Seasonal cycle of C^(16)O^(16)O, C^(16)O^(17)O, and C^(16)O^(18)O in the middle atmosphere: Implications for mesospheric dynamics and biogeochemical sources and sinks of CO_2

The isotopic anomaly of oxygen in atmospheric CO_2 is caused by exchange reactions with isotopically anomalous O(^1D) in the middle atmosphere. In the stratosphere, the major source of O(^1D) 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 O2 provides a more important source of heavy O(^1D) 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