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

Atmospheric measurement techniques, vol. 19, pp. 5267–5280

Abstract

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.

Authors 9

1. Paul Magyar

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Swiss Federal Laboratories for Materials Science and Technology (Dübendorf, Switzerland)

2. Nico Kueter Aachen Geological Institute

   ETH Zurich · RWTH Aachen University

   Affiliation as printed

   Department of Earth and Planetary Sciences, ETH Zürich, 8092 Zurich, Switzerland

   Geological Institute, RWTH Aachen University, 52062 Aachen, Germany

3. Simone Brunamonti

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

4. Naizhong Zhang

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

5. Ivan Prokhorov

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

6. Noémy Chénier

   University of Basel · Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Department of Environmental Science, University of Basel, 4056 Basel, Switzerland

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

7. Lukas Emmenegger

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

8. Béla Tuzson

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland

9. Joachim Mohn

   Swiss Federal Laboratories for Materials Science and Technology

   Affiliation as printed

   Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland