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

Mass accommodation coefficients (alpha) have been measured as a functi on of temperature in the range 260-290 K for trifluoro-, trichloro-, c hlorodifluoro-, chloro-, and dichloroacetic acids. As part of the stud y, the gas-phase diffusion coefficients in argon have also been determ ined for these species. The experimental method employs a monodisperse d train of fast moving droplets (120-250 mum in diameter) in a low-pre ssure flow reactor. Droplet-trace gas interaction times are on the ord er of milliseconds. The mass accommodation coefficients for all the sp ecies studied are on the order of 0.1 at 273 K. The mass accommodation coefficients show a negative temperature dependence which can be expr essed in terms of an observed Gibbs free energy DELTAG(double dagger)o bs = DELTAH(obs) - TDELTAS(obs) at alpha/(1 - alpha) = exp(- DELTAG(do uble 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 discuss ed in terms of a previously described uptake model. The atmospheric si gnificance of the results is examined.