cl-2 is thought to take place on ice crystals in polar stratospheric clouds. However, the detailed mechanism for the initial adsorption of HCl on ice remains unclear. Here, the influence of defects in the ice surface on this adsorption mechanism is examined by using molecular dynamics simulations. A mixed quantum-classical method is used that involves treating the vibrational and translational motion of HCl using time-dependent quantum mechanics while classical mechanics is used to describe all other degrees of freedom including those of 360 water molecules representing ice. It is found that the defects significantly enhance the attractive interaction of HCl with the ice surface and give physisorption times (ca. 10(-3) s) for the HCl on the surface which are much longer than those obtained for ice surfaces that do not contain defects. Examination of the trajectories also indicates that the defects in the ice surface may allow the HCl to be encapsulated by water molecules, and this might facilitate ionisation of HCl.