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 15Nα and 15Nβ, 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 δ15Nα and δ15Nβ (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 δ15Nα and δ15Nβ (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