Effects of ozone isotopologue formation on the clumped-isotope composition of atmospheric O2

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.