Determination of experimental diffusion coefficients of acetic acid and formic acid vapours in air using a passive sampler

A passive sampling method has been developed for acetic acid and formic acid vapours in ambient air. The procedure involves collection of the vapours in a Palmes diffusion tube (7.1 cm long and 1.1 cm i.d.) and analysis by ion chromatography. Application of Fick's first law of diffusion allows calculation of the acid vapour concentrations. To validate the procedure, the tubes have been exposed to calculated or known concentrations of acetic acid and formic acid vapours, so that the diffusion coefficients of each acid can be calculated for comparison with theoretical values. Two procedures were used to generate acid vapour concentrations in the range 6–26 mg m −3 . Solutions containing calcium chloride and acetic acid or formic acid were placed in desiccators in which replicate diffusion tubes were exposed. The derived diffusion coefficients of acetic acid and formic acid were 0.110 ± 0.003 and 0.127 ± 0.005 cm 2 s −1 , respectively. An alternative procedure used a 43 dm 3 exposure chamber and acid permeation devices to produce the acid vapour concentrations. The experimentally derived diffusion coefficients obtained with this system were 0.133 ± 0.007 and 0.120 ± 0.008 cm 2 s −1 for acetic acid and formic acid vapours, respectively. The theoretical diffusion coefficients for acetic acid and formic acid vapours are 0.105 and 0.129 cm 2 s −1 , respectively.