An absolute reference frame for nitrous oxide clumped and position-specific isotopes provided by thermal equilibration

Abstract. Clumped isotopic measurements of nitrous oxide (N₂O) have the potential to offer unique constraints on the processes governing N₂O production and destruction, building on the information provided by δ15N, δ18O and 15N site preference (SP). Extending their application requires a robust absolute reference frame. Here we show that thermal equilibration of N₂O over γ-Al₂O₃ provides such a reference frame for measurements of the isotopologues 14N15N18O and 15N14N18O, as well as for SP. Using a quantum cascade laser absorption spectroscopy (QCLAS) platform, we simultaneously quantify seven isotopologues of N₂O, including 14N15N18O, 15N14N18O, and 15N15N16O. Experiments starting from isotopically-distinct starting materials show convergence to time-invariant compositions that are in agreement with theoretically predicted temperature dependencies. These results demonstrate that γ-Al₂O₃ activated at ≥ 550 °C catalyzes isotope exchange among isotopologues of N₂O at equilibration temperatures between 153 °C and 218 °C and thereby define an absolute stochastic reference frame for Δ14N15N18O and Δ15N14N18O. In contrast, 15N15N16O does not equilibrate under these conditions, suggesting selective activation of N–O but not N–N bonds. Comparison of equilibrium SP values with theoretical predictions reveals a systematic offset relative to the current reference scale, which will require future work to reconcile.