The vapor pressure over nano-crystalline ice

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.

Unravelling the microphysics of polar mesospheric cloud formation

“…This is a result of the curvature effect, which strongly influences the equilibrium coverage for nanometre-sized particles. Instead, we find that the adsorp-tion model that we used recently (Nachbar et al, 2018a) to describe the water affinity of iron silicate particles describes the water coverage more adequately. In this work we modify this model to account for the influence of the curvature on the equilibrium coverage, which has not been done before.…”

“…In this work we performed laboratory experiments using the MICE-TRAPS apparatus which was described earlier (Duft et al, 2015;Meinen et al, 2010;Nachbar et al, 2018bNachbar et al, , 2016. In brief, sub-4 nm iron silicate nanoparticles of adjustable elemental composition are produced in a microwave plasma particle source as MSP analogues (Nachbar et al, 2018a). The nanoparticles are transferred continuously to the low pressure (p < 10 −4 mbar) Trapped Reactive Atmospheric Particle Spectrometer (TRAPS) by means of an aerodynamic lens system.…”

“…In some cases, and in order to facilitate comparison with previous studies, the saturation ratio is also given with respect to the saturation vapour pressure of hexagonal ice (S h ), for which we use the well-established parameterization given by Murphy and Koop (2005). The saturation S ASW can be obtained from S h using the following relation (Nachbar et al, 2018c):…”

Effect of nucleation on icy pebble growth in protoplanetary discs

“…using experimentally determined material constants for water (Haynes et al 1992). Higher saturated vapour pressures are expected over nano-crystalline ice, crystallised from amorphous ice at low temperatures (below T ≈ 160 K), however the exact value of S does not change the qualitative outcome of our model (Nachbar et al 2018). For a system with water vapour and water ice, S = 1 means that the vapour is saturated, which is equivalent to having an equilibrium between the two phases.…”

Response to the reviewers

“…Absorption efficiencies determined in Nachbar et al (2019) for maghemite particles with the MICE-TRAPS apparatus were used in combination with the optical extinction mea-sured in this work for iron oxide particles produced with the PAFS apparatus, in order to derive complex RIs. The experimental and analytical methods used for the MICE-TRAPS experiment have been described in detail previously (Meinen et al, 2010a, b;Duft et al, 2015;Nachbar et al, 2016), with the recent methodology for particle production outlined in Nachbar et al (2018a).…”