The Henry's law constant of trifluoroacetic acid and its partitioning into liquid water in the atmosphere

The Henry's law constant of trifluoroacetic acid [K H′ (mol kg −1 atm −1 ), for the equilibrium CF 3 COOH (g) ⇔ CF 3 000H (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(K H′ ) = 9.099 − 9.328 × 10 3 (1/T r − 1/T) where T (K) is temperature and T r is equal to 298.15 K. Accuracy is estimated to be ±25% in K H′ , partly limited by uncertainties in the pK a 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 H 2 O m −3 ), but can reside in both phases for the liquid water contents typical of aerosols (10 −5 − 10 −4 g H 2 O m t3 ) depending on aerosol pH and to some extent on composition.