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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-1101
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
05 Dec 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
The vapor pressure over nano-crystalline ice
Mario Nachbar1,2, Denis Duft2, and Thomas Leisner1,2 1Institute of Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology – KIT, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
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.

Citation: Nachbar, M., Duft, D., and Leisner, T.: The vapor pressure over nano-crystalline ice, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1101, in review, 2017.
Mario Nachbar et al.
Mario Nachbar et al.
Mario Nachbar et al.

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Short summary
The crystallization process of amorphous ice below 160 K forms nano-crystalline ice. We report high quality vapor pressure measurements over nano-crystalline ice below 160 K. We show that the vapor pressure is increased by more than 100 % compared to bulk crystalline ice and that amorphous ice always forms first followed by crystallization of nano-crystalline ice. Our findings are relevant for cold ice clouds in the atmospheres of planets, e.g. of Earth and Mars.
The crystallization process of amorphous ice below 160 K forms nano-crystalline ice. We report...
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