Journal article

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

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 mu m 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 alpha-NAT (nitric acid trihydrate) followed by the gradual transformation of alpha-to beta-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 alpha-NAT proceeded without prior formation of an amorphous HNO3 / H2O layer and always resulted in alpha-NAT. For alpha- and alpha-NAT, the temperature-dependent accommodation coefficient alpha(H2O) and alpha(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 Delta H-ev(0) of H2O and HNO3 for alpha- and beta-NAT, respectively, led to an estimate for the relative standard enthalpy difference between alpha-and beta-NAT of -6.0 +/- 20 kJ mol(-1) in favor of beta-NAT, as expected, despite a significantly larger value of E-ev for HNO3 in alpha-NAT. This in turn implies a substantial activation energy for HNO3 accommodation in alpha- compared to beta-NAT where E-acc(HNO3) is essentially zero. The kinetic (alpha(HCl), J(ev)(HCl)) and thermodynamic (P-eq(HCl)) parameters of HCl-doped alpha- and beta-NAT have been determined under the assumption that HCl adsorption did not significantly affect alpha(H2O) and alpha(HNO3) as well as the evaporation flux J(ev)(H2O). J(ev)(HCl) and P-eq(HCl) on both alpha- and beta-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.


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