A transient model of mass transfer and kinetics in a passive vapor sampler

A transient model of vapor contaminant diffusion and homogeneous reaction in a two-compartment passive sampler was formulated. The mathematical analysis considered finite reaction kinetics, bulk air boundary-layer effects, and the solubility of the vapor contaminant in the liquid absorbing/reacting medium. The model was evaluated using experimentally measured Cl2 uptake rates in a series of samplers of different dimensions containing a 0.1% aqueous sulfamic acid solution in the absorbing medium chamber. The theory predicts accurately the effects of sampler diameter, stagnant air chamber path length, and sampler orientation with respect to the flowing air on Cl2 uptake, with an average error of less than 10%. Chlorine uptake in a 33-mm diameter sampler with a 10.8-mm path length was found to be essentially independent of wind direction and varied by approximately 23% for wind speeds ranging from 5.1 to 203 cm s-1 (10-400 ft min-1). Calculations show that the sampler is sensitive to wind speed partly because of the Cl2 hydrolysis reaction, which produces HOCl and drops the vapor phase concentration of chlorine to near zero at the stagnant air/absorbing medium chamber interface, regardless of the rate of homogeneous reaction between HOCl and sulfamate anions. If the solubility of the vapor contaminant in the liquid absorbing medium were to obey Henry's Law (no hydrolysis reaction) and the homogeneous trapping reaction were sufficiently slow, then model calculations show that contaminant uptake could be controlled solely by reaction kinetics, although the sampling rates would be lower than those observed and predicted for the Cl2 case.