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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 5 | Copyright
Atmos. Chem. Phys., 18, 3419-3431, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Mar 2018

Research article | 08 Mar 2018

The vapor pressure over nano-crystalline ice

Mario Nachbar1,2, Denis Duft2, and Thomas Leisner1,2 Mario Nachbar et al.
  • 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. The 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 in which the crystallization of amorphous ices occurs, e.g., in interstellar ices or cold ice cloud formation in planetary atmospheres, but up to now is not well understood. Here, we present laboratory measurements on the saturation vapor pressure over ice crystallized from ASW between 135 and 190K. Below 160K, where the crystallization of ASW is known to form nano-crystalline ice, 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 form in the crystallization process of ASW. The high curvature of the nano-crystallites results in a vapor pressure increase that can be described by the Kelvin equation. Our measurements are consistent with the assumption that ASW is the first solid form of ice deposited from the vapor phase at temperatures up to 160K. Nano-crystalline ice with a mean diameter between 7 and 19nm forms thereafter by crystallization within the ASW matrix. The estimated crystal sizes are in agreement with reported crystal size measurements and remain stable for hours below 160K. Thus, this ice polymorph may be regarded as an independent phase for many atmospheric processes below 160K and we parameterize its vapor pressure using a constant Gibbs free energy difference of 982  ±  182Jmol−1 relative to hexagonal ice.

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The crystallization process of amorphous ice below 160 K forms nano-crystalline ice. We report high-quality vapor pressure measurements over ice crystallized from amorphous 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 the crystallization of nano-crystalline ice. Our findings are relevant for cold ice clouds in the atmospheres of planets, e.g., Earth and Mars.
The crystallization process of amorphous ice below 160 K forms nano-crystalline ice. We report...