Navigating Challenges for Isotope Ratio Measurements of Greenhouse Gases

Abstract

A driving factor of global climate change is the increased emissions of greenhouse gases, mainly carbon dioxide and methane. These emissions may originate naturally from atmospheric exchanges with the oceans, forest fires, or microbial activity, or they can also be produced by human activities like the burning of fossil fuels. In 2016, governments from around the world signed the Paris Agreement to support a reduction in anthropogenic greenhouse gas emissions. Before we can curb greenhouse gas emissions, we must first ascertain the source of these emissions - a challenging task due to the complexity of greenhouse gas sources and sinks in the global carbon cycle. Isotope ratio analysis is used as a tool to distinguish between the different sources of greenhouse gases. This technique is based on the small, but measurable, differences in isotopic signatures between the various sources of greenhouse gases. By evaluating the long-term trends of carbon isotope ratios of atmospheric carbon dioxide and methane, we can monitor the changes in anthropogenic emissions and contribute to science-based policy and decision-making. The small differences in isotope ratios between natural and anthropogenic sources of greenhouse gases require measurements of the highest precision, and the World Meteorological Organization (WMO) has set rigorous inter-laboratory data comparability goals. Unfortunately, several technical obstacles prevent us from reaching these measurement goals. One of the biggest impediments is the disagreement over the primary reference materials that set the measurement scales. Carbon isotope ratio measurements performed at the National Research Council of Canada (NRC) demonstrated that significantly different results for the same carbon dioxide sample can be obtained depending on which combination of calibration standards are used. This bias has since been independently confirmed by a multi-laboratory international comparison of carbon dioxide samples. As a result of these findings, the scientific community has recently acknowledged that carbon isotope measurements are indeed reported on two distinct measurement scales, namely VPDB and VPDB-LSVEC, depending on the calibration standards used for the measurements. Measurements with unclear traceability to either of these scales can often be found in the literature which highlights the widespread lack of attention paid to measurement scales. These measurement problems also extend to isotope measurements in methane. Not only is the difference between the measurements most pronounced for methane, there is a lack of available methane calibration standards. A joint task group of the CCQM Gas Analysis and Isotope Ratio Working Groups has engaged with the methane isotope measurement community to identify the needs and challenges associated with these measurements. The production of high-quality methane reference materials has been identified as a priority, and laboratories worldwide, including the NRC, have moved towards addressing this gap. As methane calibration standards are being developed, many of the challenges that have plagued the isotopic community could be avoided if a single measurement scale is adopted.