New reference materials for measuring the potent greenhouse gas methane
Challenge
Methane has a global warming potential 28 times more than carbon dioxide over a 100-year time scale. Since 2008, levels of methane have risen in the atmosphere and identifying the origins of these emissions is vital for reducing this.
Methane emissions come from natural sources, such as geological processes or decomposition of organic matter in wetlands, and from human activities, including leaks from fossil fuels or emissions from agriculture.
The most sensitive way to distinguish between these sources is via isotopic analysis of the gas. Methane is composed of carbon and hydrogen (CH4). Carbon has two stable isotopes, carbon-12 (12C) and carbon-13 (13C). Hydrogen has two stable isotopes, hydrogen-1(1H) and hydrogen-2 or deuterium (2H). Different emissions sources produce methane with distinct proportions of these isotopes – a so-called “isotopic signature”. Atmospheric monitoring networks can traceably link measurements of these to international scales such as the Vienna Pee Dee Belemnite (VPDB) for carbon isotopes and Vienna Standard Mean Ocean Water (VSMOW) for hydrogen.
However, at the start of 2020 there were no available isotopically characterised gas reference materials in sufficient quantities to meet the demand for calibrating field instruments measuring methane isotope ratios at atmospheric monitoring network sites. As a result, variations in standards and calibration protocols led to discrepancies of up to 0.5 per-mille (‰) for δ13C-CH4 and 13 ‰ for δ2H-CH4.
Solution
Building on the EMPIR project SIRS, the STELLAR project produced reference materials made up to a specific amount fraction and with varying isotopic signatures for methane both as a pure gas and as dilutions in synthetic air, depending on the instrument requirements.
For the pure gas, methane from a fossil source and a biogenic source were obtained. These where then used to prepare synthetic air reference materials close to the levels of methane found in ambient air, analysed by Isotope Ratio Mass Spectrometry (IRMS), and linked to the World Meteorological Organization scale WMOCH4- X2004A for amount fraction.
To link the methane standards to the VPDB and VSMOW isotope ratio scales a sample of the pure gas was measured by IRMS at BCG-IsoLab at the Max Planck Institute for Biogeochemistry. These certified synthetic air reference materials were the first ever isotopically-characterised methane reference materials at ambient amount fraction (1.85 μmol mol-1), with isotopic uncertainties better than the required uncertainties of 0.2 ‰ for δ13C-CH4 and 5 ‰ for δ 2H-CH4.
Impact
Since 2012, the UK’s Deriving Emissions linked to Climate Change (DECC) network has monitored atmospheric levels of nitrous oxide, carbon monoxide and dioxide, hydrogen, sulfur hexafluoride, halogenated hydrocarbons and methane. These data are used to estimate national emissions of greenhouse gases and are compared against the UK national atmospheric emissions inventory to verify the UK’s emissions. The UK is one of five countries in the world that verifies their emissions inventories for these GHGs.
Funded by the UK government, it has four monitoring sites in the UK and one in Ireland, located to measure greenhouse gases flowing from Europe and the outflow of these gases from the UK. Measuring instruments are situated at different heights on tall (90m – 314m) telecommunications towers and provide information on emissions over a wide area, allowing observations of sources and sinks from different spatial footprints.
The DECC site at Heathfield is now using the methane gas standards to calibrate a preconcentrator instrument, used to continuously measure the isotope ratios in atmospheric methane. These standards, provided by NPL, the UK’s NMI, allow calibration of this newly-developed, field-deployable, optical isotope ratio spectroscopy (OIRS) instrument.
The work performed in the STELLAR project will be important for the UK and other nations to verify the emissions reporting of this important greenhouse gas. In turn this will provide a greater knowledge of its origins and reasons for its increase, and help the UK and Europe meet the aim of becoming climate neutral by 2050.
