A robust method for direct calibration of isotope ratios in gases against liquid/solid reference materials, including a laboratory comparison for δ13 C-CH4

Perspective: On the discontinuity between the scale realizations VPDB2020 and VPDB2006 - The underestimated role of measurement uncertainties?

Several recent publications have discussed the discontinuity between the δ13C scale realization VPDB2020 (new IAEA carbonate reference materials (RMs) IAEA-610/611/612) and the LSVEC-traceable realization named as VPDB2006 (organic RMs normalized to LSVEC in 2006, and USGS44 with its δ13C based on those RMs and LSVEC). This looks like an unsolvable puzzle. The question from practical users is as follows: do we want to be correct, that is, use VPDB2020, or be compatible with the results obtained in previous years, that is, use VPDB2006? In metrology, the significance of any discrepancy is considered in terms of the associated combined uncertainty. Uncertainties estimated for VPDB2020 according to the metrological approaches are much lower than the discontinuity. However, the situation for VPDB2006 is unclear. By reviewing publications related to LSVEC and LSVEC-normalized organic RMs of 2006, several missing and/or underestimated uncertainty components can be identified. When these missing or underestimated uncertainties are taken together, the discontinuity at negative δ13C appears to be the same as or similar to the combined underestimated uncertainties for VPDB2006.

Isotopically characterised N2 O reference materials for use as community standards

RATIONALE

Information on the isotopic composition of nitrous oxide (N2 O) at natural abundance supports the identification of its source and sink processes. In recent years, a number of mass spectrometric and laser spectroscopic techniques have been developed and are increasingly used by the research community. Advances in this active research area, however, critically depend on the availability of suitable N2 O isotope Reference Materials (RMs).

METHODS

Within the project Metrology for Stable Isotope Reference Standards (SIRS), seven pure N2 O isotope RMs have been developed and their 15 N/14 N, 18 O/16 O, 17 O/16 O ratios and 15 N site preference (SP) have been analysed by specialised laboratories against isotope reference materials. A particular focus was on the 15 N site-specific isotopic composition, as this measurand is both highly diagnostic for source appointment and challenging to analyse and link to existing scales.

RESULTS

The established N2 O isotope RMs offer a wide spread in delta (δ) values: δ15 N: 0 to +104‰, δ18 O: +39 to +155‰, and δ15 NSP : -4 to +20‰. Conversion and uncertainty propagation of δ15 N and δ18 O to the Air-N2 and VSMOW scales, respectively, provides robust estimates for δ15 N(N2 O) and δ18 O(N2 O), with overall uncertainties of about 0.05‰ and 0.15‰, respectively. For δ15 NSP , an offset of >1.5‰ compared with earlier calibration approaches was detected, which should be revisited in the future.

CONCLUSIONS

A set of seven N2 O isotope RMs anchored to the international isotope-ratio scales was developed that will promote the implementation of the recommended two-point calibration approach. Particularly, the availability of δ17 O data for N2 O RMs is expected to improve data quality/correction algorithms with respect to δ15 NSP and δ15 N analysis by mass spectrometry. We anticipate that the N2 O isotope RMs will enhance compatibility between laboratories and accelerate research progress in this emerging field.

Atmospheric methane and nitrous oxide: challenges alongthe path to Net Zero

The causes of methane's renewed rise since 2007, accelerated growth from 2014 and record rise in 2020, concurrent with an isotopic shift to values more depleted in ¹³C, remain poorly understood. This rise is the dominant departure from greenhouse gas scenarios that limit global heating to less than 2°C. Thus a comprehensive understanding of methane sources and sinks, their trends and inter-annual variations are becoming more urgent. Efforts to quantify both sources and sinks and understand latitudinal and seasonal variations will improve our understanding of the methane cycle and its anthropogenic component. Nationally declared emissions inventories under the UN Framework Convention on Climate Change (UNFCCC) and promised contributions to emissions reductions under the UNFCCC Paris Agreement need to be verified independently by top-down observation. Furthermore, indirect effects on natural emissions, such as changes in aquatic ecosystems, also need to be quantified. Nitrous oxide is even more poorly understood. Despite this, options for mitigating methane and nitrous oxide emissions are improving rapidly, both in cutting emissions from gas, oil and coal extraction and use, and also from agricultural and waste sources. Reductions in methane and nitrous oxide emission are arguably among the most attractive immediate options for climate action. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.