Conclusions
The 253 Ultra allows interference-free clumped isotope measurements of O2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.
Methods
O2 from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to -78°C. The δ17 O, δ18 O, Δ17 O, Δ35 and Δ36 values are measured on the extracted O2 with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues.
Results
Interferences from isobars, mainly 35 Cl for 17 O18 O and H35 Cl and 36 Ar for 18 O18 O, are sufficiently resolved to enable high-precision determination of Δ35 and Δ36 . The Δ35 and Δ36 values of O2 after photochemical isotope equilibration at -63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ35 and Δ36 values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O2 differ from those expected at equilibrium. Conclusions: The 253 Ultra allows interference-free clumped isotope measurements of O2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.