Measurement of 18O18O and 17O18O in atmospheric O2 using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples

Rationale: The doubly substituted isotopologues (e.g., 18O18O, 17O18O) in atmospheric O2 are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high-resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences. 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 δ17O, δ18O, Δ17O, 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 35Cl for 17O18O and H35Cl and 36Ar for 18O18O, 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.