First investigation and absolute calibration of clumped isotopes in N O by mid-infrared laser spectroscopy.

Rationale: Unravelling the biogeochemical cycle of the potent greenhouse gas nitrous oxide (N O) is an underdetermined problem in environmental sciences due to the multiple source and sink processes involved, which complicate mitigation of its emissions. Measuring the doubly isotopically substituted molecules (isotopocules) of N O can add new opportunities to fingerprint and constrain its cycle.

Methods: We present a laser spectroscopic technique to selectively and simultaneously measure the eight most abundant isotopocules of N O, including three doubly substituted species - so called "clumped isotopes". For the absolute quantification of individual isotopocule abundances, we propose a new calibration scheme that combines thermal equilibration of a working standard gas with a direct mole fraction-based approach.

Results: The method is validated for a large range of isotopic composition values by comparison with other established methods (laser spectroscopy using conventional isotopic scale and isotope ratio mass spectrometry). Direct intercomparison with recently developed ultrahigh-resolution mass spectrometry shows clearly the advantages of the new laser technique, especially with respect to site specificity of isotopic substitution in the N O molecule.

Conclusions: Our study represents a new methodological basis for the measurements of both singly substituted and clumped N O isotopes. It has a high potential to stimulate future research in the N O community by establishing a new class of reservoir-insensitive tracers and molecular-scale insights.