Solubility of acetic acid and trifluoroacetic acid in low-temperature (207-245 k) sulfuric acid solutions: implications for the upper troposphere and lower stratosphere

The solubility of gas-phase acetic acid (CH(3)COOH, HAc) and trifluoroacetic acid (CF(3)COOH, TFA) in aqueous sulfuric acid solutions was measured in a Knudsen cell reactor over ranges of temperature (207-245 K) and acid composition (40-75 wt %, H(2)SO(4)). For both HAc and TFA, the effective Henry's law coefficient, H*, is inversely dependent on temperature. Measured values of H* for TFA range from 1.7 × 10(3) M atm(-1) in 75.0 wt % H(2)SO(4) at 242.5 K to 3.6 × 10(8) M atm(-1) in 40.7 wt % H(2)SO(4) at 207.8 K. Measured values of H* for HAc range from 2.2 × 10(5) M atm(-1) in 57.8 wt % H(2)SO(4) at 245.0 K to 3.8 × 10(8) M atm(-1) in 74.4 wt % H(2)SO(4) at 219.6 K. The solubility of HAc increases with increasing H(2)SO(4) concentration and is higher in strong sulfuric acid than in water. In contrast, the solubility of TFA decreases with increasing sulfuric acid concentration. The equilibrium concentration of HAc in UT/LS aerosol particles is estimated from our measurements and is found to be up to several orders of magnitude higher than those determined for common alcohols and small carbonyl compounds. On the basis of our measured solubility, we determine that HAc in the upper troposphere undergoes aerosol partitioning, though the role of H(2)SO(4) aerosol particles as a sink for HAc in the upper troposphere and lower stratosphere will only be discernible under high atmospheric sulfate perturbations.