Advances in clumped isotope measurements of nitrous oxide by laser spectroscopy

Nitrous oxide (N₂O), a greenhouse gas and ozone-depleting molecule with a 116-year atmospheric lifetime, is accumulating in the atmosphere at an accelerating pace. While it is known that the conversion of anthropogenic nitrogen pollutants to N₂O by the environmental nitrogen cycle predominately drives this accumulation, essential questions remain regarding the spatial and temporal balance of N₂O production and consumption. Stable isotope measurements of δ¹⁵N, δ¹⁸O, and ¹⁵N site preference (SP) in N₂O provide valuable constraints on its sources and sinks. Given the complex array of nitrogen cycle processes and their overlapping isotopic signatures, they are often not sufficient to deconvolve them completely. The ‘clumped’, or multiply-substituted, isotopologues ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, and ¹⁵N¹⁵N¹⁶O provide three additional independent constraints on N₂O sources and processing, with potential to provide insight into source partitioning and reaction mechanisms.

Spectroscopic approaches have emerged as central tools for quantification of N₂O isotopes in atmospheric and environmental samples due to their sensitivity, suitability for continuous or on-site measurement, and isotopologue-specificity. We present an updated approach to the measurement of the eight most abundant isotopic variants of nitrous oxide, including these rare clumped isotopologues, using a quantum cascade laser absorption spectrometer with a 36 m multipass cell. A dual-laser system offers the opportunity to choose a pair of spectral windows that contains strong ro-vibrational absorption lines of the rarest isotopologues, enabling precise clumped isotope ratio measurements on relatively small (<10 µmol) sample amounts. Samples are introduced to the spectrometer and compared to reference materials through a customized gas inlet system, which enables fast switching between samples and references, thereby maximizing reproducibility and sample throughput. Nitrous oxide heated in the presence of γ-alumina at 200 ºC has previously been found to approach equilibrium compositions of ¹⁵N¹⁴N¹⁸O, ¹⁴N¹⁵N¹⁸O, and SP. We constrain the controls on this catalytic reaction by varying temperature, pressure, substrate and catalyst concentrations, and catalyst activation conditions; and we confirm the equilibrium nature of this reaction under various conditions by heating reference gases prepared gravimetrically to have ¹⁵N¹⁴N¹⁸O, ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O elevated above natural abundances. Finally, equilibrating N₂O at several distinct temperatures gives multiple anchor points to an absolute reference scale for clumped and position-specific isotope measurements, enabling future measurement of samples from natural sources.