First preparation of isotopic nitrous oxide in synthetic air reference materials for underpinning measurements of δ15N-N2O, δ15N-N2OSP and δ18O-N2O

The precise measurement of the nitrous oxide (N₂O) isotope ratio in the atmosphere is required to understand global emission trends. Currently, however no internationally accepted reference materials for atmospheric amount fraction N₂O exist with characterised isotope ratio including uncertainties. Therefore, there is an urgent need for the development of reference materials to fill this traceability gap and meet the requirements to underpin global atmospheric measurements. Reference materials are typically traceable to internationally recognised stable isotope ratio scales: AIR-N₂ and VSMOW (Vienna Standard Mean Ocean Water) for δ¹⁵N and δ¹⁸O respectively. The linearly asymmetric structure of the N₂O molecule introduces an additional challenge in the measurement of the position specific δ¹⁵N^α (central N) and δ¹⁵N^β (terminal N). Reference materials are required to span the ranges of δ¹⁵N^α and δ¹⁵N^β expected across samples from global atmospheric measurements in addition to bulk δ¹⁵N.

We will present progress towards the development of atmospheric amount fraction N₂O in synthetic air reference materials with characterised isotope ratios suitable for calibration of optical isotope ratio spectrometers (OIRS). The reference materials span a wide range of δ values. The pure N₂O used to prepare the N₂O in synthetic air reference materials was prepared at Empa (Switzerland) and has traceability to the primary AIR-N₂ and VSMOW scales. The N₂O in synthetic air reference materials were certified for N₂O amount fraction and isotope ratio using OIRS against traceable reference materials. We will present on certification of N₂O isotope ratios based on two approaches: direct calibration of delta values; and calibration of isotopocule amount fractions. A comparison of the two approaches to certification was performed considering the sensitivities, uncertainty contributions, traceability and potential biases of each approach.

The sensitivities in the measured isotope ratios to commonly occurring synthetic air matrix impurities (e.g. trace N₂O), and the sensitivity of delta value certification to N₂O amount fractions have been assessed to provide a comprehensive uncertainty budget for the certification of N₂O in synthetic air reference materials using each approach.

Reproducibility within 0.3 ‰ has been demonstrated across five measurements over a 6-month period using delta and isotopocule amount fraction approaches. Standard measurement uncertainties (k=1) within 1.4 ‰ for δ¹⁵N^α, δ¹⁵N^β and 0.5 ‰ for δ¹⁵N, δ¹⁸O were achieved for certification based on a delta value approach. Standard measurement uncertainties (k=1) within 1.5 ‰ for δ¹⁵N^α, δ¹⁵N^β, δ¹⁵Nand 0.7 ‰ for δ¹⁸O were achieved for certification based on an isotopocule amount fraction approach.