UPTAKE OF GAS-PHASE HALOGENATED ACETIC-ACID MOLECULES BY WATER SURFACES

Mass accommodation coefficients (alpha) have been measured as a function of temperature in the range 260-290 K for trifluoro-, trichloro-, chlorodifluoro-, chloro-, and dichloroacetic acids. As part of the study, the gas-phase diffusion coefficients in argon have also been determined for these species. The experimental method employs a monodispersed train of fast moving droplets (120-250 mum in diameter) in a low-pressure flow reactor. Droplet-trace gas interaction times are on the order of milliseconds. The mass accommodation coefficients for all the species studied are on the order of 0.1 at 273 K. The mass accommodation coefficients show a negative temperature dependence which can be expressed in terms of an observed Gibbs free energy DELTAG(double dagger)obs = DELTAH(obs) - TDELTAS(obs) at alpha/(1 - alpha) = exp(- DELTAG(double dagger)obs/RT). The magnitudes of both DELTAH(obs) and DELTAS(obs) correlate and increase in the sequence trifluoro- < chlorodifluoro- < (dichloro-, chloro-) < trichloroacetic acid. The results are discussed in terms of a previously described uptake model. The atmospheric significance of the results is examined.