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May-2010

Monitoring carbon dioxide

Linde Gases is to supply the University of East Anglia (UEA) with a calibration gas at an exceptionally low concentration as part of Project ANDREX.

Stephen Harrison, Linde Gas
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Article Summary
Linde Gases is to supply the University of East Anglia (UEA) with a calibration gas at an exceptionally low concentration as part of Project ANDREX – an expedition to Antarctica to research the role of the Southern Ocean in the global climate system. The preparation and use of this mixture presents both Linde and the UAE with significant challenges.

A cylinder containing a speciality calibration gas mixture with 30 parts per trillion (ppt) of sulphur hexafluoride (SF6) will accompany the research team from the university’s School of Environmental Sciences on its voyage to the Weddell Sea in Antarctica in March 2010.

ANDREX (Antarctic Deep Water Rates of Export) is a joint project between scientists from four UK research centres: the UEA, the National Oceanography Centre Southampton (NOC), the British Antarctic Survey and the University of Manchester. Some measurements will also be conducted by scientists at the Woods Hole Oceanographic Institute in the USA. ANDREX is funded by the UK Antarctic Funding Initiative, with a contribution from CARBOOCEAN.

Oceanic uptake of carbon dioxide
Since the Industrial Revolution, the world’s oceans have absorbed approximately 30 percent of all anthropogenic or human derived carbon dioxide (CO2) emitted into the atmosphere. As global emissions have increased, oceanic uptake has also grown in magnitude, but concern exists about the oceans’ continued ability to absorb CO2 at the pace at which it is being added to the atmosphere. 

Since anthropogenic CO2 cannot be measured directly, tracer chemicals are used to help assess the rate of absorption. SF6 acts as a proxy for the absorption of anthropogenic CO2 in the sea water over the last 30 years. The 30 ppt gas mixture prepared by Linde will be used as a calibration gas for determining SF6 concentrations in seawater samples. At 30 ppt, the mix comprises miniscule concentrations of SF6.

“To draw an analogy, you could say the concentration of SF6 is as tiny as one second of time when compared to a period of 1000 years,” says Stephen Harrison, Head of Specialty Gases and Specialty Equipment, Linde.  “We’re very proud to be able to support the ANDREX project. As the global community works towards reducing CO2 emissions, there is growing prioritisation in monitoring and quantifying the impact it has on the environment. Accuracy and reliability in measurement has become critical.”

Dr Peter Brown of the Laboratory for Global Marine and Atmospheric Chemistry at the UEA adds: “It’s critical to improve our understanding of the oceans’ ability to absorb anthropogenic CO2 from the atmosphere and to store it over long timescales. ANDREX will make a valuable contribution to this.”

Key region for uptake
The Southern Ocean is a key region for the uptake and long term storage of gases from the atmosphere. These processes are thought to be especially strong in the Weddell Sea, where wintertime heat loss and sea ice formation increase the density of the surface water, which sinks to the bottom taking with it gases – such as CO2 — absorbed from the atmosphere, with a unique time signature imprint.

This effectively “removes” the gases from the atmosphere, transporting them away from the surface on millennial timescales. In the interior ocean, the concentrations of these gases – or tracers – can be used as a record of the behaviour of both the oceans and the atmosphere over the last 200 hundred years and how this is changing over time. Information regarding when a water mass was last at the surface is critical to understand and assess the role of the oceans in the global climate system and their ongoing capacity to absorb human-derived constituents such as CO2 from the atmosphere.

As anthropogenic CO2 cannot be measured first-hand, a variety of tracer chemicals can instead be used to estimate the water mass age and thus act as a proxy for the absorption of human-derived CO2. Of these, SF6 is increasingly being used to give information about air-to-sea gas transfer processes over the last 30 years, as chlorofluorocarbons (CFCs) – the chemicals historically used in this respect have become less useful over time. The ANDREX project will measure and analyse for SF6 and compare the results to previous CFC measurements obtained during the 1990s, providing valuable information on the absorption of human-derived CO2 in this region.

Challenges to consider in production and supply
Brown approached Linde in September 2009 to produce the calibration gas mixture and the request was handed over to Linde’s UK business, BOC, where a team of technical experts working for Dr Kevin Cleaver, Manager, Technical Services, got to work on the challenge.

“In his original request, Peter Brown gave us a range of levels with the lowest being 30 ppt,” relates Cleaver. “So he was delighted to be told that Linde had previously implemented and supplied concentrations of 100ppt to a US-based environmental consulting firm and even 50 ppt concentrations to a Canadian laboratory and could, in fact, go down to 10 ppt.”

Dr Cleaver’s team evaluated the request and in partnership with the production site’s technical experts, produced a specific set of instructions to accurately fill and analyse the mixture. When reviewing any mixture at such low concentrations BOC routinely evaluates what cylinder preparation is required, and the cylinder and valve materials, to ensure that they are compatible with the final mixture. The UK production site, Immingham, began by producing a low concentration gas mix at 5 parts per million (ppm) and carefully, so as to avoid contamination, diluted this mix down in stages to the required specification.
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