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
It is well known that the crystallization process of amorphous solid water (ASW) below about 160 K forms nano-crystalline ice with crystallite diameters between 5 and 40 nm. Using electron diffraction, Jenniskens and Blake (1996) observed crystal diameters of 10 to 15 nm between 150 and 160 K and Kumai (1968) reported diameters of 5 to 30 nm at 113 to 143 K. Dowell and Rinfret (1960) used X-ray diffraction and observed grain sizes of about 40 nm. Crystallization of the high-pressure ices II, IV, V and IX has been shown to produce nano-crystalline ice as well (Arnold et al., 1968; Kuhs et al., 1987). This nano-granular structure may have significant effects on the properties of the ice polymorph. For example, Johari and Andersson (2015) attributed a reduction in the measured thermal conductivity of ice crystallized from ASW to enhanced phonon scattering at stacking faults and grain boundaries of the crystallites. Furthermore, the nano-crystallites might impact the vapor pressure over the ice phase, but to the best of our knowledge, this effect has not been quantified yet.
Below 160 K, nano-crystalline ice is stable for several hours (Hansen et al., 2008) and thus its vapor pressure is of relevance for atmospheric processes occurring in these conditions, e.g., cloud formation processes in the terrestrial mesosphere or on other planets like Mars. At temperatures below 160 K, however, only a limited number of desorption rate measurements of ice crystallized from ASW using quadrupole mass spectrometers and quartz crystal microbalances is available that may be used to calculate the saturation vapor pressure over the ice phase (Brown et al., 1996; Bryson et al., 1974; Fraser et al., 2001; La Spisa et al., 2001; Sack and Baragiola, 1993; Smith et al., 2011; Speedy et al., 1996). Measuring water vapor desorption rates at such low temperatures is a challenging task and these measurements reveal large discrepancies among each other. This situation points to the need for high-quality saturation vapor pressure measurements of nano-crystalline ice crystallized from ASW.
In this work, we report the vapor pressure of ice samples deposited from the
gas phase below 160 K in a temperature range between 135 and 190 K using
two independent and complementary experimental setups. One setup is based on
a technique for measuring absolute saturation vapor pressures using the
growth of trapped nanoparticles in isothermal conditions as a sensitive probe
at temperatures between 135 and 160 K. This setup is briefly described in
Sect. 2.1. In order to extend the range to temperatures around 190 K, for
which the vapor pressure of crystalline ice is established within a few
percent, we also report results from an independent more conventional setup.
It allows for the measurement of the relative vapor pressure of water ice
samples with respect to hexagonal ice
The molecular flow ice cell within the trapped reactive atmospheric particle
spectrometer (MICE–TRAPS; Duft et al., 2015; Meinen et al., 2010) has been
used previously to investigate the adsorption and nucleation of CO
Radial cross section of MICE.
We generate single-charge silica (SiO
In order to extend the saturation vapor pressure measurements to temperatures
above 160 K, we used an additional experimental setup and measured the
relative vapor pressure difference of metastable crystalline ice and ice
Measured relative saturation vapor pressure of
low-temperature-deposited ices with respect to ice
Isothermal saturation vapor pressure measurements were performed with
MICE–TRAPS in the temperature range between 133 and 160 K. Non-isothermal measurements
using the hot ionization gauge setup were performed with a temperature ramp
of 0.5 K min
Above 140 K, the saturation vapor pressure is found to be independent of the
deposition temperature, suggesting that ice deposited between 140 and 160 K
forms the same ice polymorph as ice crystallized from ASW. Between 135 and
160 K the vapor pressure of this ice polymorph is elevated by a factor
between 2 and 3 with respect to hexagonal ice
We reviewed and partially reanalyzed the limited amount of available
literature data on the desorption rate of metastable ice below about 170 K
in order to compare them with our measurements. These measurements typically
employed a quadrupole mass spectrometer (QMS) and/or a quartz crystal
microbalance to measure desorption rates. Desorption rates can be used to
infer saturation vapor pressures under the well-supported assumption that the
sticking coefficient for water molecules on water ice is unity at these
temperatures (Batista et al., 2005; Brown et al., 1996; Gibson et al., 2011;
Kong et al., 2014). Measuring water vapor desorption rates at the temperatures
under investigation is a challenging task and previous experiments were
influenced by contamination issues, showed a very large degree of scattering in
the data or yielded unphysically low vapor pressures below that of ice
Hexagonal ice is the lowest energy phase of solid water below the freezing
point under typical terrestrial atmospheric conditions. The overall
thermodynamic model of ice
It is well known that the crystallization process below 166 K of ASW as well
as the high-pressure ices II, IV, V and IX form nano-crystalline ice (Arnold
et al., 1968; Backus and Bonn, 2004; Dowell and Rinfret, 1960; Jenniskens and
Blake, 1996; Kondo et al., 2007; Kuhs et al., 1987; Kumai, 1968). The
formation of nano-crystallites is believed to occur by nucleation of ice
embryos followed by their diffusional isotropic three-dimensional growth within
the remaining ASW matrix until all amorphous water is transformed to
crystalline ice. At low temperatures, the interplay of ice nucleation and ice
growth leads to nanoscale crystallites (e.g., Backus and Bonn, 2004; Kondo et
al., 2007). A nano-crystallite exhibits a large surface energy to volume
energy ratio, resulting in an increased vapor pressure above its surface. This
vapor pressure increase is described by the Kelvin equation, which at the same
time corresponds to the vapor pressure increase over a macroscopic surface
composed of spherical nano-grains:
Calculated nano-crystallite diameters as a function of temperature
in ice crystallized from ASW. The black squares (
Stacking disorder in ice
Since deposition between 140 and 160 K and the crystallization of ASW deposited at 95 and 100 K leads to identical vapor pressures, it is very likely that ice deposition up to 160 K proceeds by an initial deposition of ASW followed by rapid crystallization. This conclusion is supported by the work of Chonde et al. They used deposition rates comparable to our work and observed nonporous ASW immediately after deposition at 140 K (Chonde et al., 2006). At temperatures above 140 K, we cannot observe the crystallization process after deposition of ASW with the MICE–TRAPS setup since the time needed to perform the first experimental run exceeds the crystallization time at these temperatures.
It is well known that ASW might be deposited in a porous form, which depends on deposition angle, rate and temperature (Dohnalek et al., 2003; Hill et al., 2016; Kimmel et al., 2001a, b; Kouchi et al., 1994; Mayer and Pletzer, 1986; Mitterdorfer et al., 2014; Raut et al., 2007; Stevenson et al., 1999). Deposition of ASW at temperatures between 90 and 110 K revealed either small degrees of porosity (Brown et al., 1996; Chonde et al., 2006) or were nonporous (Kimmel et al., 2001b; Stevenson et al., 1999). Thus, reports of porosity in ASW deposited in conditions comparable to our studies are inconsistent and we cannot exclude a small degree of porosity in our ASW samples. However, due to the fact that independent of deposition temperature the same crystalline ice polymorph forms, we conclude that either all our ASW samples are nonporous or that any porosity of the ASW sample deposited at 95 and 100 K has no influence on the crystallized ice polymorph. The latter is supported by the observation of a strong decrease in the porosity of microporous ASW at annealing temperatures above 100 K with a complete absence of micropores above temperatures of 140 K (Hill et al., 2016; Kimmel et al., 2001b; Raut et al., 2007).
We present saturation vapor pressure measurements of water ices deposited from the vapor phase at temperatures below 160 K using two independent and complementary experimental approaches. One experiment is based on a novel technique using nanoparticles as sensitive probes for isothermal absolute sublimation rate measurements (135–160 K), and a more conventional setup uses a hot ionization gauge for relative vapor pressure measurements during a temperature ramp experiment (166–190 K).
Our vapor pressure measurements below 160 K show a 2 to 3 times higher
saturation vapor pressure compared to ice
Because the same nano-crystalline ice polymorph forms by vapor
deposition below 160 K and by crystallization from ASW, we conclude that even
at temperatures as high as 160 K, amorphous ice is the initial phase formed
by ice deposition from the vapor prior to crystallization. This is important
for ice cloud processes that occur below 160 K as it implies that ice
nucleation rates at these temperatures are dominated by the properties of ASW
rather than those of crystalline ice. After crystallization, however, ice
growth processes are described by the properties of nano-crystalline ice. The
mean crystallite size of 7 to 19 nm in diameter determined in this work is
stable for hours below 160 K. We therefore propose considering
nano-crystalline ice as an independent phase in ice cloud processes below
160 K. For practical reasons, we provide a parameterization for the
saturation vapor pressure over this ice polymorph and suggest it to be used
in a temperature range in which the transformation time to microscopic crystal
sizes is long compared to the processes involved. Below 160 K,
Our findings are of importance for cloud processes in the middle atmospheres
of planets. For instance, water ice clouds are frequently observed in the
middle atmosphere of Mars (Guzewich et al., 2013; Vincendon et al., 2011)
with temperatures commonly falling below 160 K (Maltagliati et al., 2011).
In the terrestrial atmosphere, noctilucent clouds form at the high-latitude
summer mesopause (Rapp and Lübken, 2004) with temperatures falling to
120 K on average (Lübken et al., 2009) with extremes down to 100 K
(Rapp et al., 2002). In addition, the vapor pressure of nano-crystalline
ice is important for modeling H
All data are available on request from the corresponding author.
In general, the mass growth rate
Particle mass
Figure A1a shows the measured particle mass as a function of trapping time in
MICE for three exemplary measurements with sample surface temperatures of
147.4, 149.7 (particle material: Fe
In this work, only spherical nuclei and ice particles are considered.
However, noctilucent clouds (NLCs) form under conditions investigated in this
work and light scattering models showed better agreement to NLC data
retrieved by satellite and lidar remote sensing instruments when analyzed
under the assumption of aspherical ice particle shapes (Eremenko et al.,
2005; Hervig et al., 2012; Kiliani et al., 2015). At the particle
temperatures investigated here (below
At
Experimental setup used for the relative saturation vapor pressure
measurements between 166 and 190 K. A hot-cathode ionization gauge (
In order to extend the saturation vapor pressure measurements to temperatures
above 160 K, an additional setup to measure the relative vapor pressure
difference between ice deposited below 160 K and hexagonal ice was built. A
schematic representation of the experimental setup is depicted in Fig. B1.
The setup consists of two interconnected vacuum chambers with a base residual
gas pressure below
Vapor pressure between 160 and 190 K after deposition at 100 and 150 K (black lines, four runs) and after the crystallization of hexagonal ice from liquid water at 260 K (gray lines, three runs). The solid blue and red lines represent calculated mean values for deposition at 100 and 150 K and hexagonal ice, respectively.
We pursued two methods for depositing water vapor onto the sample surface.
Nano-crystalline ice is produced using the same procedure as with the
MICE–TRAPS setup, either via the deposition of ASW at 100 K followed by
crystallization during warm-up or by direct deposition at 150 K resulting in
a roughly 15 To create hexagonal ice, the fine-dosing valve was opened
to the full extent with
The temperature of the sample disk was measured with a Pt-100 temperature
sensor and a distributed set of six Si diode sensors. It was found that
during warm-up the sample surface temperature is homogeneous to within 0.2 K
and the absolute uncertainty of the temperature measurement was estimated to
be 0.5 K. During warm-up,
Simultaneous residual gas measurements with a quadrupole mass spectrometer
(QMS; Pfeiffer PrismaPlus QMA 200) ensured that no significant amount of
trace gases other than H
For hexagonal ice all curves fall onto each other above 168 K, showing a decreasing difference towards the ices deposited at and below 150 K. Below 168 K, the three measurements of hexagonal ice show deviations, which can be explained by the following: during cooldown residual water desorbing from the inner surfaces of the vacuum chamber deposits onto the hexagonal ice film, forming a layer of the same ice that is created when depositing water directly at 150 K. After some time of pumping and sample temperature increase, the residual water source is depleted and the layer on top of the hexagonal ice film begins to evaporate. Eventually, the overlayer will evaporate completely and expose the hexagonal ice below. The transition from one ice phase being exposed at the surface to the other can be seen in all three measurements of hexagonal ice in Fig. B2. Therefore, the analysis of the data is restricted to temperatures above 166 K. Depending on ice thickness, all ice is evaporated somewhat above 190 K, which limits our data to temperatures between 166 and 190 K. Absolute vapor pressure measurements with the accuracy required to distinguish between different ice phases at such low temperatures are difficult to achieve with this setup. However, the measurements were reproducible and we can directly compare the unprocessed recorded vapor pressure of ices deposited below 160 K with hexagonal ice, relying for the latter on the accuracy of the well-established parameterization by Murphy and Koop (2005). In this way, we avoid many uncertainties and systematic errors occurring in absolute vapor pressure measurements. We calculated the mean and standard deviation for all runs of low-temperature vapor-deposited ice between 166 and 190 K (blue curve). For hexagonal ice, we use experiments 281 and 285 between 166 and 169 K and all three runs above 169 K (red curve). The recorded vapor pressures were highly reproducible and the ratio of the vapor pressures of the two ice phases could be determined with an accuracy of 10 %.
MN and DD designed the experiments. MN carried out the MICE–TRAPS experiments. MN and DD carried out the pressure gauge experiments. MN performed the data analysis. MN prepared the paper with contributions from all coauthors. DD and TL supervised the experiments.
The authors declare that they have no conflict of interest.
The authors thank the German Federal Ministry of Education and Research (BMBF, grant numbers 05K13VH3 and 05K16VHB) and the German Research Foundation (DFG, grant number LE 834/4-1) for financial support of this work. We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: Markus Petters Reviewed by: two anonymous referees