FY 2002

The solubility of sulfur hexafluoride in water and seawater

Bullister, J.L., D.P. Wisegarver, and F.A. Menzia

Deep-Sea Res. Pt. I, 49(1), 175–187, doi: 10.1016/S0967-0637(01)00051-6 (2002)

The concentration of sulfur hexafluoride (SF₆) in the atmosphere has been rapidly increasing during the past several decades. This long-lived compound enters the surface ocean by air-sea gas exchange and is potentially a very useful transient tracer for studying ocean circulation and mixing. SF₆ has also been directly injected into the ocean at a minimal number of locations as a part of deliberate tracer release experiments to study gas exchange and sub-surface mixing rates. In this study, laboratory measurements of the solubility of SF 6 in water and seawater were made over the temperature range of ~-0.5°C to 40°C. Volumes of water and seawater held at constant temperature in glass chambers were equilibrated with a gas mixture containing SF₆ and CFC-12 (CF₂Cl₂) at parts-per-trillion levels in nitrogen. Small volume water samples were analyzed by electron capture gas chromatography. Using the method of least squares, equations previously used in describing gas solubility as a function of temperature and salinity were fit to the SF₆ and CFC-12 measurements. The CFC-12 results were in good agreement with previous work, while substantial differences were found between these SF₆ results and those reported in earlier studies. The mean error for the analytical measurements is estimated to be ~0.5%. Based on errors in the fits and the analytical errors, we estimate the overall accuracy of the SF₆ solubility function to be of the order of 2%. The results from this work should be useful in determining equilibrium concentrations for SF₆ in ocean observation and modeling studies.