Isotopic composition of stratospheric ozone

We present a kinetic calculation for the isotopic composition of stratospheric ozone. The calculated enrichments of 49 O 3 and 50 O 3 are in agreement with atmospheric measurements made at midlatitudes. Integrating the kinetic fractionation processes in the formation and photolysis of ozone, we obtain enrichments of ∼7.5–10.5 and ∼7.5–12.5% (referenced to atmospheric O 2 ) for δ 49 O 3 and δ 50 O 3 , respectively, at altitudes between 20 and 35 km; the photolysis in the Hartley band of ozone is responsible for the observed altitude variation. The overall magnitude of the ozone enrichments (∼10%) is large compared with that commonly known in atmospheric chemistry and geochemistry. The heavy oxygen atom in ozone is therefore useful as a tracer of chemical species and pathways that involve ozone or its derived products. For example, the mass anomalies of oxygen in two greenhouse gases, CO 2 and N 2 O, are likely the consequences of the transfer of heavy oxygen atoms from ozone.