Laurence Y. Yeung, Lee T. Murray, Asmita Banerjee, Xin Tie, Yuzhen Yan, Elliot L. Atlas, Sue M. Schuaffler, and Kristie A. Boering

doi: 10.1029/2021JD034770

Abstract

Tropospheric ¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive ¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the ¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the ¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric ¹⁸O¹⁸O abundances (quantified as ∆₃₆ values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂ isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆ values are too high elsewhere. Investigations using a comprehensive model of the O-O₂-O₃ isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆ values: the anomalously efficient titration of ¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆ calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of ¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆ value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆₃₆ record, implying that the tropospheric ozone burden increased less than ∼40% over the twentieth century.

Plain-language summary

Oxygen in the air is constantly being broken apart and remade. Its constituent atoms are shuffled around by light-induced chemical reactions, which cause changes in the number of heavy oxygen atoms that are bound together. The number of these heavy-atom “clumps” is sensitive to air temperatures and the presence of air pollution; hence, their variations are being used to understand past high-altitude temperatures and atmospheric chemistry. This study incorporates oxygen clumping into an atmospheric chemistry model and compares the results to measurements of oxygen clumping in the atmosphere. We find that the model can explain all the modern-day measurements (from the surface to 33 km altitude), but only if the broader fates of oxygen atoms―i.e., their incorporation into other molecules beyond O₂―are considered. Simulations of the preindustrial atmosphere are also generally consistent with snapshots of the ancient atmosphere obtained from O₂ trapped in ice cores. The developments described herein will thus enable models to simulate heavy oxygen-atom clumping in past cold and warm climates and enable simulated high-altitude atmospheric changes to be evaluated directly against ice-core snapshots of the ancient atmosphere.