Seasonal cycle of C¹⁶O¹⁶O, C¹⁶O¹⁷O, and C¹⁶O¹⁸O in the middle atmosphere: Implications for mesospheric dynamics and biogeochemical sources and sinks of CO₂

The isotopic anomaly of oxygen in atmospheric CO 2 is caused by exchange reactions with isotopically anomalous O( 1 D) in the middle atmosphere. In the stratosphere, the major source of O( 1 D) is O 3 photolysis; O 3 is known to possess mass‐independent isotopic composition, with δ 49 O 3 ≈ δ 50 O 3 ≈ 100‰ relative to atmospheric O 2 . Higher in the mesosphere, Lyman α ‐driven photodissociation of O 2 provides a more important source of heavy O( 1 D) than O 3 photolysis. Here we present a two‐dimensional simulation of the isotopic composition of CO 2 from the surface to an altitude of ∼130 km that adequately reproduce the observed seasonal cycle of CO 2 in the upper troposphere and the age of air in the stratosphere. Our model results suggest that stratospheric‐tropospheric exchange not only modifies the level of heavy CO 2 in the troposphere, but also influences its seasonal cycle. Thus the isotopic composition of CO 2 in the troposphere/biosphere could be affected by the downwelling air from the stratosphere. The predicted size of the effect is detectable by current instrumentation. Implications for the use of the isotopic composition of CO 2 to constrain the gross carbon flux between the atmosphere and terrestrial biosphere and the dynamics in the remote mesosphere are discussed.