Observations of the anomalous oxygen isotopic composition of carbon dioxide in the lower stratosphere and the flux of the anomaly to the troposphere

Measurements of the triple oxygen isotopic composition of stratospheric CO 2 in whole air samples from the NASA ER‐2 aircraft show anomalous enrichments in 17 O and 18 O. The compact correlation of the isotope anomaly (defined as Δ 17 O = δ 17 O − 0.516 × δ 18 O) with simultaneous N 2 O measurements demonstrates that Δ 17 O CO2 is a long‐lived tracer with a stratospheric source. These characteristics, and an isotopic link to O 3 production, make Δ 17 O CO2 potentially useful as a tracer of integrated stratospheric chemistry and transport. The Δ 17 O CO2 :N 2 O correlation is also used to estimate a net Δ 17 O CO2 flux to the troposphere of 3.6 ± 0.9 × 10 15 ‰ mol CO 2 yr −1 . This flux is required to predict and understand the CO 2 and O 2 isotope anomalies in the troposphere and their use as tracers of gross carbon exchanges between the atmosphere and biosphere on interannual to glacial‐interglacial time scales.