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Experiments on the title compounds have been performed using a\udmultidiagnostic stirred-flow reactor (SFR) in which the gas phase as well as the\udcondensed phase has been simultaneously investigated under stratospheric\udtemperatures in the range 175–200 K. Wall interactions of the title\udcompounds have been taken into account using Langmuir adsorption isotherms in\udorder to close the mass balance between deposited and desorbed (recovered)\udcompounds. Thin solid films at 1 µm typical thickness have been\udused as a proxy for atmospheric ice particles and have been deposited on a Si\udwindow of the cryostat, with the optical element being the only cold point in\udthe deposition chamber. Fourier transform infrared (FTIR)\udabsorption spectroscopy in transmission as well\udas partial and total pressure measurement using residual gas mass spectrometry (MS) and sensitive\udpressure gauges have been employed in order to monitor growth and evaporation\udprocesses as a function of temperature using both pulsed and continuous gas\udadmission and monitoring under SFR conditions. Thin solid H2O ice films\udwere used as the starting point throughout, with the initial spontaneous\udformation of α-NAT (nitric acid trihydrate) followed by the gradual\udtransformation of α- to β-NAT at T > 185 K. Nitric acid dihydrate (NAD) was spontaneously formed at somewhat larger\udpartial pressures of HNO3 deposited on pure H2O ice. In contrast to\udpublished reports, the formation of α-NAT proceeded without prior\udformation of an amorphous HNO3 ∕ H2O layer and always resulted\udin β-NAT. For α- and β-NAT, the temperature-dependent\udaccommodation coefficient α(H2O) and α(HNO3), the\udevaporation flux Jev(H2O) and Jev(HNO3) and the\udresulting saturation vapor pressure Peq(H2O) and\udPeq(HNO3) were measured and compared to binary phase diagrams\udof HNO3 ∕ H2O in order to afford a thermochemical check of the\udkinetic parameters. The resulting kinetic and thermodynamic parameters of\udactivation energies for evaporation (Eev) and standard heats of\udevaporation ΔHev0 of H2O and HNO3 for α- and β-NAT, respectively, led to an estimate for the relative\udstandard enthalpy difference between α- and β-NAT of\ud−6.0 ± 20 kJ mol−1 in favor of β-NAT, as expected,\uddespite a significantly larger value of Eev for HNO3 in\udα-NAT. This in turn implies a substantial activation energy for\udHNO3 accommodation in α- compared to β-NAT where\udEacc(HNO3) is essentially zero. The kinetic (α(HCl),\udJev(HCl)) and thermodynamic (Peq(HCl)) parameters of\udHCl-doped α- and β-NAT have been determined under the\udassumption that HCl adsorption did not significantly affect α(H2O) and α(HNO3) as well as the evaporation flux\udJev(H2O). Jev(HCl) and Peq(HCl) on both\udα- and β-NAT are larger than the corresponding values for\udHNO3 across the investigated temperature range but significantly smaller\udthan the values for pure H2O ice at T < 200 K

atmospheric chemistry and physics

Heterogeneous kinetics of H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O, HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and HCl on HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; hydrates (&amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-NAT, &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-NAT, NAD) in the range 175–200 K