An absolute reference frame for nitrous oxide position-specific and clumped isotopic measurements

Stable isotopes are a powerful tool for constraining the sources and sinks of nitrous oxide (N₂O), essential for identifying and mitigating the climate and air quality impacts of N₂O emissions. Site preference (SP), the position-specific N stable isotope incorporation in N₂O (¹⁴N¹⁵N¹⁶O vs ¹⁵N¹⁴N¹⁶O), has proven especially useful. Measurements of the clumped isotopologues ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, and ¹⁵N¹⁵N¹⁶O are emerging as new constraints on the processes of N₂O formation and destruction. An advantage of clumped and position-specific isotopic systems over conventional stable isotopes is the existence of an absolute reference frame: under equilibrium conditions, isotopes are randomly distributed among molecules at high temperatures, and deviations from this random distribution at lower temperatures can be both predicted by thermodynamic modelling and measured.

We use quantum cascade laser adsorption spectroscopy to measure the seven-dimensional stable isotopic composition of N₂O (δ¹⁵N, δ¹⁸O, ∆¹⁷O, SP, ∆¹⁴N¹⁵N¹⁸O, ∆¹⁵N¹⁴N¹⁸O, and ∆¹⁵N¹⁵N¹⁶O). This spectroscopic approach provides key benefits for standardization studies, including the ability to measure each isotopologue directly without the need for the fragmentation and rearrangement corrections required by mass spectrometric methods. In addition, the ability to measure a sample in replicate (n = 3) in <30 min with precision better than ±0.3‰ for all isotopologues increases the throughput of N₂O clumped isotope measurements and improves greatly on previous analytical approaches.

We present results for N₂O equilibrated over g-alumina, which has been identified as a catalyst for the N-O isotope exchange equilibria, at temperatures between 170 °C and 230 °C. This range of temperatures represents an optimum where the kinetics of isotope exchange reactions outpace N₂O thermal decomposition but proceed fast enough for readily repeatable experiments. Additionally, this range of temperatures is associated with a predicted variation in SP of 4.6‰, suitable for evaluating the temperature dependency of reactions among N₂O isotopologues. We find the catalytic activity of g-alumina to be sensitive to its conditions of activation and to the N₂O/catalyst ratio. We report equilibration and analyses of gases of a wide variety of starting isotopic compositions to demonstrate the equilibrium nature of these reactions by the principle of bracketing, to document the kinetics of isotope exchange for each isotopologue, and to establish a set of reference gases suitable for robust two-point calibration in all isotopic dimensions.