The Henryʹs law constant of trifluoroacetic acid and its partitioning into liquid water in the atmosphere

The Henryʹs law constant of trifluoroacetic acid [KH′ (mol kg−1 atm−1), for the equilibrium CF3COOH(g) CF3000H(aq)] has been determined from measured partial pressure over aqueous solutions at 278.15, 298.15 and 308.15 K, and is given by: ln(KH′) = 9.099 − 9.328 × 103(1/Tr − 1/T) where T (K) is temperature and Tr is equal to 298.15 K. Accuracy is estimated to be ±25% in KH′, partly limited by uncertainties in the pKa of the acid (0.47 at all temperatures, from literature data). Trifluoroacetic acid in the atmosphere will partition entirely into fog and cloud water (0.05–1.0 g H2O m−3), but can reside in both phases for the liquid water contents typical of aerosols (10−5 − 10−4 g H2O mt3) depending on aerosol pH and to some extent on composition.