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

Abstract

The isotopic anomaly of oxygen in atmospheric CO2 is caused by exchange reactions with isotopically anomalous O(1D) in the middle atmosphere. In the stratosphere, the major source of O(1D) is O3 photolysis; O3 is known to possess mass-independent isotopic composition, with δ 49O3 ≈ δ 50O3 ≈ 100‰ relative to atmospheric O2. Higher in the mesosphere, Lyman α-driven photodissociation of O2 provides a more important source of heavy O(1D) than O3 photolysis. Here we present a two-dimensional simulation of the isotopic composition of CO2 from the surface to an altitude of ∼130 km that adequately reproduce the observed seasonal cycle of CO2 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 CO2 in the troposphere, but also influences its seasonal cycle. Thus the isotopic composition of CO2 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 CO2 to constrain the gross carbon flux between the atmosphere and terrestrial biosphere and the dynamics in the remote mesosphere are discussed.

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