The Influence of HCl on the Evaporation Rates of H 2 O over Water Ice in the Range 188 to 210 K at small Average Concentrations

The evaporation flux Jev(H2O) of H2O from HCl-doped typically 1.5 μm or so thick vapordeposited 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 behaviour and to be a function of the average mole fraction χHCl of HCl in the ice film ranging from 6x10 to 3x10 molecule cm s at 174 – 210 K for initial values χHCl ranging from 5x10 to 3x10 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 temporal dependence of Jev(H2O) was correlated with (a) the evaporation range r as 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 time dependence of Jev(H2O) 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 life-time 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. 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-238 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 17 April 2018 c © Author(s) 2018. CC BY 4.0 License.


HCl
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 H 2 O vapor pressure was independent of χ HCl within the measured range and equal to that of pure ice down to 80 nm thickness.The dependence of J ev (H 2 O) with increasing average χ HCl was correlated with (a) the evaporation range r b/e parameter, that is, the ratio of J ev (H 2 O) just before HCl doping of the pure ice film and J ev (H 2 O) after observable HCl desorption towards the end of film evaporation, and (b) the remaining thickness d D below which J ev (H 2 O) decreases to less than 85 % of pure ice.The dependence of J ev (H 2 O) with increasing average χ HCl from HCl-doped ice films suggests two limiting data sets, one associated with the occurrence of a twophase pure ice/crystalline HCl hydrate binary phase (set A) and the other with a single-phase amorphous HCl/H 2 O binary mixture (set B).The measured values of J ev (H 2 O) may lead to significant evaporative lifetime extensions of HClcontaminated ice cloud particles under atmospheric condi-tions, regardless of whether the structure corresponds to an amorphous or crystalline state of the HCl/H 2 O aggregate.

Introduction
HCl is among the mineral acids that control the acidity of the atmosphere, together with HNO 3 and H 2 SO 4 .The production of atmospheric HCl predominantly takes place in the middle and upper stratosphere where O 3 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 O 3 and HCl has been used to monitor stratospheric-C.Delval and M. J. Rossi: The influence of HCl on the evaporation rates tropospheric exchange processes and stratospheric O 3 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 H 2 SO 4 aerosol according to the following reaction taken as an example: HCl(ads) + ClONO 2 Cl 2 (g) + HNO 3 (ads). (R1) These reactions efficiently convert inactive Cl-containing reservoir molecules such as HCl and ClONO 2 into active photolyzable Cl-containing compounds in a single reaction.
Typical examples of such photolabile reaction products are Cl 2 , ClNO 2 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) H 2 O 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(Delzeit et al., , 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 H 2 O on the surface of ice films and may occur either as amorphous HCl/H 2 O 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 lowtemperature 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 H 3 O + Cl − (Eigen cation) or H 5 O + 2 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 NEX-AFS with photoemission (UPS, XPS) and FTIR in transmission of thin HCl/H 2 O 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" H 2 O 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/H 2 O 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/H 2 O system under steady-state conditions of both HCl and H 2 O at temperatures representative of the UT/LS (Flückiger et al., 1998;Oppliger et al., 1997).The most recent experimental work on HCl/H 2 O 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.
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 J ev (molecule cm −2 s −1 ) or rate R ev (molecule cm −3 s −1 ) and the rate of condensation k cond (s −1 ), of H 2 O in the presence in ice without perturbing the equilibrium vapor pressure of H 2 O, P eq H 2 O (Delval et al., 2003).We have furthermore shown that the way J ev of H 2 O decreases with time depends on the rate of deposition or the integral of deposited HCl, namely R HCl (molecule s −1 ) and N HCl (molecule), respectively.It appears that two observed HCl species on/in ice, namely single-phase amorphous HCl/H 2 O mixtures and a binary phase consisting of pure ice and an as yet unidentified crystalline HCl hydrate, HCl • xH 2 O, decrease J ev (H 2 O) 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 J ev (H 2 O) with the increasing average mole fraction of HCl, χ HCl , 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 J ev 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(Solomon et al., , 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 × 10 14 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 Table 1.Hardware parameters of both cryogenic sample supports of HCl-doped ice.

Si optical window QCM
Reactor temperature T r (K) 320 Reactor volume V r (cm −3 ) 2350 Conversion factor (1/RT) Conv (molec cm −3 Torr −1 ) with R = 62 398 (Torr cm 3 mol −1 K −1 ) 3.0 × 10 16 1 Sample surface area (cm2 ) 0.78 0.50 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 HNO 3 -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 HNO 3 was deposited by directed injection onto ice films supported by the quartz crystal of the QCMB as referenced above.Evap-oration 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.
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 × 10 17 molecule cm −2 s −1 under static conditions.The H 2 O equilibrium vapor pressure agreed with published values across the covered temperature range (Marti and Mauers-    , 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, R HCl , as well as its time integral, namely the number of HCl molecules deposited on ice, N HCl , have been evaluated using the method described in Delval and Rossi (2005).Typically, R HCl ranges between 8.0 × 10 11 and 4.2 × 10 13 molecule s −1 and N HCl between 1.0 × 10 14 and 5.4 × 10 15 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.J ev (H 2 O) 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.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 J QCM ev , the evaporative flux of H 2 O calculated from the raw signal at the output of the QCMB.The diamond symbol ( ) corresponds to J 18 ev evaluated from I 18 , the MS signal amplitude for H 2 O monitored at m/e = 18.Int(J 18 ev ) marked by triangles in Fig. 1a is the time integral of J 18 ev and corresponds to the total number of H 2 O molecules that have evaporated from the ice film at t. D is the label at time t D at which J ev (H 2 O) decreased from its original value corresponding to pure ice to 85 % of its original value at t = 0, and d D is the remaining thickness of the ice film at t D .H b and H e 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 t Hb and t He .The data have been treated in analogy to HNO 3 -doped ice through the formalism given in Delval and Rossi (2005).Akin to HNO 3 the mass balance between HCl deposited, N dep HCl , and HCl recovered during ice evaporation, N evap HCl , 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 χ HCl of ±18 % from the average discrepancy between N dep HCl and N evap HCl displayed in Table 2.In the following N HCl will always refer to N dep HCl 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 P / t.The present experiments cover the evaporation of a small albeit important fraction of the model ice film for which the decrease in J ev (H 2 O) is significant., monitored using QCM (molec cm −2 s −1 ), (+): I 18 MS signal for H 2 O, (×): I 36 MS signal for HCl (A), ( ): J 18 ev evaporative flux calculated from I 18 (molec cm −2 s −1 ), ( ): Int(J 18 ev ) time integral of J 18 ev (molec cm −2 ).

Results
The experimental data reported in Table 2 on the isothermal change in the evaporative flux of water, J ev (H 2 O), as a function of the average mole fraction of HCl, χ HCl , 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 H 2 O vapor during evaporation owing to the low H 2 O partial pressures in the reactor.The axes labeled "b" and "e" correspond to the values of J ev (H 2 O) at the end of ice film deposition and after desorption of most of the adsorbed HCl from the HCldoped ice film at t He , respectively, as displayed in Fig. 1b.The average mole fraction χ HCl of HCl in the remaining ice film as a function of time is calculated according to Delval and Rossi (2005).The change in χ HCl owing to H 2 O evaporation is evaluated between t = t D and t = t Hb , which corresponds to the time interval when the number of adsorbed HCl  2 colorcoded according to the corresponding experiment number in Table 2.The colored and circled numbers on axis "b" (left) correspond to J ev (H 2 O) of pure ice before HCl deposition; the ones on axis "e" (right) are J ev (H 2 O) at t = t He at the end of HCl evaporation.The colored circles in the data field mark the value of J ev (H 2 O) after HCl deposition at t = t 0 and are equal to J ev (H 2 O) of pure ice.The start of any particular J ev (H 2 O) curve as a continuous solid (bold) line occurs at t = t D at 85 % of J ev (H 2 O) at t = 0 (pure ice value, colored dot or circled number on axis "b" to the left) and ends at t Hb , the beginning of HCl evaporation as displayed in Fig. 1b.
molecules is constant, as no release of HCl is observable in the gas phase at m/e = 36 before t Hb .Table 2 also displays the initial value of the HCl mole fraction, χ 0 HCl , calculated for the ice film just at the end of HCl deposition and marked by a colored circle on the experimental trajectory of a colorcoded evaporating ice film displayed in Fig. 2. The average mole fraction of HCl in the ice film, χ HCl , increases owing to evaporation of H 2 O from the ice film without loss of HCl such that the elapsed time increases with χ HCl in Fig. 2.
The beginning of an evaporation experiment after the end of HCl doping (t = 0 in Fig. 1 or t 0 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 = t D J ev (H 2 O) has decreased to an arbitrarily chosen value of 85 % of its original value measured at t = t 0 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 t 0 (colored circle) moving to t D and finishing at t Hb 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 t He (color-coded number of experiment on axis "e" for "Halogen end").The trajectory of an experiment with values of χ HCl between t 0 (colored circle at χ 0 HCl ) and t D (beginning of bold colored line, see experiment 8 in Fig. 2) ending at t Hb (end of bold line, experiment 8) is presented as a bold dashed-dotted and bold smooth line from t 0 to t Hb , 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 t 0 , the beginning of the evaporation experiment, and also describe the post phase of evaporation starting at t Hb to t He , in order to guide the eye of the reader to imagine a complete evaporation cycle.
Two different data sets of the change in J ev (H 2 O) with χ HCl may be distinguished in Fig. 2. The first kind of data set corresponds to the curves describing J ev for experiments 1, 2, 9 and 11 and is called dataset A. These traces present a slow continuous decrease in J ev (H 2 O) as χ HCl increases during H 2 O evaporation.The second type of dataset shows an initial plateau of J ev (H 2 O) with increasing χ HCl starting at the value of pure ice evaporation followed by a sudden decrease in J ev (H 2 O) and is found for experiments 3, 4, 7 and 8, which we call dataset B. Akin to HNO 3 , we have evaluated the impact of the HCl deposition protocol on the evaporation range parameter, r b/e , which is the ratio between the evaporative flux of H 2 O at the beginning of ice evaporation, J b ev (H 2 O) reported on the left axis "b" in Fig. 2, and J ev (H 2 O) close to the end of the desorption of HCl, J e ev (H 2 O), at t = t He (the right axis "e" in Fig. 2).It describes the factor by which J ev (H 2 O) decreases within the limits of "b" and "e".The impacts of both the rate of deposition of HCl on ice, R HCl , and its time integral corresponding to the dose of deposited HCl, N HCl , 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 J ev (H 2 O), either with elapsed time or amount of adsorbed HCl expressed as the time dependence of χ HCl .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 J ev (H 2 O) within a fairly narrow range of χ HCl .Indeed, the available number of experiments clearly shows two distinct and limiting cases, whereas the search for other controlling parameters such as R HCl , N HCl and the temperature of deposition (T ice ) for dataset A failed, akin to a similar HNO 3 study (Delval and Rossi, 2005).
One may take note for instance of the low value of χ HCl at 210 K for experiment 9 where the conditions of deposition are similar to experiments 1 and 2; however, its respective values of r b/e differ significantly from experiment 9 (Fig. 3).In contrast, for dataset B the r b/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 R HCl and N HCl as for dataset A. In contrast, the r b/e values for dataset A seem widely scattered over the explored parameter space.We have also investigated the impact of the deposition protocol on d D , which is the thickness of ice that is affected by the presence of HCl, namely the remaining thickness of ice whose J ev (H 2 O) value has decreased to 85 % of

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 χ 0 HCl 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 H 2 O molecules contained in the ice film and the measured number of deposited HCl molecules, N dep HCl , for experiment 11 (Table 2).Table 2 and Fig. 1 reveal that for approximately 2.2 × 10 18 H 2 O molecules in the film and 5.4 × 10 14 molecules of deposited HCl, we obtain χ 0 HCl = 2.7 × 10 −4 .This HCl mole fraction represents an average value that takes into account all H 2 O 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 HNO 3 /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, H 2 O evaporates at fluxes J ev (H 2 O) that are characteristic of pure ice measured previously (Delval and Rossi, 2004;Pratte et al., 2006).These initial values J b ev (H 2 O) are displayed on the left-hand "b" (= beginning) axis in Fig. 2. As the evaporation proceeds J ev (H 2 O) slightly decreases with time, as displayed in Fig. 1a, to the arbitrarily chosen point where J ev (H 2 O) has decreased to 85 % of the initial pure ice value, at which point the remaining ice thickness d D has decreased by approximately one-third to 771.7 nm remaining ice thickness as displayed in Fig. 1b and Table 2. Further evaporation of H 2 O leads to a continuous decrease in J ev (H 2 O) at a corresponding increase in χ HCl up to point H b defined above ("Halogen beginning") at t Hb (Fig. 1b) where HCl starts to desorb from the ice film as monitored using the residual MS signal at m/e = 36.
For t < t Hb , χ HCl is given by the number of originally deposited HCl molecules that remain adsorbed on the ice film up to t Hb and the remaining H 2 O molecules in the film.In contrast, for t > t Hb the composition of the remaining ice film must be determined by taking into account the loss by evaporation of both H 2 O and HCl.The present experimental configuration is not adapted to quantitatively measure HCl loss.Therefore, we have chosen to display the temporal development of J ev (H 2 O) for t < t Hb in Fig. 2 as a function of the average value of the HCl mole fraction χ HCl .However, the value of J ev (H 2 O) at t = t He where most of the HCl has desorbed from the ice film is plotted on the right axis labeled "e" (= end) as J e ev (H 2 O) in Fig. 2 in order to provide a limit for the minimum value of the evaporation rate J ev (H 2 O) at an ice film thickness d He of approximately 80±10 nm as displayed in Fig. 1b.We have observed in the past that J ev (H 2 O) 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 t He .The ratio r b/e = J b ev (H 2 O)/J e ev (H 2 O) is displayed in Table 2 and is an operational evaporation range parameter that estimates the extent of decrease in J ev (H 2 O) 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 H 2 O, P eq (H 2 O), is that of pure ice (Delval et al., 2003;Delval and Rossi, 2004;Pratte et al., 2006) owing to the small values of χ 0 HCl .Raoult's law applies to such small values of χ HCl but leads to unmeasurably small deviations from the observed vapor pressure of H 2 O 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 H 2 O/HCl system.This value of P eq (H 2 O) is observed throughout the evaporation up to t He as the film is apparently sufficiently H 2 O-rich to support an equilibrium vapor pressure characteristic of pure ice consistent with the published, albeit revised, HCl/H 2 O-phase diagram by Iannarelli and Rossi (2014).In view of the decreasing values of J ev (H 2 O) displayed in Fig. 2 the equilibrium vapor pressure of pure ice can only be maintained if the condensation rate coefficient k c for H 2 O adsorption decreases to the same extent as J ev (H 2 O), 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 H 2 O at m/e = 18 ( ).Akin to the HNO 3 /H 2 O system studied previously (Delval and Rossi, 2005) we obtain a perfect match between the two signals for t<t D , whereas for t > t D 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 HNO 3 /H 2 O, 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 C f 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 J ev (H 2 O) at t > t D akin to the previous study on HNO 3 /H 2 O.
The accuracy with which both t Hb and t He 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 t Hb that is signalled by an increase in the MS intensity, whereas t He 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 t Hb is determined to ±10 s, whereas t He 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, R HCl , in the range 1 × 10 13 to 5 × 10 13 molecule s −1 for the 0.78 cm 2 surface area of the Si window leads to the formation of a crystalline HCl hydrate, HCl • xH 2 O, whereas values outside of this range seemed to favor the formation of an amorphous HCl/H 2 O 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 H 3 O + X − suggests that the ν 1 and ν 3 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•6H 2 O) under the present experimental conditions could be safely excluded, however, the FTIR absorption spectrum www.atmos-chem-phys.net/18/15903/2018/Atmos.Chem.Phys., 18, 15903-15919, 2018 clearly shows the presence of dissociated HCl within the ice film (Iannarelli and Rossi, 2014).Akin to HCl • 6H 2 O 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/H 2 O deposit, either crystalline or amorphous, and the relevant HCl deposition parameters.Previous work has revealed a distinctly different temporal dependence of J ev (H 2 O) between the crystalline and amorphous HCl hydrates with the extent of H 2 O 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 J ev (H 2 O) as a function of increasing χ HCl owing to H 2 O 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 > t D J ev (H 2 O) for experiment 7 decreases at once with χ HCl , in contrast to experiment 11, whose J ev (H 2 O) value gradually starts to decrease at roughly the same value of χ HCl as experiment 7.In addition, in both cases the extent of the decrease in J ev (H 2 O) is roughly equal between t D and t Hb within less than a factor of 2. Set B data are in marked contrast to set A independent of the magnitude of χ HCl , 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 J ev (H 2 O) for set B as well as the gradual decline for set A, both at t D , occur before HCl starts to evaporate from the sample at t Hb and appears therefore to be independent of χ HCl within the range explored in the present work.
If we consider the mean value d D 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 × 10 17 molecules of H 2 O spread out over 0.50 cm 2 .These H 2 O molecules are impacted by the presence of HCl to some extent because J ev (H 2 O) 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 • xH 2 O was confirmed by FTIR absorption led to the conclusion that on average the amount of "trapped" H 2 O within d D corresponded to 1.2 × 10 18 molecules starting with an original 1 µm thick ice film that was subsequently doped with HCl.This quantity of H 2 O, when scaled from the 0.78 cm 2 area of the Si window used for FTIR absorption to the area of 0.5 cm 2 of the QCMB, leads to 7.7 × 10 17 H 2 O, which is in satisfactory agreement with the present measurement of d D or 8.5 × 10 17 H 2 O in the present work.We may add that the previous value of 1.2 × 10 18 H 2 O from the work of Delval et al. (2003) corresponding to d D 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 d D 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 J ev (H 2 O) of pure H 2 O ice before it slows down.This decrease in J ev (H 2 O) 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 = t He when the sample runs out of H 2 O 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 d D is proportional to T ice 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 J ev (H 2 O) characteristic of pure ice before it decreases to lower values when the presence of HCl slows down J ev (H 2 O).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 gascondensed 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 H 2 O 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 HNO 3 , 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 d D at higher temperatures and higher HCl doses (Table 2).Because of higher presumed interfacial HCl concentrations these samples experience a decrease in J ev (H 2 O) 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 d D .Both high HCl doses and high temperatures favor HCl contamination of deeper layers of the HCl film, and hence high values of d D .
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/H 2 O 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 J ev (H 2 O) as well as the corresponding mass accommodation coefficient or the adsorption rate coefficient for H 2 O 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/H 2 O mixture (Fig. A3) compared to crystalline HCl hexahydrate (Fig. A4).
Figure 3 displays the range parameter r b/e as a function of R HCl for all data displayed in Table 2.It is noteworthy that r b/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 d D and r b/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 d D = 549.0± 120.0 nm together with a similarly restricted range of r b/e between 20 and 27 as well as the substantial overlap in R HCl 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/H 2 O mixture.In contrast, the scatter of the set A data across the range of r b/e and d D 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/H 2 O-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 T ice , R HCl , N dep HCl do apparently not control J ev (H 2 O) of both datasets.

Atmospheric implications
The evaporation range parameter r b/e may be used to quantitatively evaluate the upper limit of the evaporative lifetime extension of thin ice films under conditions of H 2 O vapor subsaturation.In the interest of applying the data of the present work to atmospheric conditions we make the as-sumption 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 t ev 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): 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 H 2 O, r and a are the ice particle radius and the distance between two molecular layers in H 2 O(ice), respectively (Iannarelli and Rossi, 2016a).Equation ( 1) is based on a simple layerby-layer evaporation model of H 2 O(ice) from a spherical ice particle following a zero-order rate law for J ev or a first order rate law for its inverse, namely H 2 O adsorption or condensation.For a 10 µm diameter ice particle approximated by thin film experiment 1 (Table 2) at rh = 80 %, T = 192 K, J ev = 3 × 10 16 molecule s −1 cm −2 (Petrenko and Whitworth, 1999) and a = 4×10 −8 cm we obtain t ev = 2050 s or 34 min.This is the value for a pure ice particle as J ev (H 2 O) 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 r b/e = 43 for experiment 1 t ev 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, HNO 3 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 andRossi, 2004, 2005;Pratte et al., 2006).
We would like to stress that the variable r b/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 = t He 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 = t Hb , 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 andRossi, 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 r b/e and d D in dataset A displayed in Figs. 3 and 4 and the apparent lack of influence of the deposition parameters (T ice , R HCl , N dep HCl on J ev (H 2 O), we may state several key points from the present work.
a. We observe two types of behavior, both complex, as far as the temporal change in J ev (H 2 O) with ongoing evaporation of H 2 O from a HCl/H 2 O condensate is concerned.We have named it sets A and B that represent limiting behavior as not all performed experiments fit into this scheme.
b.At low temperature or low dose of deposited HCl N dep HCl set A samples, especially samples 1 and 10, reveal an "ice-like" behavior that corresponds to a low value of d D .This means that the HCl/H 2 O condensate evaporates a large fraction of the sample thickness at a value of J ev (H 2 O) characteristic of pure ice before slowing down at an increasing mole fraction of HCl upon H 2 O evaporation.This corresponds to a twophase system consisting of a major ice-like and minor e. Set B samples are tentatively identified as single-phase binary amorphous mixtures of HCl/H 2 O 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 r b/e and d D is consistent with these observations and manifests itself as a continuous yet gradual decrease in J ev (H 2 O) with increasing χ HCl .It is in distinct contrast to Set A, where J ev (H 2 O) values are those of pure ice until the ice phase has completely evaporated followed by a gradual decline of J ev (H 2 O) when the crystalline HCl hydrate starts to decompose.
f.It must be recalled that the vapor pressure of H 2 O remained that of pure ice during most of the thickness of the H 2 O/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 H 2 O saturation vapor pressure unmeasurably small.The present results therefore primarily address the kinetics of H 2 O evaporation which changes with the total mass of the thin film condensate and the concomitant increase in HCl concentration and/or mole fraction.
Data availability.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).Table A1.Brief summary of the amount of a molecular monolayer (coverage) of HCl adsorbed on H 2 O ice.
10 24 MS calibration factor for H 2 O (m/z = 18, dynamic) C dyn 18 (molec s −1 A −1 ) 1.7 × 10 25 HCl collision frequency with ice sample ω HCl (s −1 ) 3.59 2.31 HCl effusion rate constant of calibrated leak k esc (HCl) (s −1 ) 0.047 MS calibration factor for HC1 (m/z = 36, stirred flow) Representative experimental results for the kinetics of H 2 O evaporation in the presence of HCl for increasing HCl deposition temperatures at given rates of deposition R HCl and doses of HCl N dep HCl .In the first column the number refers to the corresponding experiment and identifies the data displayed in Fig.2

Figure 2 .
Figure 2. Change in the evaporative flux J ev (H 2 O) as a function of the HCl mole fraction (χ HCl ) for the cases presented in Table2colorcoded according to the corresponding experiment number in Table2.The colored and circled numbers on axis "b" (left) correspond to J ev (H 2 O) of pure ice before HCl deposition; the ones on axis "e" (right) are J ev (H 2 O) at t = t He at the end of HCl evaporation.The colored circles in the data field mark the value of J ev (H 2 O) after HCl deposition at t = t 0 and are equal to J ev (H 2 O) of pure ice.The start of any particular J ev (H 2 O) curve as a continuous solid (bold) line occurs at t = t D at 85 % of J ev (H 2 O) at t = 0 (pure ice value, colored dot or circled number on axis "b" to the left) and ends at t Hb , the beginning of HCl evaporation as displayed in Fig.1b.

Figure 3 .
Figure 3. Synopsis of the dependence of the evaporation range parameter r b/e on the rate of deposition R HCl of HCl for temperatures between 188 and 210 K.Each point is marked with the total number of HCl molecules (N HCl ) 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 r b/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.
J ev (H 2 O) of pure ice.The results on d D as a function of R HCl and N dep HCl 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.

Figure 4 .
Figure 4. Synopsis of the dependence of d D on the rate of deposition R HCl of HCl for temperatures between 188 and 210 K.Each point is marked with the total number of HCl molecules (N HCl ) 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 d D values for dataset B (experiments3, 4, 7, and 8).The color code goes from low (blue) over medium (green) to high (red) temperatures.
HCl/H 2 O phase, both with significantly different values of J ev (H 2 O).c. High values of d D are observed at high T ice or N dep HCl values for set A samples.This means that the sample evaporates H 2 O at J ev (H 2 O) characteristic of pure ice for a relatively short time of its evaporation history because the quantity of HCl is sufficient to decrease J ev (H 2 O) already at high values of d D by rapidly diffusing to deeper layers of the ice film.An equivalent way of expressing the point would be to state that d D which is an indicator of the total mass of the ice film, is proportional to T ice for Set A. d.Set A samples generally show scattered values of both d D and r b/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 • xH 2 O, 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 J ev (H 2 O) until the pure ice phase has disappeared, followed by the crystalline HCl/H 2 O phase at a lower rate of J ev (H 2 O) to attain the characteristic value for the evaporation of the crystalline HCl • xH 2 O phase.

Figure A1 .
Figure A1.Graph of the dependence of the evaporation range parameter, r b/e , on the number of adsorbed HCl, N dep HCl , 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.

Figure A2 .
Figure A2.Graph of the dependence of the remaining thickness d D on the number of adsorbed HCl, N dep HCl , 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.

Figure A3 .
Figure A3.Synopsis of kinetic and thermodynamic results for an amorphous H 2 O/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).

Figure A4 .
Figure A4.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).