The vapor pressure over nano-crystalline ice
Abstract. Crystallization of amorphous solid water (ASW) is known to form nano-crystalline ice. The influence of the nanoscale crystallite size on physical properties like the vapor pressure is relevant for processes where crystallization of amorphous ices occurs e.g. in interstellar ices or cold ice cloud formation in planetary atmospheres, but up to now not well understood. Here, we present laboratory measurements on the saturation vapor pressure over nano-crystalline ice between 135 K and 190 K. Below 160 K, where nano-crystalline ice is known to be metastable for extended periods, we obtain a saturation vapor pressure that is 100 % to 200 % higher compared to stable hexagonal ice. This elevated vapor pressure is in striking contrast to the vapor pressure of stacking disordered ice which is expected to be the prevailing ice polymorph at these temperatures with a vapor pressure at most 18 % higher than that of hexagonal ice. This apparent discrepancy can be reconciled by assuming that nanoscale crystallites with mean diameter between 7 nm and 19 nm form in the crystallization process of ASW. The high curvature of these nano-crystallites results in a vapor pressure increase which can be described by the Kelvin equation. Our measurements show, that at temperatures up to 160 K, ASW is the first solid form of ice deposited from the vapor phase and that nano-crystalline ice forms thereafter by crystallization within the ASW matrix. The size of the nano-crystallites remains stable for hours below 160 K and thus nano-crystalline ice may be regarded as an independent phase for many atmospheric processes below 160 K. We parameterize the vapor pressure of nano-crystalline ice using a constant Gibbs free energy difference of (982 ± 182) J mol−1 relative to hexagonal ice.