The vapor pressure over nano-crystalline ice
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Published:2018-03-08
Issue:5
Volume:18
Page:3419-3431
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Nachbar Mario, Duft DenisORCID, Leisner Thomas
Abstract
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
190 K. Below 160 K, 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 160 K. Nano-crystalline ice with a mean
diameter between 7 and 19 nm 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 160 K. Thus,
this ice polymorph may be regarded as an independent phase for many
atmospheric processes below 160 K and we parameterize its vapor pressure
using a constant Gibbs free energy difference of
(982 ± 182) J mol−1 relative to hexagonal ice.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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