Uptake and Reaction of ClONO₂on Water Ice and HCl Trihydrate at Low Temperatures

Abstract

Chlorine nitrate adsorption kinetics and uptake have been measured on ordered ice and HCl trihydrate films at temperatures below 150 K. Reaction was followed using a thermal molecular beam, with mass spectrometric detection of gas-phase products and temperature-programmed desorption (TPD) and IR to identify adsorbed species. The sticking probability (S) on pure water ice is (0.98 ( 0.03) at 85 K and remains near unity for temperatures up to 145 K. Initially S is independent of the ClONO 2 uptake, indicating a trapping mechanism for reaction. A molecular state which desorbs near 120 K during TPD is identified as a precursor state and above this temperature molecular ClONO 2 is not stable on the surface and adsorption forms HOCl and nitric acid hydrate. On a clean ice surface the reaction probability starts to decrease after 0.1 monolayer of ClONO 2 has adsorbed, reaction ceasing (S < 5 × 10 -2 ) at an uptake of (0.25 ( 0.05) monolayer, independent of temperature. Reaction occurs even at low temperature, where the surface is immobile, indicating that ClONO 2 hydration can occur on the ice surface and does not require an extensive hydrate cage. The saturation stoichiometry is consistent with a surface covered with HOCl and an amorphous nitric acid trihydrate film. The initial reaction probability for ClONO 2 uptake onto pure HCl trihydrate films was similar, (0.98 ( 0.05) for temperatures between 85 and 145 K with some chlorine desorbing promptly into the gas phase even at 85 K. The uptake of ClONO 2 increases from 0.5 to ca. 1 monolayer above 125 K, the temperature at which HCl starts to transport into water ice films, reaction extending beyond the top layer of the HCl trihydrate surface.

HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K

“…The blue trace in Figure 1 reports the behavior of the HCl flux reflected from a 50 ML coverage crystalline ice film, prepared as described above, but maintained at 80 K. The initial sticking coefficient for HCl is indistinguishable from unity within experimental error up to 4 s but the reflected HCl flux rises sharply thereafter until it eventually coincides with the flux reflected from Pt(111) at 300 K. This observation is consistent with previous reports 29,47À49 indicating that HCl adsorbs with high probability (S ≈ 1) onto crystalline ice up to a saturation coverage of ∼1 ML even for temperatures above the HCl condensation temperature (i.e., 72 K at this molecular beam flux). Molecular beam reflection experiments were performed on crystalline ice at various temperatures up to 140 K and the observations reported by Hodgson and co-workers 48,50 were largely reproduced. Survey RAIRS spectra obtained during incremental dosing of 0.2 ML aliquots of gaseous HCl, up to 2.0 ML total cumulative HCl coverage, onto a 50 ML coverage crystalline ice film at 20 K are displayed in Figure 2.…”

Probing the Interaction of Hydrogen Chloride with Low-Temperature Water Ice Surfaces Using Thermal and Electron-Stimulated Desorption

“…There are numerous investigations into the TPD of HCl from ice over a wide range of dosing temperatures, HCl dose rates, ice phase, and annealing history of the ice. ,− ,− Both Harnett et al and Graham and Roberts measured lower temperature features that were dependent on several factors such as total HCl dose, saturation of the underlying ice film, annealing history, and dosing temperature. Harnett et al found that HCl dosing below 130 K results in an HCl enriched surface layer that leads to HCl desorption before water desorption and proposed the formation of HCl trihydrate at the surface.…”

Interactions of Atmospheric Trace Gases with Ice Surfaces: Adsorption and Reaction

“…Detailed studies involving vapor pressure measurements and infrared characterization of the condensed phase have since shown that supercooled solutions of nitric acid initially form on ice surfaces as a result of reactions (3) and (4). , These thin films inefficiently nucleate to nitric acid hydrates, only doing so when they are concentrated and the supersaturation level above the hydrate is extremely high. Deactivation of fresh ice surfaces via exposure to either ClONO 2 or N 2 O 5 has been observed by a number of other research groups and experimental techniques. ,− …”

“…Deactivation of fresh ice surfaces via exposure to either ClONO 2 or N 2 O 5 has been observed by a number of other research groups and experimental techniques. 133,[136][137][138][139] There has been considerable effort expended, in particular on reaction (3), to validate these measurements and understand the mechanism at the molecular level. Experimentally, the large uptake coefficient for reaction (3) was confirmed by low-pressure Knudsen cell measurements conducted in both a steady-state and pulsed manner, e.g., γ react ) 0.2 from 180 to 200 K. 137 Other flow-tube measurements conducted with low partial pressures at similar temperatures report a slightly lower uptake coefficient, γ react ≈ 0.1.…”