Trifluoromethyl Sulfur Pentafluoride (SF₅CF₃) and Sulfur Hexafluoride (SF₆) from Dome Concordia

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Investigators

W. T. Sturges,¹ T. J. Wallington,² M. D. Hurley,² K. P. Shine,³ K. Sihra,³ A. Engel,⁴ D. E. Oram,¹ S. A. Penkett,¹ R. Mulvaney,⁵ and C. A. M. Brenninkmeijer⁶

¹School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom
²Ford Motor Company, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053, U.S.A.
³Department of Meteorology, University of Reading, Reading RG6 6BB, United Kingdom
⁴Institute for Meteorology and Geophysics, Johann Wolfgang Goethe University of Frankfurt, D-60325 Frankfurt, Germany
⁵British Antarctic Survey, Natural Environmental Research Council, Cambridge CB3 0ET, United Kingdom
⁶Atmospheric Chemistry Division, Max Planck Institute for Chemistry, D-55060 Mainz, Germany

Period of Record

1965-1999

Methods

The sampling and analytical methods are described more fully in Sturges et al. (2000). In summary, air samples were pumped from consolidated deep snow (firn) at Dome Concordia (eastern Antarctica) in December 1998 and January 1999, from the surface to a depth of approximately 100 m. Air samples were analyzed with a gas chromatograph - mass spectrometer, with a detection limit of about 0.001 parts per trillion (ppt). A diffusive transport model was used to calculate the age of samples as a function of depth. Measurements of SF₆ were used to determine the mean age of the firn air by comparison with extrapolated measurements from Cape Grim, Tasmania combined with estimates from industrial emissions (Maiss and Brenninkmeijer 1998, adapted by Sturges et al. 2000). Dates for SF₅CF₃ are different than for SF₆ due to the lower diffusivity of SF₅CF₃: the SF₆ ages were multiplied by the ratio of the free-air diffusion coefficient of SF₅CF₃ to that of SF₆ (1.18). Free-air diffusion coefficients were determined by a semi-empirical formula based on molecular volumes (Fuller et al. 1966). Note that mean ages represent a very wide distribution of probable ages spanning many years, with an increasing spread of ages at increasing depth.

Trends

The measured concentration of SF₅CF₃ increased from zero in 1965-1966 to about 0.12 ppt in 1999, with a current growth rate of about 0.008 ppt per year (about 6% per year). Given the similarity of the growth curves of SF₅CF₃ and SF₆ (which increased from 0.18 ppt in 1970 to 4.0 ppt in 1999), Sturges et al. (2000) speculate that the former may originate as a breakdown product of the latter in high-voltage equipment. While the current radiative forcing of SF₅CF₃ may be minor, the high growth rate and long atmospheric residence time suggest that the greenhouse significance of this gas could increase markedly in the future. Conversely, SF₅CF₃ appears not to have any natural sources, so control might be feasible, once the sources are identified.

References

• Fuller, E.N., P.D. Schettler, and J.C. Giddings. 1966. A new method for prediction of binary gas-phase diffusion coefficients. Industrial Engineering and Chemistry 58:19- 27.
• Maiss, M., and C.A.M. Brenninkmeijer. 1998. Atmospheric SF₆: Trends, sources, and prospects. Environmental Sciences and Technology 32:3077-3086.
• Sturges, W.T., T.J. Wallington, M.D. Hurley, K.P. Shine, K. Sihra, A. Engel, D.E. Oram, S.A. Penkett, R. Mulvaney, and C.A.M. Brenninkmeijer 2000. A potent greenhouse gas identified in the atmosphere: SF₅CF₃. Science 289:611-613.