H2O and HCl trace gas kinetics on crystalline HCl hydrates and amorphous HCl / H2O in the range 170 to 205 K: the HCl / H2O phase diagram revisited
Other literature type
Rossi, M. J.
In this laboratory study, H<sub>2</sub>O 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 H<sub>2</sub>O 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 H<sub>2</sub>O : HCl = 5.8 ± 0.7 has
been measured for HCl · 6H<sub>2</sub>O,
and a mass balance ratio between HCl adsorbed
onto ice and the quantity of HCl measured using FTIR absorption
(<i>N</i><sub>in</sub> – <i>N</i><sub>esc</sub> – <i>N</i><sub>ads</sub>) /
<i>N</i><sub>FTIR</sub> = 1.18 ± 0.12 has been obtained.
The rate of evaporation </i>R</i><sub>ev</sub>(HCl) for crystalline HCl hexahydrate
(HCl · 6H<sub>2</sub>O) films and amorphous HCl / H<sub>2</sub>O mixtures
has been found to be lower by a factor of 10 to 250 compared to
<i>R</i><sub>ev</sub>(H<sub>2</sub>O) in the overlapping temperature range 175 to 190 K.
Variations of the accommodation coefficient α(HCl) on pure
HCl · 6H<sub>2</sub>O up to a factor of 10 at nominally identical
conditions have been observed. The kinetics (α, <i>R</i><sub>ev</sub>) are
thermochemically consistent with the corresponding equilibrium vapour
pressure. In addition, we propose an extension of the HCl / H<sub>2</sub>O phase
diagram of crystalline HCl · 6H<sub>2</sub>O 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 / H<sub>2</sub>O and crystalline HCl · 6H<sub>2</sub>O – is
performed in favour of the amorphous phase.