Measurement of O O and O O in atmospheric O using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples.

Rationale: The doubly substituted isotopologues (e.g., O O, O O) in atmospheric O 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: O from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to -78°C. The δ O, δ O, Δ O, Δ and Δ values are measured on the extracted O 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 Cl for O O and H Cl and Ar for O O, are sufficiently resolved to enable high-precision determination of Δ and Δ . The Δ and Δ values of O after photochemical isotope equilibration at -63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ and Δ values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O differ from those expected at equilibrium.

Conclusions: The 253 Ultra allows interference-free clumped isotope measurements of O at medium mass resolution. The Δ and Δ signatures in atmospheric O are mainly governed by O photochemistry, temperature and atmospheric transport. Tropospheric O is isotopically well mixed and retains a significant stratospheric signature.