The solubility of sulfur hexafluoride in water and seawater

Abstract

The concentration of sulfur hexafluoride (SF 6) 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. SF 6 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 SF 6 and CFC-12 (CF 2Cl 2) 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 SF 6 and CFC-12 measurements. The CFC-12 results were in good agreement with previous work, while substantial differences were found between these SF 6 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 SF 6 solubility function to be of the order of 2%. The results from this work should be useful in determining equilibrium concentrations for SF 6 in ocean observation and modeling studies.