Laboratory and atmospheric measurements of the isotopic composition of ozone have shown large and unusual ‘non-mass-dependent’ enrichments in 17O and 18O of ∼10% relative to the O2 from which it is formed. We present measurements of the bath gas and pressure dependence of the isotope enrichments in ozone formed by photolysis of O2 in the presence of Ar, O2, CO2, and SF6 from 50 to 760 Torr. The results provide new insights into the origin of the dynamically-driven symmetry-related isotope effect involved and new benchmarks for theory, including elimination of all isotope selectivity for ozone formation in SF6 by 700 Torr.Graphical abstractHighlights► Bath gas and pressure dependence of heavy isotope enrichments in O3 were measured. ► All isotope selectivity is eliminated for O3 formed in an SF6 bath by 700 Torr. ► Results provide new insights into the non-mass-dependent isotope effects in ozone formation and new benchmarks for theory.

We report observations of stratospheric CO2 that reveal surprisingly large anomalous enrichments in 17O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone–CO2 isotope exchange demonstrates that non–mass-dependent isotope effects in ozone formation alone quantitatively account for the 17O anomaly in CO2 in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O3, O(1D), or CO2 and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of 17O anomalies in tropospheric CO2, O2, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO2.