Interaction of formic and acetic acid with ice surfaces between 187 and 227 K. Investigation of single species- and competitive adsorption.

The physical adsorption of formic (HC(O)OH) and acetic (CH(3)C(O)OH) acid on ice was measured as a function of concentration and temperature. At low concentrations, the gas-ice interaction could be analysed by applying Langmuir adsorption isotherms to determine temperature dependent partition constants, K(Lang). Using temperature independent saturation coverages (N(max)) of (2.2 +/- 0.5) x 10(14) molecule cm(-2) and (2.4 +/- 0.6) x 10(14) molecule cm(-2) for HC(O)OH and CH(3)C(O)OH, respectively, we derive K(Lang)(HC(O)OH) = 1.54 x 10(-24) exp (6150/T) and K(Lang)(CH(3)C(O)OH) = 6.55 x 10(-25) exp (6610/T) cm(3) molecule(-1). Via a van't Hoff analysis, adsorption enthalpies were obtained for HC(O)OH and CH(3)C(O)OH. Experiments in which both acids or HC(O)OH and methanol interacted with the ice surface simultaneously were adequately described by competitive adsorption kinetics. The results are compared to previous measurements and used to calculate the equilibrium partitioning of these trace gases to ice surfaces under conditions relevant to the atmosphere.