Heterogeneous kinetics of H₂O, HNO₃ and HCl on HNO₃ hydrates (α-NAT, β-NAT, NAD) in the range 175–200 K

Abstract. Experiments on the title compounds have been performed using a multidiagnostic stirred-flow reactor (SFR) in which the gas phase as well as the condensed phase has been simultaneously investigated under stratospheric temperatures in the range 175–200 K. Wall interactions of the title compounds have been taken into account using Langmuir adsorption isotherms in order to close the mass balance between deposited and desorbed (recovered) compounds. Thin solid films at 1 µm typical thickness have been used as a proxy for atmospheric ice particles and have been deposited on a Si window of the cryostat, with the optical element being the only cold point in the deposition chamber. Fourier transform infrared (FTIR) absorption spectroscopy in transmission as well as partial and total pressure measurement using residual gas mass spectrometry (MS) and sensitive pressure gauges have been employed in order to monitor growth and evaporation processes as a function of temperature using both pulsed and continuous gas admission and monitoring under SFR conditions. Thin solid H₂O ice films were used as the starting point throughout, with the initial spontaneous formation of α-NAT (nitric acid trihydrate) followed by the gradual transformation of α- to β-NAT at T > 185 K. Nitric acid dihydrate (NAD) was spontaneously formed at somewhat larger partial pressures of HNO₃ deposited on pure H₂O ice. In contrast to published reports, the formation of α-NAT proceeded without prior formation of an amorphous HNO₃ ∕ H₂O layer and always resulted in β-NAT. For α- and β-NAT, the temperature-dependent accommodation coefficient α(H₂O) and α(HNO₃), the evaporation flux J_ev(H₂O) and J_ev(HNO₃) and the resulting saturation vapor pressure P_eq(H₂O) and P_eq(HNO₃) were measured and compared to binary phase diagrams of HNO₃ ∕ H₂O in order to afford a thermochemical check of the kinetic parameters. The resulting kinetic and thermodynamic parameters of activation energies for evaporation (E_ev) and standard heats of evaporation ΔH_ev⁰ of H₂O and HNO₃ for α- and β-NAT, respectively, led to an estimate for the relative standard enthalpy difference between α- and β-NAT of −6.0 ± 20 kJ mol⁻¹ in favor of β-NAT, as expected, despite a significantly larger value of E_ev for HNO₃ in α-NAT. This in turn implies a substantial activation energy for HNO₃ accommodation in α- compared to β-NAT where E_acc(HNO₃) is essentially zero. The kinetic (α(HCl), J_ev(HCl)) and thermodynamic (P_eq(HCl)) parameters of HCl-doped α- and β-NAT have been determined under the assumption that HCl adsorption did not significantly affect α(H₂O) and α(HNO₃) as well as the evaporation flux J_ev(H₂O). J_ev(HCl) and P_eq(HCl) on both α- and β-NAT are larger than the corresponding values for HNO₃ across the investigated temperature range but significantly smaller than the values for pure H₂O ice at T < 200 K.