Relating the current science of ion-defect behavior in ice to a plausible mechanism for directional charge transfer during ice particle collisions

Abstract

A melding of modern experimental results descriptive of fundamental ion defect properties of ice is presented as a logical basis of a mechanism for the preferential transfer of positive charge from large to small colliding ice particles. The result may relate to the electrification of storm clouds. It is broadly agreed that such localized charge transfer during collision of small upwardly mobile ice particles with falling ice granules (i.e., graupel/hail) can lead to macroscopic charge separation capable of initiating lightning strikes during the expansion stage of a storm cell. Though the larger particles are thought to become negatively charged during the collisions neither a generally favored charge-exchange agent nor a preferred mechanism for the directional particle-to-particle charge transfer exists. Nevertheless, should ionic point defects of ice play a key role, the fundamental properties of ice defects considered here must apply. They include: (1) above 140 K protons move readily within and on the surface of ice while hydroxide ions are orders-of-magnitude less mobile, (2) whether generated by dissociation of HCl buried in ice, during neat ice particle growth, or at platinum-ice interfaces, interior protons move to and apparently collect at the ice-vacuum interface, and (3) proton activity and populations are orders-of-magnitude greater at the surface of ice films and free-standing ice particles than in the interior. From these fundamentals an untested argument is developed that within an ensemble of free floating ice particles the proton density at the surface is greater for larger particles. This implies a plausible proton-based mechanism that is consistent with current concepts of ice particle charging through collisions.