| Summary: | Tropospheric (OO)-O-18-O-18 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 (OO)-O-18-O-18 abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the (OO)-O-18-O-18 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 (OO)-O-18-O-18 proxy, we developed a scheme to simulate atmospheric (OO)-O-18-O-18 abundances (quantified as increment (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 O-2 isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled increment (36) values are too high elsewhere. Investigations using a comprehensive model of the O-O-2-O-3 isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to increment (36) values: the anomalously efficient titration of (OO)-O-18-O-18 to form ozone. Incorporating these effects into the online increment (36) calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of (OO)-O-18-O-18 in O-2, the modeled change in the mean tropospheric increment (36) value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice-core increment (36) record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.
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