Clumped isotope laser spectroscopy of non-CO2 GHGs

Abundances of doubly-substituted (i.e. clumped) isotopic species of non-CO₂ greenhouse gases (GHGs) are novel tracers, especially in cases where the average (or bulk) isotopic composition is ambiguous. Under equilibrium conditions, they represent intra-species thermometers to trace the molecular temperature history. For kinetically controlled formation and removal processes, clumped isotopic signatures are linked to molecular mechanisms, reflecting reversibility or combinatorics, and can thus be applied to refine biogeochemical cycles. On the planetary scale, model predictions indicate that atmospheric measurements of clumped isotopic species provide constraints on trends in global source and sink magnitudes beyond current knowledge.

We present our recent advances in mid-IR quantum-cascade laser spectroscopy for rapid and high-precision analysis of ¹³CH₃D and ¹²CH₂D₂ in methane (CH₄) and ¹⁵N¹⁴N¹⁸O, ¹⁴N¹⁵N¹⁸O and ¹⁵N¹⁵N¹⁶O in nitrous oxide (N₂O). Analytical developments include selection of spectral regions with improved selectivity, customization of spectrometers with inlet systems for automated sample dilution and injections, and referencing of measurements to stochastic distributions. The two laser spectrometers offer a rapid and precise approach for investigating various aspects of CH₄ and N₂O cycles and origins. Currently, we target first applications in the thermal history of methane-rich sedimentary rocks and key microbial formation pathways of nitrous oxide.