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An EMPIR project has delivered a range of new reference materials for measuring N₂O and CO₂ – gases that are potent drivers of climate change

The main drivers of climate change are atmospheric greenhouse gases (GHGs) such as carbon dioxide (CO₂) and nitrous oxide (N₂O). These arise from both natural and man-made sources, and it is essential to discriminate between these to enable governments to develop accurate emissions inventories, climate change models and to comply with environmental legislation.

Both carbon and nitrogen have isotopes that differ in the number of neutrons in their nuclei. The ratio of the different isotopes present in the atmosphere can help trace the origin of these gases, including the burning of fossil fuels or the use of fertilisers in agriculture.

Accurate measurement of isotopic ratios requires robust reference materials that are accessible to environmental monitoring stations.

This includes ‘ambient air reference materials’ with known trace amounts of the target and accompanying greenhouse gases, required to obtain an accurate reading when calibrating analysers and ‘zero’ gas materials with the same composition as air but without the target substance. Both gases, must also have trace amounts of the accompanying greenhouse gases and the main air constituents present - as even small deviations from the target composition will affect the reading of the analysers.

However, existing materials were expensive to produce, not readily available and often had high uncertainties and - for N₂O - available commercial gases only had limited isotopic variability.

This lack was addressed by the recently completed EMPIR project Metrology for stable isotope reference standards (16ENV06, SIRS).

During its lifetime project results included:

• The development of seven N₂O isotope references materials and ambient amount fraction N₂O isotope ratio reference materials, and three CO₂ reference materials anchored to international isotope ratio scales.
• The characterisation and validation of different commercial and research-grade laser spectrometers for N₂O isotope analysis by project partner EMPA. The published results will help more accurate isotope quantification with these types of instruments. The paper drew much scientific interest and was named one of the journal’s ‘research highlights’.
• The development of an analyser for measurement of CO₂ isotopes by VTT MIKES, Finland’s National Measurement Institute (NMI) and the characterisation of a spectroscopy system for CO₂ analysis by PTB, Germany’s NMI. Both instruments are capable of field deployment to areas where no monitoring stations exist - or at the stations themselves to back up their own monitoring results.
• A new range of stable isotope ratio reference gases for atmospheric monitoring analysis: Synthetic air with the same composition as natural air without CO₂ produced at NPL, the UK’s NMI,  and CO₂ in air produced at NPL, INRiM – the Italian NMI and TUBITAK – the Turkish NMI.. These materials allow the calibration of detection instruments and, in addition, contain argon which reduces measurement artefacts such as ‘peak-broadening’ when measuring ratios spectroscopically.

Members of the project consortium are also taking part in the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology (CCQM) inter-comparison CCQM-P204. This started in 2020 and is the CCQM’s first inter-comparison aimed at evaluating the participating laboratories’ capability in assigning isotope ratios in pure CO₂ gas samples. The results of this comparison will go out to environmental monitoring stations and will be supplied to the World Meteorological Organization (WMO).
The new reference materials are already being utilised by laboratories around Europe and are being supplied to monitoring stations. Work on improved CO₂ measurements – and the production of the first ever methane reference materials – is continuing in the current EMPIR project Stable isotope metrology to enable climate action and regulation (19ENV05, STELLAR).

This project has developed a new infrastructure including methods and instrumentation to underpin measurements of stable isotopes of carbon dioxide and nitrous oxide, two major greenhouse gases, which enable their origin to be identified. This will be instrumental in providing more accurate, comparable data to separate various man-made sources of greenhouse gas emissions from each other, as well as from natural sources. This will enable governments to develop accurate emissions inventories and models to comply with legislation and inform new policy and better abatement strategies to help prevent dangerous climate change.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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