Heterogeneous Kinetics of H2O, HNO3 and HCl on HNO3 hydrates (α-NAT, β-NAT, NAD) in the range 175–200 K
Authors: Iannarelli Riccardo , Rossi Michel J. 
Published: April 2016

Abstract. Experiments on the title compounds have been performed using a multidiagnostic stirred-flow reactor (SFR) in which the gas- as well as the condensed phase has been simultaneously investigated under stratospheric temperature conditions 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 micron or so typical thickness have been used as a proxy for atmospheric ice particles and have been deposited on a Si window of the cryostat where the optical element was the only cold point in the deposition system. FTIR absorption spectrometry in transmission as well as partial and total pressure measurement using residual gas MS and sensitive pressure gauges have been employed in order to monitor growth and evaporation processes as a function of temperature using both pulsed gas admission and continuous monitoring under SFR conditions. Thin solid H2O ice films were used as the starting point throughout, with the initial formation of α-NAT followed by the gradual transformation of α → β-NAT starting at 185 K. NAD was formed at once 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 Jev(H2O) and Jev(HNO3) and the resulting saturation vapor pressure Peq(H2O) and Peq(HNO3) were measured and compared to binary phase diagrams of HNO3/H2O in order to afford thermochemical control of the kinetic parameters. The resulting kinetic and thermodynamic parameters of activation energies for evaporation (Eev) and standard heats of evaporation ΔHev0 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 in favor of β-NAT, as expected, despite a significantly larger value of Eev for HNO3 in α-NAT. This in turn implies a substantial activation energy for HNO3 accommodation in α- compared to β-NAT where Eacc(HNO3) is essentially zero. The kinetic (α(HCl), Jev(HCl)) and thermodynamic (Peq(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 Jev(H2O). Jev(HCl) and Peq(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. This means that once contaminated with HCl the "impurity" HCl will persist along with HNO3 upon complete evaporation of the atmospheric ice particle. We comment on recent laboratory results involving the HNO3/H2O system using Chilled Mirror Hygrometers (CMH) in light of the present kinetic results.