ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-15903-2018The influence of HCl on the evaporation rates of H2O over water ice in
the range 188 to 210 K at small average concentrationsThe influence of HCl on the evaporation ratesDelvalChristopheRossiMichel J.michel.rossi@psi.chhttps://orcid.org/0000-0003-3504-695XLaboratory of Air and Soil Pollution Studies (LPAS), ENAC Faculty,
Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, SwitzerlandAtmospheric Particle Research Laboratory (APRL), ENAC Faculty, Swiss
Federal Institute of Technology (EPFL), 1015 Lausanne, SwitzerlandLaboratory of Atmospheric Chemistry (LAC), Paul Scherrer Institute
(PSI), 5232 Villigen-PSI, Switzerlandpresent address: Patent Examiner – Directorate 1657, Dir. 1.6.5.7,
European Patent Office, Patentlaan 3-9, 2288 EE Rijswijk, the NetherlandsMichel J. Rossi (michel.rossi@psi.ch)7November20181821159031591912March201817April20189October20186October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/15903/2018/acp-18-15903-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/15903/2018/acp-18-15903-2018.pdf
The evaporation flux Jev(H2O) of H2O
from HCl-doped typically 1.5 µm or so thick vapor-deposited
ice films has been measured in a combined quartz crystal
microbalance (QCMB)–residual gas mass spectrometry (MS) experiment.
Jev(H2O) has been found to show complex behavior and to
be a function of the average mole fraction χHCl of HCl in
the ice film ranging from 6×1014 to
3×1017 molecule cm-2 s-1 at 174–210 K for initial
values χHCl0 ranging from 5×10-5 to
3×10-3 at the start of the evaporation. The dose of HCl on
ice was in the range of 1 to 40 formal monolayers and the H2O vapor
pressure was independent of χHCl within the measured range and
equal to that of pure ice down to 80 nm thickness. The dependence of
Jev(H2O) with increasing average χHCl was
correlated with (a) the evaporation range rb/e
parameter, that is, the ratio of Jev(H2O) just before HCl
doping of the pure ice film and Jev(H2O) after observable
HCl desorption towards the end of film evaporation, and (b) the
remaining thickness dD below which Jev(H2O)
decreases to less than 85 % of pure ice. The dependence of
Jev(H2O) with increasing average χHCl from
HCl-doped ice films suggests two limiting data sets, one associated
with the occurrence of a two-phase pure ice/crystalline HCl hydrate binary
phase (set A) and the other with a single-phase amorphous HCl/H2O
binary mixture (set B). The measured values of Jev(H2O)
may lead to significant evaporative lifetime extensions of
HCl-contaminated ice cloud particles under atmospheric conditions,
regardless of whether the structure corresponds to an amorphous or
crystalline state of the HCl/H2O aggregate.
Introduction
HCl is among the mineral acids that control the acidity of the atmosphere,
together with HNO3 and H2SO4. The production of
atmospheric HCl predominantly takes place in the middle and upper
stratosphere where O3 is formed owing to photolysis of
halogen-containing source gases such as CFCs (chlorofluorocarbons). However,
there are no known sources of HCl in the upper troposphere (UT) because
scavenging processes of HCl throughout the troposphere are very
efficient, which leads to HCl background concentrations of less than
0.1 ppb (Graedel and Keene, 1995). The absence of significant sources in the
troposphere, the long photolytic lifetime of HCl and the fact that the
production region is well separated from the regions of interest, namely the
UT and the lower stratosphere (LS), all contribute to the fact that
HCl is an excellent tracer for stratospheric ozone in the UT (Marcy et
al., 2004). Owing to the frequent occurrence of cirrus clouds in this
atmospheric region it is of obvious interest to study the interaction of
HCl with atmospheric ice particles at relevant temperature and
pressure conditions (Jensen et al., 2001; Zerefos et al., 2003). The compact
correlation between O3 and HCl has been used to monitor
stratospheric–tropospheric exchange processes and stratospheric O3
intrusions into the troposphere that are still an active field of
investigation (Houghton et al., 2001).
HCl is of importance in the LS as it partakes in heterogeneous reactions on
polar stratospheric ice clouds (PSCs) as well as on background stratospheric
H2SO4 aerosol according to the following reaction taken as an
example:
HCl(ads)+ClONO2Cl2(g)+HNO3(ads).
These reactions efficiently convert inactive Cl-containing reservoir
molecules such as HCl and ClONO2 into active photolyzable Cl-containing
compounds in a single reaction. Typical examples of such photolabile
reaction products are Cl2, ClNO2 and HOCl that will change the
atmospheric composition owing to the high reactivity of the photolysis
products such as atomic Cl (Solomon et al., 1986; Tolbert et al., 1987; WMO,
2003). It thus follows that HCl is of stratospheric importance and is
frequently used as a model compound for heterogeneous reactions on ices that
has inspired many laboratory kinetic studies (Leu et al., 1991; Hanson and
Ravishankara, 1992; Chu et al., 1993; Flückiger et al., 1998; Hynes et
al., 2001; Abbatt, 2003).
HCl forms hydrates of variable stoichiometry when exposed to ice depending on
the temperature of deposition and the partial pressure of HCl (Graham
and Roberts, 1997; Ortega et al., 2004). X-ray diffraction has allowed the
identification of four crystalline hydrates containing one (Yoon and
Carpenter, 1959), two (Lundgren and Olovson, 1967), three (Lundgren and
Olovson, 1967a) and six (Taesler and Lundgren, 1978) H2O per
HCl molecule. In addition, amorphous mono-, tetra- and hexa-hydrates
have been reported under various experimental conditions (Yoon and Carpenter,
1959; Delzeit et al., 1993a). The control of growth conditions of a specific
HCl hydrate is sometimes elusive, but the formation of a saturated
HCl hexahydrate phase has been reported at sufficiently large
HCl exposure (Graham and Roberts, 1995) using amorphous ice as a
starting point despite the fact that the hexahydrate is said to nucleate with
difficulty, at least in thin films (Ortega et al., 2004). However, the
molecular and dynamic details of the crystallization process have not been
investigated as yet.
Fourier transform IR (FTIR) absorption measurements have enabled the
characterization of both amorphous as well as crystalline HCl hydrates
at growth conditions that are sometimes significantly different compared to
the samples investigated using X-ray diffraction. Vibrational spectra of
HCl hydrates in the mid-IR have been routinely used for identification
purposes for some time (Ferriso and Hornig, 1955; Gilbert and Sheppard,
1973). Recently, the mid-IR absorption spectra of the four HCl
hydrates mentioned above have been assigned in a comprehensive and definitive
way, albeit without simultaneous proof of the crystalline structure using
X-ray diffraction (Buch et al., 2002; Xueref and Dominé, 2003). More
recently, the reflection absorption IR spectrum (RAIR) of crystalline
HCl hexahydrate in the mid-IR range has been recorded and assigned
using theoretical calculations based on density functional theory that
results in a refinement of the geometric structure of the HCl hydrates
and a prediction of the vibrational modes of the crystal (Ortega et al.,
2004). It must be recalled that FTIR spectra in transmission and reflection
may in most cases not be directly compared across the mid-IR range.
Regarding the nature of the HCl-ice adsorbate one of the important
questions is whether adsorbed HCl is ionized or exists as a molecular
adsorbate under atmospherically relevant conditions of the UT/LS. This will
determine the mechanism of the heterogeneous reaction which constitutes
necessary knowledge for the extrapolation of heterogeneous reaction rates
measured in the laboratory to atmospheric conditions. Thermal desorption of
HCl monitored by IR absorption in the mid-IR range revealed a
molecularly adsorbed state of HCl desorbing below 50 K (Delzeit et
al., 1993b). IR studies performed by Banham et al. on HCl-ice films
failed to detect molecularly adsorbed HCl at T≥90 K despite the
high rate of HCl adsorption in that temperature range (Banham et al.,
1995). In contrast, Graham and Roberts attributed a characteristic
Temperature Programmed Desorption (TPD) peak of a HCl/ amorphous ice
adsorbate monitored by residual gas MS and occurring at 150 K to molecularly
adsorbed HCl (Graham and Roberts, 1995). However, they did not report
the IR absorption spectrum of the adsorbate in the mid-IR nor did they
explain why molecular adsorption of HCl exclusively occurred on amorphous,
but not on crystalline ice. Most recent results seem to point towards the
existence of molecularly adsorbed HCl on ice below 50 K and at submonolayer
coverages in coexistence with ionized solvated HCl whose fraction
increases with increasing ice temperature (Delzeit et al., 1993b, 1997; Uras
et al., 1998; Lu and Sanche, 2001; Buch et al., 2002; Devlin et al., 2002).
Kang et al. (2000) discovered that both molecularly adsorbed as well as
ionized HCl coexisted on ice that was deposited under Ultra-High
Vacuum (UHV) conditions in the temperature range 50 to 140 K and under
conditions of low HCl exposure (Kang et al., 2000).
Although theoretical electronic structure calculations predict spontaneous
ionization of adsorbed HCl (Gertner and Hynes, 1996; Bolton and
Petterson, 2001), most experiments point towards a seemingly thermally
activated ionization process that may be enabled by structural factors of the
ice matrix that are themselves a function of temperature. Consistent with
these results concentration profiling experiments of HCl/ice
adsorbates using static secondary ionization mass spectroscopy (SIMS)
techniques failed to discover molecularly adsorbed HCl on ice in the
range 90–150 K (Donsig and Vickerman, 1997). In conclusion, both
experimental and theoretical studies clearly point to the absence of
significant quantities of molecularly or covalently adsorbed HCl under
stratospheric conditions. Instead, HCl is ionized and solvated by
H2O on the surface of ice films and may occur either as amorphous
HCl/H2O hydrates of undefined stoichiometry or as crystalline
HCl hydrates. However, these facts do not rule out the presence of
small amounts of molecularly adsorbed HCl on ice that may be
intermediates in the complex mechanism of HCl adsorption on ice, as
evidenced by the negative temperature dependence of the rate of uptake of
HCl on ice (Flückiger et al., 1998). In fact, such an intermediate
has been invoked in the description of HCl adsorption on ice under
atmospheric conditions using a chemical kinetic model based on a multitude of
experimental observables collected upon HCl uptake on ice
(Flückiger and Rossi, 2003).
Work by Parent and coworkers uses near-edge X-ray absorption
spectroscopy (NEXAFS) of HCl-doped low-temperature ice substrates in
order to determine the relative population of ionic and covalently bound
HCl and distinguish between bulk and HCl surface states in the
temperature range 20 to 150 K (Bournel et al., 2002; Parent and Laffon,
2005). The results seem to confirm the consensus on the low-temperature
existence of molecularly adsorbed HCl up to 90 K beyond which an
increasing amount of HCl is converted into an ionic form, such as
H3O+Cl- (Eigen cation) or H5O2+Cl- (Zundel
cation) formed by spontaneous ionization of adsorbed HCl on ice, up to
completion at 150 K (Buch et al., 2008). The newest work by Parent compares
NEXAFS with photoemission (UPS, XPS) and FTIR in transmission of thin
HCl/H2O films (Parent et al., 2011). The results are roughly
consistent but surprising in the sense that these workers find 92 %
ionically dissolved HCl in/on ice at 50 K in contrast to Kang et
al. (2000) and Devlin et al. (2000) under similar exposure (dose) and
temperature conditions. In addition, Parent et al. (2011) perform the NEXAFS
experiment on a (thick) 100 ML “crystalline” H2O ice substrate
deposited at 150 K, whereas the photoemission and FTIR absorption
experiments used a 4 ML thin ice slab deposited at 120 K. The question has
to be raised whether the two types of used ice films may be responsible for
some of the discrepancies in the results because both the density and the
structure of ice are known to be a strong function of temperature and
deposition conditions (Kuhs et al., 2012; Schriver-Mazzuoli et al., 2000).
The most recent work of Parent et al. (2011) sparked an interesting
controversy in the assignment of the FTIR absorption spectrum of thin
HCl/H2O films and led to two comments showcasing the difficulties of
intercomparison of nominally identical experiments (Devlin and Kang, 2012;
Parent et al., 2012).
Furthermore, the results indicate that the “dangling bonds” of the ice
surface attributed to isolated OH groups are not the unique site of
HCl adsorption, even in the range 20–90 K (Flückiger and Delval,
2002). The present work suggests that maiden uptake of HCl onto pure
ice weakens and perturbs the crystal structure of the ice matrix in an
irreversible way such that additional sites for HCl adsorption and
ionization are created akin to Parent et al. (2011). Initial HCl
uptake on pure ice therefore has a catalytic effect on the following
HCl uptake. This irreversible nature of initial HCl dosing has
been known for several years and was observed some time ago in Knudsen flow
reactor studies on the HCl/H2O system under steady-state conditions
of both HCl and H2O at temperatures representative of the
UT/LS (Flückiger et al., 1998; Oppliger et al., 1997). The most recent
experimental work on HCl/H2O at an atmospherically relevant
(“warm”) temperature (253 K) has examined the HCl depth profile
using XPS spectroscopy and finds molecularly adsorbed (physisorbed)
HCl at its outermost layer and ionic dissociation in deeper layers
(Kong et al., 2017). Complementary X-ray absorption results also point
towards a perturbation of the crystal structure of ice in the aftermath of
HCl adsorption/dissolution into deeper layers of ice.
Hardware parameters of both cryogenic sample supports of
HCl-doped ice.
Si optical windowQCM Reactor temperature Tr (K)320 Reactor volume Vr (cm-3)2350 Conversion factor (1/RT) Conv (molec cm-3 Torr-1) with R= 62 398 (Torr cm3 mol-1 K-1)3.0×10161Sample surface area (cm2)0.780.50 H2O collision frequency with ice sample ωH2O (s-1)5.083.26 H2O effusion rate constant of calibrated leak kesc(H2O) (s-1)0.064 MS calibration factor for H2O (m/z=18, stirred flow) C18s-Flow (molec s-1 A-1)2.4×1024MS calibration factor for H2O (m/z=18, dynamic) C18dyn (molec s-1 A-1)1.7×1025HCl collision frequency with ice sample ωHCl (s-1)3.592.31 HCl effusion rate constant of calibrated leak kesc(HCl) (s-1)0.047 MS calibration factor for HC1 (m/z=36, stirred flow) C36s-Flow (molec s-1 A-1)3.9×1024MS calibration factor for HC1 (m/z=36, dynamic) C36dyn (molec s-1 A-1)6.3×1024Calculated escape orifice area Aesc (mm2)1.0 d=104 Å or 1.0 µmCalibration factor for O.D. = 1.082at 3260 cm-1Temperature (K) ratio31709.01808.01907.81936.02052.02081.9
1 Wall temperature of the reactor at T=320 K.
2 See Delval et al. (2003). 3 Corresponds to the ratio between the
true number of molecules present on the QCM support and the number of
molecules displayed by the IC5 controller (Delval et al., 2004).
We have concluded from recent work that HCl doping in quantities of a
submonolayer to several monolayers of HCl leads to the decrease in
both the evaporative flux Jev (molecule cm-2 s-1) or
rate Rev (molecule cm-3 s-1) and the rate of
condensation kcond (s-1), of H2O in the presence in
ice without perturbing the equilibrium vapor pressure of H2O,
PH2Oeq (Delval et al., 2003). We have furthermore shown
that the way Jev of H2O decreases with time depends on
the rate of deposition or the integral of deposited HCl, namely
RHCl (molecule s-1) and NHCl (molecule),
respectively. It appears that two observed HCl species on/in ice,
namely single-phase amorphous HCl/H2O mixtures and a binary phase
consisting of pure ice and an as yet unidentified crystalline HCl
hydrate, HCl⋅xH2O, decrease
Jev(H2O) to a different extent, as proposed in Delval et
al. (2003). These results have led us to perform systematic experiments in
this work using the quartz crystal microbalance (QCMB) combined with residual
gas mass spectrometry (MS) that we have used successfully in the past (Delval
and Rossi, 2004) in order to investigate the temporal change in
Jev(H2O) with the increasing average mole fraction of
HCl, χHCl, remaining in the ice. One of the goals of the
present work is to determine the influence of the HCl deposition
parameters on the temporal change in Jev and the mass
accommodation coefficient α during evaporation of a HCl-doped
ice film and its consequence for the lifetime of atmospheric ice particles
contaminated by HCl. This issue is key in relation to the importance of
heterogeneous vs. homogeneous atmospheric reactions at midlatitudes, as has
been pointed out in the past (Solomon et al., 1986, 1997).
Experimental
The emphasis of the present experiments was placed on the deposition of small
amounts of HCl ranging in doses from 1 to 40 formal monolayers of
HCl where a formal monolayer of adsorbed HCl corresponded to a
surface concentration of 2.5×1014 molecule cm-2 (Table A1),
which is a consensus value obtained from several selected experiments. The
apparatus as well as the methods used for calibration and the HCl
deposition procedure have been described in detail elsewhere (Delval and
Rossi, 2005). The experimental conditions are generally identical to the ones
presented in Delval and Rossi (2005) and the instrumental parameters are
summarized in Table 1. The only significant difference between the study of
HNO3-doped ice and the present condensed-phase investigation of
HCl-doped ice lies in the mode of trace gas admission. HCl was
deposited by backfilling the reactor under stirred flow conditions with the
inlet tubing used for trace gas injection oriented towards one side of the Si
window of the cryostat set at ambient temperature, whereas HNO3 was
deposited by directed injection onto ice films supported by the quartz
crystal of the QCMB as referenced above. Evaporation experiments have been
performed isothermally on samples in the temperature range 174–210 K under
dynamic pumping conditions, that is, at maximum pumping speed (gate valve
open) in order to prevent readsorption of HCl on the ice substrate.
Representative experimental results for the kinetics of H2O
evaporation in the presence of HCl for increasing HCl
deposition temperatures at given rates of deposition RHCl and doses
of HClNHCldep. In the first column the number
refers to the corresponding experiment and identifies the data displayed in
Fig. 2.
First, an approximately 1.5 µm thick ice film was grown at 190 K
on the quartz crystal of the QCMB by deposition of bidistilled water vapor at
a rate of 1×1017 molecule cm-2 s-1 under static
conditions. The H2O equilibrium vapor pressure agreed with published
values across the covered temperature range (Marti and Mauersberger, 1993;
Mauersberger and Krankowsky, 2003). Subsequently, the system was set to the
desired temperature given in Table 2 (second column from the left) and a
metered amount of HCl was deposited under stirred flow conditions. The
rate of deposition of HCl, RHCl, as well as its time
integral, namely the number of HCl molecules deposited on ice,
NHCl, have been evaluated using the method described in Delval and
Rossi (2005). Typically, RHCl ranges between 8.0×1011 and
4.2×1013 molecule s-1 and NHCl between
1.0×1014 and 5.4×1015 molecules. The experimental
conditions of HCl deposition as well as important experimental
parameters are reported in Table 2. Finally, the system was set to dynamic
pumping conditions by opening the gate valve to the turbopump.
Jev(H2O) was measured isothermally using both the QCBM
and residual gas MS. Figure 1 illustrates a typical experimental protocol of
the evaporation at 192 K of a HCl-doped ice film labeled as
experiment 11 in Table 2 and performed as a multidiagnostic experiment where
both the gas as well as condensed phases are simultaneously monitored.
Typical experimental protocol of the evaporation at 192 K of an
approximately 1.2 µm thick ice film doped with 5.4×1014 molecules of HCl. This illustration corresponds to experiment 11 of
Table 2. (∘): ice thickness monitored by QCM (Å), (□):
“apparent” H2O evaporative flux, JevQCM ,
monitored using QCM (molec cm-2 s-1), (+): I18 MS signal for
H2O, (×): I36 MS signal for HCl (A),
(♢): Jev18 evaporative flux calculated from
I18 (molec cm-2 s-1), (Δ): Int(Jev18)
time integral of Jev18 (molec cm-2).
At t=0, the system is set from stirred flow to dynamic pumping that
starts the evaporation experiment. The continuous curve marked with the empty
squares symbol in Fig. 1a corresponds to JevQCM, the
evaporative flux of H2O calculated from the raw signal at the output
of the QCMB. The diamond symbol (♢) corresponds to
Jev18 evaluated from I18, the MS signal amplitude for
H2O monitored at m/e=18. Int(Jev18) marked by
triangles in Fig. 1a is the time integral of Jev18 and
corresponds to the total number of H2O molecules that have evaporated
from the ice film at t. D is the label at time tD at which
Jev(H2O) decreased from its original value corresponding
to pure ice to 85 % of its original value at t=0, and dD is the
remaining thickness of the ice film at tD. Hb and
He in Fig. 1b correspond to the time when HCl evaporation
begins and ceases to be observed, respectively, using gas-phase residual mass
spectrometry (× symbols in Fig. 1b), and are labeled tHb
and tHe. The data have been treated in analogy to
HNO3-doped ice through the formalism given in Delval and Rossi
(2005). Akin to HNO3 the mass balance between HCl deposited,
NHCldep, and HCl recovered during ice evaporation,
NHClevap, agrees to within less than a factor of 2 under
dynamic pumping conditions. We therefore estimate the average uncertainty
(2σ) of the HCl mole fraction χHCl of ±18 % from the average discrepancy between NHCldep
and NHClevap displayed in Table 2. In the following
NHCl will always refer to NHCldep derived from
the measurement of HCl at deposition because it refers to a directly
measured quantity originating from a measured pressure decrease in a given
volume and time interval ΔP/Δt. The present experiments cover
the evaporation of a small albeit important fraction of the model ice film
for which the decrease in Jev(H2O) is significant.
Change in the evaporative flux Jev(H2O) as a
function of the HCl mole fraction (χHCl) for the cases
presented in Table 2 color-coded according to the corresponding experiment
number in Table 2. The colored and circled numbers on axis “b” (left)
correspond to Jev(H2O) of pure ice before HCl
deposition; the ones on axis “e” (right) are Jev(H2O)
at t=tHe at the end of HCl evaporation. The colored
circles in the data field mark the value of Jev(H2O)
after HCl deposition at t=t0 and are equal to
Jev(H2O) of pure ice. The start of any particular
Jev(H2O) curve as a continuous solid (bold) line occurs
at t=tD at 85 % of Jev(H2O) at t=0 (pure
ice value, colored dot or circled number on axis “b” to the left) and
ends at tHb, the beginning of HCl evaporation as displayed
in Fig. 1b.
Results
The experimental data reported in Table 2 on the isothermal change in the
evaporative flux of water, Jev(H2O), as a function of the
average mole fraction of HCl, χHCl, in the remaining ice
film during the evaporation process under dynamic conditions, are presented
in Fig. 2. Dynamic pumping conditions ensure the absence of any readsorption
of H2O vapor during evaporation owing to the low H2O partial
pressures in the reactor. The axes labeled “b” and “e” correspond to the
values of Jev(H2O) at the end of ice film deposition and
after desorption of most of the adsorbed HCl from the HCl-doped
ice film at tHe, respectively, as displayed in Fig. 1b. The
average mole fraction χHCl of HCl in the remaining ice
film as a function of time is calculated according to Delval and
Rossi (2005). The change in χHCl owing to H2O
evaporation is evaluated between t=tD and t=tHb, which
corresponds to the time interval when the number of adsorbed HCl
molecules is constant, as no release of HCl is observable in the gas
phase at m/e=36 before tHb. Table 2 also displays the initial
value of the HCl mole fraction, χHCl0, calculated for
the ice film just at the end of HCl deposition and marked by a colored
circle on the experimental trajectory of a color-coded evaporating ice film
displayed in Fig. 2. The average mole fraction of HCl in the ice film,
χHCl, increases owing to evaporation of H2O from the
ice film without loss of HCl such that the elapsed time increases with
χHCl in Fig. 2.
The beginning of an evaporation experiment after the end of HCl doping
(t=0 in Fig. 1 or t0 in Fig. 2) is marked by a colored circle of a
given experiment whose parameters are displayed in Table 2 and Fig. 2 (see
experiment 8). As pointed out above, at t=tDJev(H2O) has decreased to an arbitrarily chosen value of
85 % of its original value measured at t=t0 that corresponds to the
beginning of the bold color-coded smooth curve of a given experiment.
Figure 2 essentially displays trajectories of evaporation experiments from
t0 (colored circle) moving to tD and finishing at tHb
between the two limiting values for pure ice (color coded number of a given
experiment on axis “b” for “beginning”) and the remaining ice film at the
end of measurable HCl desorption tHe (color-coded number
of experiment on axis “e” for “Halogen end”). The trajectory of an
experiment with values of χHCl between t0 (colored circle
at χHCl0) and tD (beginning of bold colored line, see
experiment 8 in Fig. 2) ending at tHb (end of bold line,
experiment 8) is presented as a bold dashed-dotted and bold smooth line from
t0 to tHb, respectively, in order to emphasize the
quantitative portion of the experiment. Thinner (color-coded) dotted lines
connect the end of ice film deposition (colored circle on axis “b”) and
HCl dosing with t0, the beginning of the evaporation
experiment, and also describe the post phase of evaporation starting at
tHb to tHe, in order to guide the eye of the reader
to imagine a complete evaporation cycle.
Synopsis of the dependence of the evaporation range parameter
rb/e on the rate of deposition RHCl of
HCl for temperatures between 188 and 210 K. Each point is marked with
the total number of HCl molecules (NHCl) deposited on the
ice film, the temperature of the ice film at HCl deposition and the
experiment number (bold) referring to Table 2. The hashed area encompasses
rb/e values for dataset B (experiments 3, 4, 7, and
8). The color code goes from low (blue) over medium (green) to high (red)
temperatures.
Two different data sets of the change in Jev(H2O) with
χHCl may be distinguished in Fig. 2. The first kind of data
set corresponds to the curves describing Jev for experiments 1,
2, 9 and 11 and is called dataset A. These traces present a slow continuous
decrease in Jev(H2O) as χHCl increases
during H2O evaporation. The second type of dataset shows an initial
plateau of Jev(H2O) with increasing χHCl
starting at the value of pure ice evaporation followed by a sudden decrease
in Jev(H2O) and is found for experiments 3, 4, 7 and 8,
which we call dataset B. Akin to HNO3, we have evaluated the impact
of the HCl deposition protocol on the evaporation range parameter,
rb/e, which is the ratio between the evaporative flux
of H2O at the beginning of ice evaporation,
Jevb(H2O) reported on the left axis “b” in
Fig. 2, and Jev(H2O) close to the end of the desorption
of HCl, Jeve(H2O), at t=tHe
(the right axis “e” in Fig. 2). It describes the factor by which
Jev(H2O) decreases within the limits of “b” and “e”.
The impacts of both the rate of deposition of HCl on ice,
RHCl, and its time integral corresponding to the dose of deposited
HCl, NHCl, are presented in Figs. 3 and A1 (Appendix),
respectively.
It appears from these figures that we have not succeeded in finding a simple
experimental parameter that controls Jev(H2O), either
with elapsed time or amount of adsorbed HCl expressed as the time
dependence of χHCl. Instead, the data may roughly be classified
along the two cases presented above, namely datasets A and B. The distinction
between both data sets seems to be the rate of change (slope) in
Jev(H2O) within a fairly narrow range of
χHCl. Indeed, the available number of experiments clearly shows
two distinct and limiting cases, whereas the search for other controlling
parameters such as RHCl, NHCl and the temperature of
deposition (Tice) for dataset A failed, akin to a similar
HNO3 study (Delval and Rossi, 2005).
One may take note for instance of the low value of χHCl at
210 K for experiment 9 where the conditions of deposition are similar to
experiments 1 and 2; however, its respective values of
rb/e differ significantly from experiment 9 (Fig. 3).
In contrast, for dataset B the rb/e values are similar
for the whole set and range from 20 to 27.2, staying within a fairly narrow
band. Moreover, they seem to be independent of RHCl and
NHCl as for dataset A. In contrast, the rb/e
values for dataset A seem widely scattered over the explored parameter space.
We have also investigated the impact of the deposition protocol on dD,
which is the thickness of ice that is affected by the presence of HCl, namely
the remaining thickness of ice whose Jev(H2O) value has
decreased to 85 % of Jev(H2O) of pure ice. The
results on dD as a function of RHCl and
NHCldep are presented in Figs. 4 and A2 (Appendix),
respectively. Taking the results of Figs. 3, 4, A1 and A2 together, we arrive
at the following two conclusions.
Tice, RHCl and NHCldep are not controlling
parameters or predictors for Jev(H2O) of either set.
The evaporation range parameters rb/e and dD are not
characterizing set A. In contrast, for dataset B, rb/e
and dD values fall into a narrow range with values varying from 460.7 to
636.0 nm compared to the original ice thickness d0 of 1500 nm or so
(exact numbers in Table 2).
Synopsis of the dependence of dD on the rate of deposition
RHCl of HCl for temperatures between 188 and 210 K. Each
point is marked with the total number of HCl molecules
(NHCl) deposited on the ice film, the temperature of the ice film
at HCl deposition and the experiment number (bold font) referring to
Table 2. The hashed area encompasses dD values for dataset B
(experiments 3, 4, 7, and 8). The color code goes from low (blue) over medium
(green) to high (red) temperatures.
Discussion
Figure 1 displays the evaporation history of sample 11 as an example whose
deposition parameters are listed in Table 1. The initial average mole
fraction χHCl0 of HCl, once deposition on the
1.44 µm thick ice film under stirred flow reactor conditions is
terminated, has been estimated from the total number of H2O molecules
contained in the ice film and the measured number of deposited HCl
molecules, NHCldep, for experiment 11 (Table 2). Table 2
and Fig. 1 reveal that for approximately 2.2×1018H2O
molecules in the film and 5.4×1014 molecules of deposited
HCl, we obtain χHCl0=2.7×10-4. This
HCl mole fraction represents an average value that takes into account
all H2O molecules contained in the ice film, whereas in reality there
will be a HCl gradient across the ice film, as has been observed in
the case of the HNO3/ice system (Delval and Rossi, 2005).
After the HCl deposition process on the typically 1.5 µm
thick ice film the gate valve is opened in order to initiate the isothermal
evaporation experiment under dynamic pumping conditions. Initially,
H2O evaporates at fluxes Jev(H2O) that are
characteristic of pure ice measured previously (Delval and Rossi, 2004;
Pratte et al., 2006). These initial values
Jevb(H2O) are displayed on the left-hand “b”
(= beginning) axis in Fig. 2. As the evaporation proceeds
Jev(H2O) slightly decreases with time, as displayed in
Fig. 1a, to the arbitrarily chosen point where Jev(H2O)
has decreased to 85 % of the initial pure ice value, at which point the
remaining ice thickness dD has decreased by approximately one-third to
771.7 nm remaining ice thickness as displayed in Fig. 1b and Table 2.
Further evaporation of H2O leads to a continuous decrease in
Jev(H2O) at a corresponding increase in χHCl
up to point Hb defined above (“Halogen beginning”) at
tHb (Fig. 1b) where HCl starts to desorb from the ice film
as monitored using the residual MS signal at m/e=36.
For t<tHb, χHCl is given by the number of
originally deposited HCl molecules that remain adsorbed on the ice
film up to tHb and the remaining H2O molecules in the
film. In contrast, for t>tHb the composition of the remaining
ice film must be determined by taking into account the loss by evaporation of
both H2O and HCl. The present experimental configuration is
not adapted to quantitatively measure HCl loss. Therefore, we have
chosen to display the temporal development of Jev(H2O)
for t<tHb in Fig. 2 as a function of the average value of the
HCl mole fraction χHCl. However, the value of
Jev(H2O) at t=tHe where most of the
HCl has desorbed from the ice film is plotted on the right axis
labeled “e” (= end) as Jeve(H2O) in Fig. 2
in order to provide a limit for the minimum value of the evaporation rate
Jev(H2O) at an ice film thickness dHe of
approximately 80±10 nm as displayed in Fig. 1b. We have observed in the
past that Jev(H2O) for a pure ice film of an approximate
thickness of 80 nm or less also slows down, presumably owing to island
formation at the very end of pure thin ice film evaporation (Delval and
Rossi, 2005). Therefore, results are becoming more difficult to interpret,
such that we halted the experiment at tHe. The ratio
rb/e=Jevb(H2O)/Jeve(H2O) is displayed in Table 2 and is an
operational evaporation range parameter that estimates the extent of decrease
in Jev(H2O) for a thick HCl-doped ice film of
µm size down to thicknesses of approximately 80 nm.
At the start of the evaporation experiment the equilibrium vapor pressure of
H2O, Peq(H2O), is that of pure ice (Delval et
al., 2003; Delval and Rossi, 2004; Pratte et al., 2006) owing to the small
values of χHCl0. Raoult's law applies to such small values of
χHCl but leads to unmeasurably small deviations from the
observed vapor pressure of H2O which is that of pure ice. In fact, we
have never observed an equilibrium vapor pressure that did not correspond to
pure ice in the course of the present work that seems to be the consequence
of the small average mole fractions of HCl in the H2O/HCl
system. This value of Peq(H2O) is observed throughout the
evaporation up to tHe as the film is apparently sufficiently
H2O-rich to support an equilibrium vapor pressure characteristic of
pure ice consistent with the published, albeit revised, HCl/H2O-phase
diagram by Iannarelli and Rossi (2014). In view of the decreasing values of
Jev(H2O) displayed in Fig. 2 the equilibrium vapor
pressure of pure ice can only be maintained if the condensation rate
coefficient kc for H2O adsorption decreases to the same
extent as Jev(H2O), in agreement with previous work
(Delval et al., 2003; Delval and Rossi, 2004; Pratte et al., 2006) and the
concept of microscopic reversibility.
Figure 1a displays both the QCMB signal (□) as well as the corresponding
MS signal for evaporating H2O at m/e=18 (♢). Akin to
the HNO3/H2O system studied previously (Delval and Rossi, 2005) we
obtain a perfect match between the two signals for t<tD,
whereas for t>tD there is a significant discrepancy, especially at t>300 s, amounting to typically less than a factor of 2. Such a
disagreement has been noted before for HNO3/H2O, albeit to a larger
extent. The reason for this behavior of the QCMB signal has not been studied
in detail but may well lie in a structural rearrangement of the condensed
phase during evaporation that will lead to a change in the calibration factor
Cf defined in Table 1 and in Delval and Rossi (2005). In view of
the straightforward interpretation of the calibrated MS signal at m/e=18
we have used it for the measurement of Jev(H2O) at t>tD akin to the previous study on HNO3/H2O.
The accuracy with which both tHb and tHe can be
determined depends on the temporal change in the background MS signal for
HCl at m/e=36 displayed in Fig. 1b following the dosing of the thin
ice film under stirred flow conditions. Figure 1b displays the MS signal at
m/e=36 as a function of time just before the start of HCl
desorption at tHb that is signalled by an increase in the MS
intensity, whereas tHe corresponds to the return of the
HCl signal to the decaying HCl background in comparison to a
reference experiment in which the HCl background was monitored as a
function of time following the admission of the same HCl dose in the
absence of an ice film. We estimate that tHb is determined to
±10 s, whereas tHe may only be estimated to ±100 s by
virtue of the vanishing intensity of the HCl MS signal compared to its
slowly decaying background.
Previous work has established that the rate of deposition of HCl,
RHCl, in the range 1×1013 to
5×1013 molecule s-1 for the 0.78 cm2 surface area of
the Si window leads to the formation of a crystalline HCl hydrate,
HCl⋅xH2O, whereas values outside of this range seemed
to favor the formation of an amorphous HCl/H2O mixture (Delval et
al., 2003). The exact nature of this undoubtedly crystalline solid is still
unknown. However, IR spectroscopic work on hydroxonium salts of the type
H3O+X- suggests that the ν1 and ν3 peak
positions of the symmetric and antisymmetric O–H stretch vibrations must
correspond to a molecular structure in which the distance between the cation
and anion is unusually large (Desbat and Huong, 1975; Iannarelli and Rossi,
2016). Recent work has shown that the presence of HCl hexahydrate
(HCl⋅6H2O) under the present experimental conditions could be
safely excluded, however, the FTIR absorption spectrum clearly shows the
presence of dissociated HCl within the ice film (Iannarelli and Rossi,
2014). Akin to HCl⋅6H2O that is known to nucleate with
difficulty, crystallization of this unknown HCl hydrate seems to occur
only under specific conditions of temperature and/or HCl deposition.
Owing to the quantitative control of HCl deposition on the ice film in
this work we infer the presence of at least two forms of HCl hydrates
in the temperature range chosen in analogy to previous work (Delval et al.,
2003).
We clearly point out that the present work has been performed without
simultaneous spectroscopic control of the HCl/ice deposit that would
have allowed the identification and/or quantification of the molecular
composition of the condensate. Because we lack a spectroscopic probe for the
ice film deposited on the QCMB in the present work, we are seeking a
correlation between the type of HCl/H2O deposit, either crystalline
or amorphous, and the relevant HCl deposition parameters. Previous
work has revealed a distinctly different temporal dependence of
Jev(H2O) between the crystalline and amorphous HCl
hydrates with the extent of H2O evaporation from the film, at both
low (Delval et al., 2003) and high temporal resolution (Iannarelli and Rossi,
2014).
Datasets A and B have been characterized above in terms of a difference in
the temporal dependence of Jev(H2O) as a function of
increasing χHCl owing to H2O evaporation. Taking one
example of each set, Fig. 2 reveals a distinct difference between
experiments 7 (set B) and 11 (set A) performed at T=195 and 192 K,
respectively, despite comparable HCl deposition parameters (Table 2).
At t>tDJev(H2O) for experiment 7 decreases
at once with χHCl, in contrast to experiment 11, whose
Jev(H2O) value gradually starts to decrease at roughly
the same value of χHCl as experiment 7. In addition, in both
cases the extent of the decrease in Jev(H2O) is roughly
equal between tD and tHb within less than a factor of 2. Set
B data are in marked contrast to set A independent of the magnitude of
χHCl, which is highlighted by a comparison of experiments 11
(set A) and 4 (set B) at 192 and 190 K, respectively. The abrupt decrease in
Jev(H2O) for set B as well as the gradual decline for
set A, both at tD, occur before HCl starts to evaporate from the
sample at tHb and appears therefore to be independent of
χHCl within the range explored in the present work.
If we consider the mean value 〈dD〉 for dataset B (Figs. 4
and A2) we find 549.0±120.0 nm compared to the 1500 nm or so original
ice thickness which corresponds to approximately 8.5×1017
molecules of H2O spread out over 0.50 cm2. These H2O
molecules are impacted by the presence of HCl to some extent because
Jev(H2O) is slowed down significantly compared to pure
ice. Previous results (Delval et al., 2003) on the deposition of HCl
on ice under conditions where the presence of an as yet unidentified
crystalline hydrate HCl⋅xH2O was confirmed by FTIR
absorption led to the conclusion that on average the amount of “trapped”
H2O within dD corresponded to 1.2×1018 molecules
starting with an original 1 µm thick ice film that was
subsequently doped with HCl. This quantity of H2O, when scaled
from the 0.78 cm2 area of the Si window used for FTIR absorption to the
area of 0.5 cm2 of the QCMB, leads to 7.7×1017H2O,
which is in satisfactory agreement with the present measurement of dD or
8.5×1017H2O in the present work. We may add that the
previous value of 1.2×1018H2O from the work of Delval et
al. (2003) corresponding to dD obtained in that work has been derived
using He–Ne interferometry, which is a crude method for measuring the film
thickness.
Specifically, considering the low value of dD of experiments 1 and 10
(Table 2, Fig. 4), we may define the behavior of these condensates as
“ice-like” because roughly 80 % of the ice sample of roughly
1.5 µm thickness has evaporated at Jev(H2O) of
pure H2O ice before it slows down. This decrease in
Jev(H2O) is a kinetic effect and acts on both the rate of
evaporation as well as on the mass accommodation coefficient, the ratio of
which remains constant because the characteristic vapor pressure of pure ice
is maintained until t=tHe when the sample runs out of
H2O and HCl. For sample 1 this conclusion is not too
surprising owing to its extremely low HCl dose of 0.8 formal
HCl monolayers. Sample 10 in comparison with the other members of
dataset A allows us to conclude that dD is proportional to
Tice for dataset A. Low temperatures prevent rapid diffusion of
HCl into the bulk of the ice film, which leaves the majority of the
total mass of the thin film deposited void of any HCl. Therefore, a
large fraction of the total mass of the thin film deposit evaporates at
values of Jev(H2O) characteristic of pure ice before it
decreases to lower values when the presence of HCl slows down
Jev(H2O). Although our experiment does not reveal the
location of the thin layer of HCl-contaminated ice, plausibility
suggests that it is located on top of the ice film at the gas-condensed
interface. The corollary of this is that it is impossible to “cap” a pure
ice sample with a thin layer of an atmospheric condensable gas of lower vapor
pressure in the hope to lower the vapor pressure of the condensate or slow
down H2O evaporation. This capping has been attempted many times, and
examples abound. However, all attempts to lower the ice vapor pressure of the
condensate using low amounts of polar contaminants of ice, such as
HNO3, HCl or HBr, have proven futile to date (Biermann et al.,
1998).
The other members of dataset A are examples (experiments 2, 9, and 11) with
high values of dD at higher temperatures and higher HCl doses
(Table 2). Because of higher presumed interfacial HCl concentrations
these samples experience a decrease in Jev(H2O) owing to
rapid diffusion of HCl into ice that affects the kinetics of
evaporation to some depths of the ice film corresponding to higher values of
dD. Both high HCl doses and high temperatures favor HCl
contamination of deeper layers of the HCl film, and hence high values
of dD.
Tentatively, we assign a crystalline, yet unknown molecular structure and
stoichiometry to samples A in contrast to samples of dataset B that we
identify with an amorphous structure in terms of a liquid HCl/H2O
mixture of variable composition. The main argument in favor of this
assignment comes from recent kinetic work performed by Iannarelli and
Rossi (2016a), who show that both Jev(H2O) as well as the
corresponding mass accommodation coefficient or the adsorption rate
coefficient for H2O adsorption are highly scattered for crystalline
HCl hexahydrate, whereas the amorphous mixture shows a significantly
smaller scatter of the experimental and thermodynamic values (Iannarelli and
Rossi, 2014). Figures A3 and A4 in the Appendix show this substantial
difference in experimental scatter for the amorphous HCl/H2O mixture
(Fig. A3) compared to crystalline HCl hexahydrate (Fig. A4).
Figure 3 displays the range parameter rb/e as a
function of RHCl for all data displayed in Table 2. It is
noteworthy that rb/e is in the range 20 to 27 for
set B experiments 3, 4, 7 and 8 compared to set A data that seem to be
scattered throughout the range. Members of dataset B show a common average
range for both dD and rb/e, which is the reason
we tentatively assign these structures to amorphous liquid mixtures of high
viscosity at the prevailing temperatures.
In conclusion, we take the simultaneous occurrence of the restricted range of
the measured remaining thickness of ice dD=549.0±120.0 nm
together with a similarly restricted range of rb/e
between 20 and 27 as well as the substantial overlap in RHCl
between the present and previous work (Delval et al., 2003) as an indication
that set B evaporation experiments imply the presence of an amorphous
HCl/H2O mixture. In contrast, the scatter of the set A data across
the range of rb/e and dD values suggests the
presence of an as yet unidentified crystalline HCl hydrate. If, and only if,
the HCl deposition conditions rapidly establish thermodynamic
equilibrium, experiment 2 (low HCl flow rate) lies in the “ice”
region in the temperature interval 192–210 K, whereas experiment 11 (high
HCl flow rate) should access crystalline HCl hexahydrate at
192 K but not at 210 K according to the revised HCl/H2O-phase
diagram of Iannarelli and Rossi (2014). It remains to be seen whether or not
the published FTIR absorption spectrum in Delval et al. (2003) turns out to
be identical to the expected crystalline HCl hexahydrate invoked as
condensate in set A molecules, similar HCl deposition parameters
notwithstanding. This proposal awaits further confirmation from FTIR
spectroscopic work that will be combined in the future with the QCMB
measurement. At this point we reiterate our earlier statement that
Tice, RHCl, NHCldep do apparently
not control Jev(H2O) of both datasets.
Atmospheric implications
The evaporation range parameter rb/e may be used to
quantitatively evaluate the upper limit of the evaporative lifetime extension
of thin ice films under conditions of H2O vapor subsaturation. In the
interest of applying the data of the present work to atmospheric conditions
we make the assumption that typical atmospheric cirrus cloud particles of
several µm diameter may be approximated by macroscopic thin films
used to obtain the present data. The time tev in seconds to
complete evaporation of an ice particle of radius r at a given relative humidity (rh) is given in Eq. (1) (Chiesa and
Rossi, 2013; Iannarelli and Rossi, 2016a):
tev=ρNLM2/3raJev(1-rh),
where ρ is the density of ice (0.916 and 0.925 g cm-3 at 273 and
173 K, respectively), M= 18 g mol-1 for H2O, r and a
are the ice particle radius and the distance between two molecular layers in
H2O(ice), respectively (Iannarelli and Rossi, 2016a). Equation (1) is
based on a simple layer-by-layer evaporation model of H2O(ice) from a
spherical ice particle following a zero-order rate law for Jev or
a first order rate law for its inverse, namely H2O adsorption or
condensation. For a 10 µm diameter ice particle approximated by
thin film experiment 1 (Table 2) at rh = 80 %, T=192 K,
Jev=3×1016 molecule s-1 cm-2 (Petrenko and
Whitworth, 1999) and a=4×10-8 cm we obtain tev=2050 s or 34 min. This is the value for a pure ice particle as
Jev(H2O) for pure ice has been used at the outset of the
evaporation experiment and is a lower limit to the true evaporation time
owing to the competition of mass transfer and heterogeneous chemistry
(Seinfeld and Pandis, 1998). Using rb/e=43 for
experiment 1 tev is calculated to be 15 min and 24 h for a
100 nm and 10 µm diameter particle, respectively, whereas the
evaporative lifetime of an analogous pure ice particle would be only 21 s
for the 100 nm diameter pure ice particle. Cirrus ice particles are
frequently in the lower tens of µm size range resulting in a longer
evaporation time considering that the simple evaporation model scales
linearly with the radius of the ice particle. In conclusion we may state
that, owing to the lifetime extension of ice particles contaminated by
HCl, HNO3 or other volatile atmospheric trace gases such as
HOCl, HOBr or HONO, small particles may have a chance to survive subsaturated
regions of the atmosphere so as to function as cloud condensation or ice
nuclei for the following cloud cycle (Delval and Rossi, 2004, 2005; Pratte et
al., 2006).
We would like to stress that the variable rb/e factor
displayed in Table 2 leads to a significant increase in the evaporative
lifetime of a contaminated ice particle and amounts to a kinetic effect that
does not affect the equilibrium vapor pressure of the ice particle in
question: it is that of pure ice from the start of the evaporation experiment
to t=tHe and therefore affects both the rate of evaporation and
accommodation equally. However, in cases where the sample has lost most of
its mass, the vapor pressure decreases and becomes somewhat uncertain. In the
present case the above statement is correct for t=tHb, that is,
before halogen evaporation. Of note is the fact that the accommodation
coefficient α is frequently less than unity, in contrast to what is
often assumed, which will lower the rate of evaporation for pure ice, hence
increasing the evaporative lifetime of pure ice particles for T≥180 K,
as proposed in previous work (Delval and Rossi, 2004, 2005; Pratte et al.,
2006).
As a token example of the potential atmospheric importance of the measured
evaporative lifetimes of ice particles laced with condensable atmospheric
trace gases, we may take the formation, persistence and evaporation of
contrails and cirrus clouds in the UT/LS. These are ice clouds forming on
non-volatile ice nuclei at the corresponding temperature and relative
humidity conditions and that also frequently serve as reaction sites for
heterogeneous atmospheric reactions in connection with ozone depletion and
chlorine activation chemistry in the LS. Under certain conditions, Schumann
and coworkers used the concept of the increase in the evaporative lifetimes
of contaminated ice particles in aviation contrails occurring mostly in the
UT, but sometimes also in the LS, in order to explain the persistence of ice
clouds below ice saturation conditions up to a certain time duration. Ice
clouds have a significant radiative forcing effect that is of interest in
evaluating the climate forcing of high-flying aircraft in future aviation
scenarios (Lewellen, 2014; Schumann et al., 2017a, b). However, the results
of the present work show that the rate of evaporation of ice films doped with
small amounts of acidic trace gases significantly slows down in a complex
manner over the evaporation history of the film or particle, and that the
application of Eq. (1) to atmospheric situations should be carried out with
caution.
Conclusions
Despite the scatter of the values of rb/e and dD
in dataset A displayed in Figs. 3 and 4 and the apparent lack of influence of
the deposition parameters Tice, RHCl,
NHCldep on Jev(H2O), we
may state several key points from the present work.
We observe two types of behavior, both complex, as far as the temporal
change in Jev(H2O) with ongoing evaporation of
H2O from a HCl/H2O condensate is concerned. We have named it
sets A and B that represent limiting behavior as not all performed
experiments fit into this scheme.
At low temperature or low dose of deposited HClNHCldep set A samples, especially samples 1
and 10, reveal an “ice-like” behavior that corresponds to a low value of
dD. This means that the HCl/H2O condensate evaporates a large
fraction of the sample thickness at a value of Jev(H2O)
characteristic of pure ice before slowing down at an increasing mole fraction
of HCl upon H2O evaporation. This corresponds to a two-phase
system consisting of a major ice-like and minor HCl/H2O phase, both
with significantly different values of Jev(H2O).
High values of dD are observed at high Tice or NHCldep
values for set A samples. This means that the sample evaporates H2O
at Jev(H2O) characteristic of pure ice for a relatively
short time of its evaporation history because the quantity of HCl is
sufficient to decrease Jev(H2O) already at high values of
dD by rapidly diffusing to deeper layers of the ice film. An equivalent
way of expressing the point would be to state that dD which is an
indicator of the total mass of the ice film, is proportional to
Tice for Set A.
Set A samples generally show scattered values of both dD and
rb/e values that we attribute to the existence of a
two-phase binary system, namely a pure ice phase and a crystalline HCl
hydrate phase of as yet unknown stoichiometry HCl⋅xH2O, but probably HCl hexahydrate. At first the pure ice phase
starts to evaporate as a whole for a fairly long time at characteristic
values of Jev(H2O) until the pure ice phase has
disappeared, followed by the crystalline HCl/H2O phase at a lower
rate of Jev(H2O) to attain the characteristic value for
the evaporation of the crystalline HCl⋅xH2O phase.
Set B samples are tentatively identified as single-phase binary amorphous
mixtures of HCl/H2O whose kinetic properties are uniform and thus
fairly independent of the HCl concentration at the gas-condensed phase
interface. The observation of a medium-sized average value for both
rb/e and dD is consistent with these observations
and manifests itself as a continuous yet gradual decrease in
Jev(H2O) with increasing χHCl. It is in
distinct contrast to Set A, where Jev(H2O) values are
those of pure ice until the ice phase has completely evaporated followed by a
gradual decline of Jev(H2O) when the crystalline
HCl hydrate starts to decompose.
It must be recalled that the vapor pressure of H2O remained that of
pure ice during most of the thickness of the H2O/HCl condensate down
to approximately 80 nm, at which point we halted the evaporation experiment.
This result is expected based on Raoult's law owing to the small average
HCl mole fractions in doped ice used in the present work: it would
make the decrease in the H2O saturation vapor pressure unmeasurably
small. The present results therefore primarily address the kinetics of
H2O evaporation which changes with the total mass of the thin film
condensate and the concomitant increase in HCl concentration and/or
mole fraction.
Additional details on raw data may be found in the PhD thesis of Delval (2005).
The URL from which the publication may be retrieved is at:
http://doc.rero.ch/record/4686 (Delval, 2004).
Graph of the dependence of the evaporation range parameter,
rb/e, on the number of adsorbed HCl,
NHCldep, adsorbed on ice for temperatures between 188 and
210 K. Each point is marked with the deposition rate of HCl molecules
in molec s-1 on the ice film, the temperature of the ice film and the
experiment running number (bold) referring to Table 2.
Graph of the dependence of the remaining thickness dD on the
number of adsorbed HCl, NHCldep, dispensed on ice
for temperatures between 188 and 210 K. Each point is marked with the
deposition rate of HCl in molec s-1 on the ice film, the
temperature of the ice film and the experiment running number (bold)
referring to Table 2.
Synopsis of kinetic and thermodynamic results for an amorphous
H2O/HCl mixture using HCl as a probe gas. The symbols/colors
used correspond to different experimental runs and the graphs show the
scatter of the individual measurements within a series. Original data are
published in Iannarelli and Rossi (2014).
Synopsis of kinetic and thermodynamic results for crystalline
HCl hexahydrate (HH) using X=HCl as a probe gas. The
symbols/colors used correspond to different experimental runs and the graphs
show the scatter of the individual measurements within a series. Original
data are published in Iannarelli and Rossi (2014).
Brief summary of the amount of a molecular monolayer (coverage) of
HCl adsorbed on H2O ice.
Coverage(molec cm-2)Temperature (K)Bibliographic reference5.0×1015200Hanson and Mauersberger (1990)1.0×1015200Abbatt et al. (1992)(2.0–3.0) ×1014191Hanson and Ravishankara (1992)1.15×1015183Foster et al. (1997)2.5×1014208Abbatt (1997)3.1×1014185Flückiger et al. (1998)(1.1±0.6)×1014201Lee et al. (1999)(2.0±0.7)×10142001Hynes et al. (2001)1.7×1014190Flückiger and Rossi (2003)1.3 ×10142006.7 ×10132102.3–2.7 ×1014180–200Henson et al. (2004)
CD performed all measurements and evaluated the data. MJR evaluated the data and wrote the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
We sincerely thank Riccardo Iannarelli
for Figs. A3 and A4 displayed in the Appendix. We also would like to thank
the Swiss National Science Foundation (SNSF) for unfailing support over the
years. This work has been performed under SNSF grant nos. 20-65299.01 and
200020-105471. Edited by: Daniel
Knopf Reviewed by: J. Paul Devlin and one anonymous referee
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