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The rate and extent of physical adsorption of methanol, acetone and formaldehyde on ice were measured as a function of concentration and temperature. The gas-ice interaction was analysed by applying adsorption isotherms to determine temperature dependent Langmuir constants, K(T) and saturation surface coverage, N max . At low coverage a partitioning constant K # (T) was derived. The dependence of K # on temperature is given by K # (T) ¼ 6.24 Â 10 À12 exp(6178/T) cm for methanol and K # (T) ¼ 1.25 Â 10 À10 exp(5575/T) cm for acetone. For formaldehyde a temperature independent expression, K # ¼ 0.7 cm was derived. From these data adsorption enthalpies DH ads of (À46 AE 7) and (À51 AE 10) kJ mol À1 were obtained for acetone and methanol, respectively. The results were used to calculate the equilibrium partitioning of these trace gases to ice surfaces under conditions relevant to the atmosphere.

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Abstract. The interaction of mineral dust with N 2 O 5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO 3 ) were used to derive reactive uptake coefficients (γ ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10 −2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ (SDCV)=(3.7±1.2)×10 −2 , γ (ATD)=(2.2±0.8)×10 −2 and γ (CaCO 3 )=(5±2)×10 −2 . The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N 2 O 5 .

Abstract. The uptake and reaction of HOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOBr is efficiently taken up onto the frozen surfaces at temperatures between 253 and 233 K where it reacts to form the di-halogens BrCl and Br 2 , which are subsequently released into the gas-phase. The uptake coefficient for HOBr reacting with a frozen, mixed salt surface of similar composition to sea-spray was ≈ 10 −2 . The relative concentration of BrCl and Br 2 released to the gas-phase was found to be strongly dependent on the ratio of Cl − to Br − in the solution prior to freezing / drying. For a mixed salt surface of similar composition to sea-spray the major product at low conversion of surface reactants (i.e. Br − and Cl − ) was Br 2 .Variation of the pH of the NaCl / NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms of initial formation of BrCl in a surface reaction of HOBr with Cl − , and conversion of BrCl to Br 2 via reaction of surface Br − . Experiments on the uptake and reaction of BrCl with frozen NaCl / NaBr solutions served to confirm this hypothesis. The kinetics and products of the interactions of BrCl, Br 2 and Cl 2 with frozen salt surfaces were also investigated, and lower limits to the uptake coefficients of > 0.034, > 0.025 and > 0.028 respectively, were obtained. The uptake and reaction of HOBr on dry salt surfaces was also investigated and the results closely resemble those obtained for frozen surfaces. During the course of this study the gas diffusion coefficients of HOBr in He and H 2 O were also measured as (273 ± 1) Torr cm 2 s −1 and (51 ± 1) Torr cm 2 s −1 , respectively, at 255 K. The implications of these results for modelling the chemistry of the Correspondence to: J. N. Crowley (Crowley@mpch-mainz.mpg.de) Arctic boundary layer in springtime are discussed.

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.

The uptake and reaction of HOI and with dry and frozen NaCl/NaBr salt surfaces was investigated IONO 2 using a coated-wall Ñow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOI and IONO 2 react with both surfaces to form the di-halogens IBr and ICl, which are released into the gas phase. On mixed Cl~/Br~surfaces in which the concentration of Cl~greatly exceeds that of Br~, both HOI and react IONO 2 to form ICl or IBr. ICl reacts further with Br~to form the observed gas-phase product, IBr. Formation and release of ICl is only observed once the Br~concentration has been chemically depleted. The sum of IBr ] ICl in the gas phase was found to remain constant during the reaction, and to account for all of the HOI/IONO 2 taken up, indicating no loss of iodine to the surface, and a catalytic role of iodine in the activation and release of bromine and chlorine from mixed salt surfaces. The uptake coefficient of HOI on a frozen, mixed salt surface of similar composition to sea-water at 243 K was [10~2. Similar results (c [ 10~2) were obtained for the uptake of HOI onto dry, mixed salt surfaces of similar composition at 298 and 243 K. Lower limits for the accommodation coefficient of HOI on frozen salt (243 K), dry salt (253 K) and (253 K) of 0.05, 0.12 and H 2 SO 4 0.3, respectively were obtained. The implications of these results for tropospheric halogen chemistry are brieÑy discussed.

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.