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

Abstract

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.

Effects of Ozone Isotopologue Formation on the Clumped‐Isotope Composition of Atmospheric O2

“…The Δ 36 values reported in Laskar et al. (2019) are all 0.3–0.4‰ higher than the Ft. Sumner and SEAC 4 RS data (samples from September 2004 to 2013, respectively), despite the common latitude range and season (34°N–42°N, 10–20 km, sampled September 2016). Reported Δ 36 values for surface air from Utrecht, The Netherlands are also higher than the values reported for surface air in earlier papers (Li et al., 2019; Yeung, 2016; Yeung et al., 2012, 2014).…”

“…Consequently, the Δ 36 values reported in Laskar et al. (2019) are incompatible with their atmospheric context. The origin of the interlaboratory disagreement remains unknown, but could plausibly be related to differences in calibration protocols such as the pressures of photolysis of photochemical standards.…”

“…At face value, the uniformly higher tropospheric and lower‐stratospheric Δ 36 values in Laskar et al. (2019) might favor the O 2 ‐only photochemical scheme; however, such a result would imply a profound inaccuracy in the Rice data. Importantly, a large offset in the Rice data would imply that the midlatitude middle stratosphere is far from isotopic equilibrium, contrary to expectations of fast isotopic reordering based on high local O( 3 P ) concentrations (Anderson, 1975; see also Figure 5c).…”

“…Furthermore, the narrow spread in measured ∆ 36 values along surfaces of constant potential temperature suggests that dispersive mixing is pervasive and mutes any variability arising from meridional temperature gradients. Consequently, the ∆ 36 values reported in Laskar et al (2019) interlaboratory disagreement remains unknown, but could plausibly be related to differences in calibration protocols such as the pressures of photolysis of photochemical standards.…”

Laser Absorption Spectroscopy of Rare and Doubly Substituted Carbon Dioxide Isotopologues

“…Particularly interesting is the temperature-dependent behavior of 13 C and 18 O isotopes during carbonates precipitation and in gaseous carbon dioxide, as these isotopes show a slightly higher tendency to form a bond at lower temperatures. − The effect is used as a temperature proxy for paleoclimate reconstruction, , biogenic carbonate formation, , quantification of nonequilibrium effects, quantification of CO 2 sources and sinks in the Earth atmosphere. , Similar effects are also observed and studied in other molecular systems, e.g. methane , or molecular oxygen. , The analytical techniques for these studies generally rely on isotope ratio mass spectrometry (IRMS) of gaseous samples, , with the drawback that most isotopologue signals such as m / z = 45 or 47 suffer from isobaric interferences (see Table 1 in Prokhorov et al). For example, the analysis of 12 C 16 O 17 O (relative amount fraction 0.75‰) at m / z = 45 is hampered by the 15 times more abundant 13 C 16 O 2 .…”