Atmospheric N₂O mole fraction has been increasing since preindustrial times, predominately due to increased agricultural activity. Owing to the approximately 300 times higher GWP compared to CO₂, this greenhouse gas currently accounts for 6% of total anthropogenic radiative forcing. Recent estimates showed that N₂O is, in addition, the single most important ozone-depleting substance.

N₂O is a linear, non-symmetric molecule (N–N–O). The incorporation of the stable isotopes of nitrogen (¹⁴N and ¹⁵N) and oxygen (¹⁶O, ¹⁷O, and ¹⁸O) into these three molecular sites yields twelve distinct isotopic variants of N₂O. The singly and most abundant doubly substituted isotopic species are:

¹⁴N¹⁴N¹⁶O (9.90 x 10^-1)

¹⁵N¹⁴N¹⁶O (3.64 x 10^-3)

¹⁴N¹⁵N¹⁶O (3.64 x 10^-3)

¹⁴N¹⁴N¹⁸O (3.64 x 10^-3)

¹⁴N¹⁴N¹⁷O (3.64 x 10^-4) 

¹⁵N¹⁵N¹⁶O (1.34 x 10^-5) 

¹⁴N¹⁵N¹⁸O (7.30 x 10^-6) 

¹⁵N¹⁴N¹⁸O (7.30 x 10^-6)

The nitrogen atom at the center (α site) and the one at the end (β site) lead to two structural isomers, namely ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O, referred to as ¹⁵N^α and ¹⁵N^β, respectively. The natural abundance of nitrogen and oxygen isotopes in nitrous oxide (N₂O) as well as their intramolecular distribution can be used to obtain important information on its geochemical cycle, because many biological and chemical processes lead to distinct isotopic signatures.

Employing recently available quantum cascade lasers (QCL), we are able to perform continuous and precise analysis of the isotope ratios δ¹⁵N and δ¹⁸O-N₂O and the site-specific isotope ratios δ15N^α and δ15N^β (e.g. Wächter et al. 2008). By coupling the QCL spectrometer to a fully-automated preconcentration unit, we achieve quasi-continuous and high-precision analysis of N₂O isotopic species at ambient mixing ratios (Mohn et al. 2010, 2012). This approach has been successfully applied to measurements of in situ isotopic signatures of N₂O emissions (Wolf et al., 2015, Harris et al., 2017). In an inter-laboratory campaign, we demonstrated excellent compatibility of QCL-based N₂O isotopomer analysis with isotope-ratio mass-spectrometry (IRMS) for δ¹⁵N and δ¹⁸O and superior performance for δ¹⁵N^α and δ¹⁵N^β (Mohn et al. 2014).

 

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Trace-N₂O: N₂O from the Swiss midlands – regional sources and hot spots

Global N₂O cycle: Assessment of the global N₂O budget based on seasonal and long-term isotope measurements at Jungfraujoch and the Cape Grim Air Archive

Clumped isotopes as a novel tracer for the N₂O cycle

ISOMOL: Advanced understanding of autotrophic nitrogen removal and associated N₂O emissions in mixed nitritation-anammox systems through combined stable ISOtopic and MOLecular constraints

SIRS: Metrology for stable isotope reference standards