Gases at the extreme

The concentration of anthropogenic carbon dioxide in the oceans could hold many answers about the global climate system - but how can it be studied?

SINCE the Industrial Revolution, the world’s oceans have absorbed approximately 30% of all anthropogenic carbon dioxide 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 carbon dioxide at the pace at which it is being added to the atmosphere. 

Since anthropogenic carbon dioxide (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, sulphur hexafluoride (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.

Project ANDREX (Antarctic Deep Water Rates of EXport) is a joint project between scientists from four UK research centres: the University of East Anglia (UEA), the National Oceanography Centre Southampton (NOC), the British Antarctic Survey and the University of Manchester. In March, the team will embark on an expedition to Antarctica to research the role of the Southern Ocean in the global climate system. 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.

Linde Gases will supply the research team from the UEA’s School of Environmental Sciences with a cylinder containing a speciality calibration gas mixture with 30 parts per trillion (ppt) of sulphur hexafluoride (SF6) – the preparation and use of this mixture presents both Linde and the UAE with significant challenges.

The 30 ppt gas mixture will be used as a calibration gas by the ANDREX project to measure and analyse 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. 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 at 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.”

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 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 from the atmosphere.

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 of 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 100 ppt and even 50 ppt concentrations – 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 in 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.

 “This was a very challenging exercise, because of the extremely low concentration levels required and the temperatures in Antarctica where the mixture is to be used. We needed to take account of the customer’s unique analytical and operational requirements,” said Dr Cleaver.  “When filling, the cylinder preparation comprising of heat drying and evacuation, the cleanliness of the filling system and the purity of the component gases are critical to avoid contamination which could result in the final mixture being out of specification.”

To analyse and certify the very low SF6 level in this mixture, BOC used a gas chromatograph with a Poropak column and an Electron Capture Detector (ECD) due to its sensitivity for measuring halogenated species like SF6, down to ppt levels. The most significant challenge in accurately certifying the mixture was to overcome the lack of available reference standards at such low concentrations that this mixture could be compared against. However BOC routinely produce internal standards through volumetric dilution methods in accordance with ISO standard 6144: 2003 ‘Gas analysis – preparation of calibration gas mixtures – static volumetric method’.

The challenges in using the mixture for accurate calibration are also complex.

  The proper pressure regulator and gas supply equipment must be used and the correct purge and carrier instrumentation gases are required. Parts of the gas supply equipment that come into contact with the gas mixture must be free of contaminants that could de-sorb and enter the instrumentation causing background noise. They must also not adsorb the SF6 from the mixture; otherwise the concentration of SF6 will decay from the correct value in the cylinder to a lower value at the point of calibration and analysis.  Using an ultra high purity stainless steel regulator ensures that metal particles or other material impurities will not be added to the gas stream and the selection of correctly rated soft goods such as seals and seats ensures the regulator will perform even under sub-zero  temperatures, such as those found in Antarctica.  The appropriate high purity grades of purge and carrier instrumentation gases must be used through the calibration and measurement processes to avoid the introduction of oxygen and moisture that can interfere with the ECD instrumentation during the analysis.  Use of the correct high purity instrumentation gases also avoids unintended introduction of SF6 from sources other than the precise amount contained in the calibration gas mixture.

After selection of all the correct gases and materials, the whole system, once connected, must be purged through to eliminate contamination from naturally occurring atmospheric gases that can interfere with readings at ppt levels.  Finally, to maintain accurate results in the longer term, leak integrity of the gas supply system must be of a high standard to avoid inward contamination from naturally occurring atmospheric gases that could influence the measurement and calibration.  With the constant instability and vibration inherent with operating at sea, this means pipe work and connections of the highest standards and regular leak testing.

Operating temperature is always considered during the design, certification and supply of calibration gas mixtures. For example gas mixture components with low vapour pressures can condense within the cylinder at very low temperatures and this must be taken account of when preparing the mixture. This means that the maximum levels that low vapour pressure components can be filled within the cylinder are limited by their physical properties and the minimum temperature at which the cylinder will be used.  At the opposite climatic extreme, high ambient temperatures in tropical environments cause the gas pressure inside the cylinder to increase. In these circumstances, the initial fill pressure of the cylinder is reduced so that the inevitable pressure increase caused by the high ambient temperature does not affect the gas mixture or integrity of the gas cylinder.

“Working at sea presents a different set of challenges to those faced on land,” comments Peter Brown, “Firstly, the unstable working location – with shifting sea conditions, ship movement and continual engine vibration – means we need technically and physically robust equipment that can withstand such a sustained, difficult environment.”

“We rely on high specification and superior quality equipment – for both cylinders and flow regulators in our gas analyses. As the only spares available are those that we take with us or can be fashioned while we’re there, equipment breakdown can lead to a wasted trip and lost research opportunities.”
“Gas cylinders are often stored on deck on research ships. These could experience a multitude of temperature and climatic conditions, particularly in the Southern Ocean where the strength of the sun can be fierce during the day, while biting winds, freezing sea spray and sub-zero air temperatures are not far away,” says Brown.

“For Project ANDREX we’ll make use of a laboratory container – a standard shipping container that has been converted for chemical analysis, with workbenches, electricity, air conditioning and water connected – that enables our analyses to be conducted at approximately 20-25°C. Our standard gas cylinders will be securely mounted within the container, to minimise movement and to minimise the effects of any inclement weather outside to ship roll and pitch.

“Nitrogen cylinders will, however, be secured on the ship’s deck outside, where they will face the full force of local conditions. The regulators will be protected by thick plastic covers, but the gas lines and cylinders themselves will not. There is a constant risk of these freezing, either to each other, the container or the ship, thus making their movement about the ship for safe installation whilst at sea a more problematic exercise, overcome mainly by the ingenuity and experience of the crew and scientists on board.”

“Conventional calibration gas mixtures rarely go lower than 1 ppm,” says Harrison, “Being able to provide Project ANDREX with a mix as low as 30 ppt demonstrates our absolute cutting edge ability to handle, prepare and supply mixtures at such low concentrations”

“Supporting Project ANDREX is in line with one of Linde’s key corporate strategies, that is, reduced environmental impact” concludes Harrison, “This initiative is driven by the increasing stress being placed on the environment by a growing population and increasing consumerism. This awareness causes consumers as well as political decision-makers to prefer products and processes that minimise water and energy usage, and that have minimal releases to the environment.”