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

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 (N_in – N_esc – N_ads) / N_FTIR = 1.18 ± 0.12 has been obtained. The rate of evaporation R_ev(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 R_ev(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, R_ev) 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.