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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.