Co-adsorption of acetic acid and nitrous acid on ice.

The co-adsorption of acetic and nitrous acids was investigated in a packed ice bed flow tube at atmospheric pressure in the temperature range of 213-243 K. The uptake of acetic acid (CH3COOH) on ice was probed by means of a chemical ionisation quadrupole mass spectrometer. For the interaction of HONO with ice, molecules labelled with the short-lived radioisotope 13N (t1/2 = 10 min) were used. First, the retention time of acetic acid in the packed ice beds was measured in the absence of HONO and used to calculate the linear partition coefficient, K(Lin,C,AA) and the dimensionless adsorption partition coefficient, K0(p,AA). The temperature dependence of KLin,C,AA was found to be K(Lin,C,AA)[m] = 1.9 x 10(-14) exp(7.4 x 10(3)/T) in good agreement with previous studies. No evidence of diffusion of acetic acid in the bulk of the ice matrix was observed. The co-adsorption measurements are well reproduced by a competitive Langmuir adsorption model and showed that the partition coefficients derived for single compounds can also be applied to this more complex system. Further, the measurements revealed that the bulk uptake of HONO is not influenced by the presence of acetic acid. The results are discussed in terms of partitioning of trace gases in snowpacks.