Simultaneous determination of sulfur hexafluoride and three chlorofluorocarbons in water and air

doi:10.1016/S0304-4203(02)00015-4

Marine Chemistry

Volume 78, Issues 2–3, May 2002, Pages 137-148

https://doi.org/10.1016/S0304-4203(02)00015-4 Get rights and content

Abstract

We have developed an analytical technique for the simultaneous measurement of the four trace gases sulfur hexafluoride (SF₆) and the chlorofluorocarbons CCl₂F₂ (CFC-12), CCl₃F (CFC-11) and CCl₂FCClF₂ (CFC-113) in water and air. Water samples are flame sealed into 350-ml glass ampoules which allow storage and sampling in locations where field measurements are not practical. For analysis, these ampoules are stripped of dissolved gases after their stems are cracked in an enclosed chamber such that the headspace fraction in the ampoule is included in the measurement. The extracted gases are then trapped cryogenically and are separated on packed columns. CFC-11 and CFC-113 are measured on one electron capture detector (ECD), while SF₆ and CFC-12 are cryofocussed on a second trap and analyzed on a second ECD. Detection limits for seawater samples are about 0.015 fmol kg⁻¹ for SF₆, 0.010 pmol kg⁻¹ for CFC-12, 0.014 pmol kg⁻¹ for CFC-11, and 0.024 pmol kg⁻¹ for CFC-113. This analytical technique also allows for analysis of air samples with low concentrations or at low pressures. Results from a profile in the northeastern Pacific Ocean show that SF₆ partial pressure ages are consistent with those of CFC-11 and CFC-12 over the age range covered by this profile. From this, we infer that SF₆ is useful for the dating of recently ventilated waters, thus complementing the dating of older waters using CFCs. Earlier reports of the degradation of CFC-113 in oxygenated water are supported by our results for samples stored in ampoules.

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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Citation 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.
Transient tracer data (CFC-12 and SF₆) from three oceanographic field campaigns to the Mauritanian Upwelling area conducted during winter, spring and summer from 2005 to 2007 is presented. The transient tracers are used to constrain a possible solution to the transient time distribution (TTD) along 18°N and to quantify the mean ages in vertical sections perpendicular to the coast. We found that an Inverse Gaussian distribution where the ratio of the moments Δ and Γ equals 1.2 is a possible solution (Δ/Γ = 1.2) of the TTD. The transient tracers further show considerable under-saturation in the mixed layer during the winter and spring cruises that can only be maintained by mixing or upwelling by tracer-poor water from below the mixed layer. We use dissipation data from microstructure measurements and the tracer depth distribution to quantify the flux of tracers to the mixed layer by vertical diffusivity and wind data from the ship to quantify the air–sea flux. We then use the magnitude of the under-saturation in the mixed layer to estimate the advective upwelling velocity which is the balance the first two processes, in a steady state assumption. We find that the upwelling velocities range from less than 1 to 5.6 × 10^- 5 m s^- 1 (< 0.8–4.8 m d^- 1), with generally higher values close to the coast, but with comparable upwelling velocities during spring and winter. During the summer cruise the transient tracers were close to equilibrium with the atmosphere, suggesting no upwelling. We have shown the use of CFC-12 and SF₆ transient tracer data for calculating upwelling velocity, and found an overall uncertainty of roughly ± 50%.

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Simultaneous determination of sulfur hexafluoride and three chlorofluorocarbons in water and air

Abstract

Introduction

Section snippets

Gas separation and detection

Results and discussion

Apparent tracer ages

Concluding remarks

Acknowledgments

Deep-Sea Res., Part I

Deep-Sea Res.

Deep-Sea Res., Part I

Deep-Sea Res.

Mar. Chem.

Atmos. Environ.

Deep-Sea Res.

Stratospheric CO2 isotopic anomalies and SF6 and CFC tracer concentrations in the Arctic polar vortex

Geophys. Res. Lett.

A capillary-column chromatographic system for efficient chlorofluorocarbon measurement in ocean waters

J. Geophys. Res.

Use of chlorofluorocarbon (CCl3F and CCl2F2) as hydrologic tracers and age-dating tools: the alluvium and terrace system of Central Oklahoma

Water Resour. Res.

Dating young groundwater with sulfur hexafluoride: natural and anthropogenic sources of sulfur hexafluoride

Water Resour. Res.

Drier for field use in the determination of trace atmospheric gases

Anal. Chem.

Chlorofluoromethanes in the northeast Pacific Ocean: measured vertical distributions and application as transient tracers of upper mixing

J. Geophys. Res.

Chlorofluorocarbon-113 in the northeast Atlantic

J. Geophys. Res.

Natural CF4 and SF6 on Earth

Geophys. Res. Lett.

Determination of Persian Gulf Water transport and oxygen utilisation rates using SF6 as a novel transient tracer

Geophys. Res. Lett.

Stratospheric CO₂ isotopic anomalies and SF₆ and CFC tracer concentrations in the Arctic polar vortex

Use of chlorofluorocarbon (CCl₃F and CCl₂F₂) as hydrologic tracers and age-dating tools: the alluvium and terrace system of Central Oklahoma

Natural CF₄ and SF₆ on Earth

Determination of Persian Gulf Water transport and oxygen utilisation rates using SF₆ as a novel transient tracer