Large and unexpected enrichment in stratospheric ^(16)O^(13)C^(18)O and its meridional variation

The stratospheric CO_2 oxygen isotope budget is thought to be governed primarily by the O(1D)+CO_2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO_2 isotopologue ^(16)O^(13)C^(18)O, in concert with ^(18)O and ^(17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO_2 sample to date. We show, through photochemical experiments, that lower ^(16)O^(13)C^(18)O proportions observed in the midlatitudes are determined primarily by the O(1D)+CO_2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher ^(16)O^(13)C^(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(1D)+CO_2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO_2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric ^(16)O^(13)C^(18)O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change

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

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

A 'clumped-isotope' study of stratospheric CO_2 reveals a new atmospheric process

The stable isotope composition of stratospheric CO_2 is a long-lived tracer of stratospheric photochemical processing. Although the stratospheric CO_2 isotopologue budget is thought to be governed primarily by the O(^1D)+CO_2 isotope exchange reaction, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the rare 'clumped' isotopologue ^(13)C^(18)O^(16)O, in concert with ^(12)C^(18)O^(16)O and ^(12)C^(17)O^(16)O abundances, provides sensitivities to these additional processes, and thus is a valuable test of current models

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

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 CE 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–that is, 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. Key Points Online calculations of atmospheric 18O18O variations have been implemented in GEOS‐Chem, which reproduces all atmospheric observations well Titration of 18O18O into heavy ozone was determined to be an important missing component of previous 18O18O budgets Change in atmospheric 18O18O abundance since 1850 CE is still consistent with <40% increase in tropospheric ozone burde