Simultaneous determination of sulfur hexafluoride and three chlorofluorocarbons in water and air
Introduction
For the past two decades, measurements of the dissolved atmospheric chlorofluorocarbon (CFC) transient tracers, CCl2F2 (CFC-12), CCl3F (CFC-11), and to a lesser extent CCl2FCClF2 (CFC-113), have helped elucidate the subsurface dynamics and circulation of the oceans Gammon et al., 1982, Weiss et al., 1985, Doney and Bullister, 1992, Wisegarver and Gammon, 1988 and of deep lakes (Weiss et al., 1991). The transient histories of these gases in the atmosphere, and hence in surface water, have provided a basis for dating water masses and tracing their origins. However, since CFC production and release have now been restricted under the Montreal Protocol and its subsequent amendments, tropospheric CFC concentrations are no longer rising steeply and some have begun to decline (Prinn et al., 2000). This has introduced ambiguity in determining the apparent ages of newly ventilated water masses. Recently, the use of dissolved atmospheric sulfur hexafluoride (SF6) as a transient tracer has been introduced (Law et al., 1994). This predominantly anthropogenic gas has a relatively short, well-known tropospheric concentration history Maiss et al., 1996, Maiss and Brenninkmeijer, 1998, Maiss and Brenninkmeijer, 2000 which shows a roughly quadratic increase over the last two decades to a current northern hemispheric value of about 5.0 ppt (dry gas mole fraction in parts-per-trillion, or parts in 1012). Owing to its very long atmospheric lifetime of 800–3200 years Morris et al., 1995, Ravishankara et al., 1993, the tropospheric concentration of SF6 is expected to continue rising despite first signs of a decreasing growth rate (Maiss and Brenninkmeijer, 2000). Its tropospheric history, its inertness in water, and the feasibility of high-precision measurements at low concentration have made SF6 a favorable time-dependent tracer for recently ventilated water masses. Combining SF6 and CFC measurements allows the determination of apparent ages ranging from about the 1950s to the present, and this is likely to remain the case for several decades to come.
To date, oceanographic and limnological studies combining SF6 and CFC data have been sparse Law and Watson, 2001, Tanhua et al., 2002 and to our knowledge the analytical technique presented here is the first allowing simultaneous measurements of these multiple tracers from individual water samples. This method described below is optimized to yield the highest measurement precisions for SF6 and the three listed CFCs on limited numbers of water samples collected in remote areas with subsequent analysis in the laboratory. The method was developed without regard to analysis time, but modifications are also discussed for those applications where analysis time is also a constraint.
Section snippets
Gas separation and detection
The measurements are made using a purge-and-trap technique and electron capture detector (ECD) gas chromatography. The flow scheme is shown in Fig. 1, where the valves are numbered V1 to V9. Sample Traps A and B and all separation columns are made of 1/8 in. outer diameter (OD) stainless steel (SS) tubing and are packed with 80–100 mesh absorbents held in place by glass wool. Ultra-high purity nitrogen (N2) is used as the stripping and carrier gas. To remove SF6 and CFC impurities in this gas,
Results and discussion
To test the large-volume ampoule sampling technique and to explore the limitations of the analytical system, we have measured a set of samples which covers a large concentration range for all gases. These samples were taken aboard the R/V Atlantis at the Ocean Drilling Program (ODP) Site 892 (44°40.5′N, 125°7.9′W) on the Oregon continental margin in the northeastern Pacific Ocean in June 1998. The ampoule samples were collected from 10-l Niskin sampling bottles for which standard precautions
Apparent tracer ages
One of the primary objectives of aqueous transient trace gas measurements is to determine the “age” of a water parcel, commonly defined as the time elapsed since this parcel has left the mixed surface layer and thus lost direct contact with the atmosphere. Because there is subsurface mixing, the estimation of the true age of a mixed water parcel is not straightforward and in general can only be computed with the help of a mixing model. Transient anthropogenic tracers can only help to estimate
Concluding remarks
Our analytical technique allows for the simultaneous determination of four trace gases from a single sample of water or air. Compared to using two separate instrumental analyses with two independent samples, our technique is advantageous in that the water sampling effort is reduced. This can also improve the sample integrity of subsequent samples drawn from the same Niskin bottle. Fewer ampoules allow the delay between filling and sealing to be minimized, thus reducing the chance of
Acknowledgments
We are indebted to many colleagues for their help. Arnold Krause manufactured the ampoule cracking device, Christina Harth prepared the primary standards, and Peter Salameh developed the software for the chromatographic analyses. Miriam Kastner provided the opportunity for participation in the R/V Atlantis expedition, and Gregory Michalski and Joshua Plant carefully assisted with the shipboard ampoule sampling. Discussion with Stephen Walker has been very helpful in interpreting tracer age
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2015, Journal of Marine SystemsCitation Excerpt :The procedure to measure the samples in the lab is based on purge-and-trap followed by chromatographic separation and detection on an electron capture detector (ECD), with the process similar to the one described by Vollmer and Weiss (2002). The ampoules were placed in an ampoule cracking device—the cracker—that allows for quantitative measurements of all the tracer in the ampoules including the headspace (Vollmer and Weiss, 2002). This is necessary since most of the tracer will be in the headspace of the ampoule due to the low solubility of the tracer.