H 2O and HCl trace gas kinetics on crystalline HCl hydrates and amorphous HCl / H 2O in the range 170 to 205 K: the HCl / H 2O phase diagram revisited
In this laboratory study, H 2O 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 2O 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 2O : HCl = 5.8 ± 0.7 has been measured for HCl · 6H 2O, 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 R ev(HCl) for crystalline HCl hexahydrate (HCl · 6H 2O) films and amorphous HCl / H 2O mixtures has been found to be lower by a factor of 10 to 250 compared to Rev(H 2O) in the overlapping temperature range 175 to 190 K. Variations of the accommodation coefficient α(HCl) on pure HCl · 6H 2O 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 / H 2O phase diagram of crystalline HCl · 6H 2O 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 2O and crystalline HCl · 6H 2O – is performed in favour of the amorphous phase.