Automated vacuum analysis of sulphur hexafluoride in seawater: derivation of the atmospheric trend (1970–1993) and potential as a transient tracer

A fully-automated analysis system for the tracer gas sulphur hexafluoride (SF6) is described. The system could be readily adapted to analyse seawater samples of varying volume and a concentration range exceeding 7 orders of magnitude. Automation facilitated a high precision despite continual analysis for periods of 30 days, and the performance of the instrument over a two year period of intensive use is discussed. Background SF6 profiles from different oceans are compared and the surface concentrations utilised to obtain atmospheric concentrations. These are incorporated with other published data to derive a history for atmospheric SF6 concentration. The data suggest an initial exponential rise in atmospheric SF6 to 1979, followed by a linear increase to late 1993 at a rate of 5.5% p.a. With the sensitive analytical system and documented atmospheric history described in this paper, SF6 has potential application as a transient tracer of recently ventilated waters, as demonstrated by comparison with the analytical parameters of the chlorofluorocarbons.