The interactions of HOCl and ClONO2 with pure and HCl-doped water ice have been reinvestigated using infrared spectroscopy in conjunction with static and thermal desorption mass spectrometry in the temperature range 140−180 K to probe the detailed mechanisms of their heterogeneous atmospheric reactions. In agreement with earlier studies, HOCl was found to react with hydroxonium chloride species to directly produce molecular chlorine. This molecular chlorine desorbed from the surface above ca. 155 K either directly or in subsequent thermal desorption experiments, depending upon substrate temperature at the time of reaction. ClONO2 was observed to heterogeneously hydrolyze upon water ice to produce HOCl and hydrated H3O+NO3 -. Below ca. 155 K, HOCl remains adsorbed on the ice surface, while above ca. 155 K, it desorbs directly into the gas phase. These results suggest that a long-lived, adsorbed state of HOCl is unlikely to play a direct role in heterogeneous chemistry at stratospheric temperatures. The direct reaction of ClONO2 with an ice surface saturated with ionic hydrates of HCl was observed to result in the production of molecular chlorine, which, as for the HOCl reaction, either remained adsorbed or desorbed from the surface depending upon temperature. This provides convincing evidence for the direct heterogeneous reaction of Cl- with ClONO2 under stratospheric conditions. A novel explanation for the enhanced reactivity of ClONO2 toward atmospherically relevant substrates is presented, in which a partially ionized precursor state is formed. Partial ionization of ClONO2 along the Cl−O bond serves to increase the electrophilicity of the chlorine atom, making the site highly prone to nucleophilic attack by either water or adsorbed chloride ions.

We have examined the reaction of with water ice at 140 K and found evidence for the formation of molecular nitric acid ClONO 2 under conditions of reduced surface water. This is very di †erent from the behaviour of on ice at 180 K, where substrate-ClONO 2 induced pre-reaction ionisation of leads to a reaction mechanism involving the intermediacy of and nitrate ClONO 2 [H 2 OCl]ì ons. These two results are not inconsistent with a single mechanism if a subtle change in the interplay between the availability of water molecules at the surface and the variation of the substrate/adsorbate interactions of with temperature are ice/ClONO 2 taken into account. This implies that reaction mechanisms and product branching ratios in the atmosphere may vary widely over a range of temperatures and reactant partial pressures.

Grazing angle reflection/absorption FTIR spectroscopy has been employed to characterise thin layers of nitric acid hydrates and ammonium nitrate in a high-vacuum system. Ordered films of frozen nitric acid di-and tri-hydrate were produced by slow effusive vapour deposition onto a polycrystalline gold foil at 80 K followed by annealing to 190 K. On the basis of the metal surface selection rule, analysis of the observed IR band intensities suggests that the nitrate and oxonium ions align preferentially with their C , axes parallel to each other and perpendicular to the underlying substrate. A similar orientation was observed for the nitrate ions of thin ammonium nitrate films.

Reflection-absorption infrared spectroscopy has been employed in order to investigate the low-temperature photochemistry (90-140 K) of thin films of nitric acid and ammonium nitrate grown in Vacuo. Photolysis of amorphous nitric acid hydrate, the crystalline dihydrate (NAD) and trihydrate (NAT) at λ > 230 nm resulted in the formation of molecular nitric acid due to rapid protonation of the excited nitrate ion. Secondary photolysis of HONO 2 produced NO 2 and NO. If a neat film of molecular, anhydrous nitric acid was irradiated, nitrate and nitronium ions were observed. In contrast, ammonium nitrate photolysis at 140 K did not result in a proton transfer to produce NH 3 and HONO 2 but in the formation of the peroxynitrite ion (ONOO -) as a precursor for NO 2 -. Molecular dinitrogen tetraoxide and nitrous oxide were also detected in the film. Mechanistic details and possible implications for the chemistry of the polar atmosphere are discussed.

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The effects of photolysis on frozen, thin films of water-ice containing nitrogen dioxide (as its dimer dinitrogen tetroxide) have been investigated using a combination of Fourier transform reflection-absorption infrared (FT-RAIR) spectroscopy and mass spectrometry. The release of HONO is ascribed to a mechanism in which nitrosonium nitrate (NO+NO3-) is formed. Subsequent solvation of the cation leads to the nitroacidium ion, H2ONO+, i.e., protonated nitrous acid. The pathway proposed explains why the field measurement of HONO at different polar sites is often contradictory.

When low-temperature thin films of either ionic or covalent dinitrogen pentaoxide, N 2 O 5 , are exposed to gaseous HCl and water, the only products observed in the solid phase by reflection-absorption infrared spectroscopy (RAIRS) are molecular nitric acid and the oxonium ion. Nitryl chloride, ClNO 2 , is not detectable. When dinitrogen pentaoxide is co-deposited with hydrogen chloride and water at 85 K and annealed to 140 K, the resultant RAIR spectra indicate that the film is composed of H 3 O + Cl -, N 2 O 5 , HNO 3 , and D 2h -N 2 O 4 . When nitryl chloride is co-deposited with either water or HCl/water mixtures, infrared spectra indicative of solid D 2h -N 2 O 4 are measured, as well as peaks corresponding to nitrate ions and cis-ClONO (chlorine nitrite). Reaction between ClNO 2 and its isomer, cis-ClONO, is proposed as an explanation for the formation of dinitrogen tetraoxide in both systems. The proposed reaction mechanism for this hydrolysis is extended to the N 2 O 5 /H 2 O/HCl deposits in order to explain the lack of observable ClNO 2 in such thin films.

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