H<sub>2</sub>O and HCl trace gas kinetics on crystalline HCl hydrates and amorphous HCl / H<sub>2</sub>O in the range 170 to 205 K: the HCl / H<sub>2</sub>O phase diagram revisited

Abstract

Abstract. In this laboratory study, H2O ice films of 1 to 2 μm thickness have been used as surrogates for ice particles at atmospherically relevant conditions in a stirred flow reactor (SFR) to measure the kinetics of evaporation and condensation of HCl and H2O on crystalline and amorphous HCl hydrates. A multidiagnostic approach has been employed using Fourier transform infrared spectroscopy (FTIR) absorption in transmission to monitor the condensed phase and residual gas mass spectrometry (MS) for the gas phase. An average stoichiometric ratio of H2O : HCl = 5.8 ± 0.7 has been measured for HCl · 6H2O, and a mass balance ratio between HCl adsorbed onto ice and the quantity of HCl measured using FTIR absorption (Nin – Nesc – Nads) / NFTIR = 1.18 ± 0.12 has been obtained. The rate of evaporation Rev(HCl) for crystalline HCl hexahydrate (HCl · 6H2O) films and amorphous HCl / H2O mixtures has been found to be lower by a factor of 10 to 250 compared to Rev(H2O) in the overlapping temperature range 175 to 190 K. Variations of the accommodation coefficient α(HCl) on pure HCl · 6H2O up to a factor of 10 at nominally identical conditions have been observed. The kinetics (α, Rev) are thermochemically consistent with the corresponding equilibrium vapour pressure. In addition, we propose an extension of the HCl / H2O phase diagram of crystalline HCl · 6H2O based on the analysis of deconvoluted FTIR spectra of samples outside its known existence area. A brief evaluation of the atmospheric importance of both condensed phases – amorphous HCl / H2O and crystalline HCl · 6H2O – is performed in favour of the amorphous phase.

The influence of HCl on the evaporation rates of H2O over water ice in the range 188 to 210 K at small average concentrations

“…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.…”

“…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 C. Delval and M. J. Rossi: The influence of HCl on the evaporation rates 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.…”

The mid‐IR Absorption Cross Sections of α‐ and β‐NAT (HNO3 · 3H2O) in the range 170 to 185 K and of metastable NAD (HNO3 · 2H2O) in the range 172 to 182 K

“…The experiments performed in this work have been conducted using a multidiagnostic stirred flow reactor (SFR) whose detailed characteristics, design, and operational functions have been reported before [Chiesa and Rossi, 2013;Iannarelli and Rossi, 2014].…”

“…HNO 3 is an extremely "sticky" molecule that readily interacts Journal of Geophysical Research: Atmospheres 10.1002/2015JD023903 with the internal surfaces of both the reservoir vessel of the inlet system and the reactor walls of the SFR. In order to minimize the internal surface of the inlet system we modified the inlet line used for HNO 3 [Iannarelli and Rossi, 2014] by shortening it and replacing the dosing Swagelok double pattern low-pressure metering valve (SS-SS2-D-TVVH) with a simpler Swagelok S series shutoff valve.…”

“…where N TOT is the maximum number of molecules that can be adsorbed on the total internal surface of the SFR vessel and K L (cm À3 molecule À1 ) is the Langmuir constant, which is the equilibrium constant for the adsorption process (additional details for the SFR vessel in use are given in Iannarelli and Rossi [2014]). The values of N TOT and K L used in the present configuration are reported in Table 1.…”