Experimental and theoretical study of isotope effects on ozone decomposition

Experimental and theoretical studies of O 3 decomposition are reported which resolve kinetic mechanisms for isotopic fractionations in O‐O 2 ‐O 3 chemistry. The thermal gas phase decomposition of O 3 at high temperatures (90°C, 110°C) produces isotopically heavy O 2 with respect to precursor O 3 with a non‐mass‐dependent fractionation pattern (δ 17 O ≠ 0.5δ 18 O). At 90°C, a noticeable component of heterogeneous decomposition is present, but is insignificant at 110°C. The product O 2 from O 3 photolysis by visible (532 nm) and UV light is depleted in the heavy isotopes, opposite from thermal decomposition. These well controlled experiments permit a quantitative kinetic analysis. A one‐box, time‐dependent model was used to simulate the evolution of the isotopic composition of the O 2 and O 3 . The comparison of thermal and photolysis experiments indicates that the initial decomposition step is most likely responsible for the observed isotopic fractionations. Based upon detailed kinetic model calculations, the roles of isotope exchange reactions, initial O 3 isotopic distribution, and various branching ratios of isotopic reactions involved in the isotopic fractionation have been examined. The possible importance of these conclusions for understanding stratospheric isotopically heavy O 3 , and possible implications for non‐mass‐dependent isotopic fractionations and chemical kinetic theory are discussed.