Isotopic ordering in atmospheric O₂ as a tracer of ozone photochemistry and the tropical atmosphere

Abstract

The distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as Δ36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped‐isotope budget of O2: Stratospheric air has higher Δ36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower Δ36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high‐Δ36 air from the stratosphere. These observations agree with predictions derived from the GEOS‐Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where Δ36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in Δ36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a ±10% range since 1978.

Extreme enrichment in atmospheric 15 N 15 N

“…The bottles were allowed to degas and equilibrate on an orbital shaker (110 rpm) for 48 hours before the water was removed, and the headspace gas was collected onto silica gel for purification. Dissolved N 2 was separated from the other gases at Rice University on a molecular sieve 5A column (3.05 m × 1/8″ OD, 80/100 mesh) at −80°C and recollected on a silica gel U-trap using a method described previously ( 41 ). We also isolated atmospheric N 2 using this method, and the results were indistinguishable from gases collected and purified at UCLA.…”

“…The data were outputs of present-day control runs of the Community Earth System Model version 1.0 (data set f.e10.FWX.f19_f19.control.001) available from the Earth System Grid (www.earthsystemgrid.org). Globally integrated species concentrations and reaction rates were calculated using methods similar to those previously described ( 41 ). Briefly, reaction rates were calculated at each grid point using monthly-averaged species mixing ratios, temperatures, and pressures.…”

Scale distortion from pressure baselines as a source of inaccuracy in triple‐isotope measurements

“…The air purification line used in this study was described briefly in a previous publication, but we will describe it more thoroughly here. A turbo‐pumped high‐vacuum line (10 −6 mbar) was constructed and interfaced with a gas chromatograph (model 7890B, with a thermal conductivity detector; Agilent Technologies, Santa Clara, CA, USA) situated on a rolling cart made of aluminum extrusion (Figure ).…”

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

“…The atmospheric O 2 reservoir exchanges with the biosphere on a timescale of roughly 1200 years . Its isotopic composition is affected by biological, hydrological and photochemical processes . However, only the bulk isotopic ratios of O 2 (i.e., 18 O/ 16 O and 17 O/ 16 O) have been studied extensively.…”

“…However, only the bulk isotopic ratios of O 2 (i.e., 18 O/ 16 O and 17 O/ 16 O) have been studied extensively. In nature, these ratios vary primarily in response to biological oxygen cycling, although the influence of stratospheric photochemistry has also been observed . The doubly substituted isotopologues, 18 O 18 O, 17 O 18 O and potentially also 17 O 17 O, also called clumped isotopes, can be useful to independently constrain the cycling of oxygen in the Earth's atmosphere …”