Oxygen Isotopic Composition in Atmospheric Carbon Dioxide

The mass-independent nature of the isotopic composition of ozone is caused by none-statistical partition in its intermediate state to the formation of ozone, resulting in δ^{49}O3 ~ δ^{50}O3 ~ 10% relative to atmospheric O2. This isotopic anomaly is large compared with other commonly known biogeochemical processes, and can transfer to other trace molecules such as CO2 [Yung et al., 1991] through the exchange reaction with O(^1D), a photolytic product of O3. Higher in the upper stratosphere and mesosphere, the Lyman-α photolysis of O2 provides a source of heavy O(^1D) more important than O3. Here, we present a two-dimensional simulation for the isotopic composition of CO2 from the surface to an altitude ~130 km. The model can adequately reproduce the observed seasonal cycle of CO2 in the upper troposphere and the age of air in the stratosphere. We find that the isotopic composition of CO2 originally in the troposphere/biosphere can be affected by the downwelling air from the stratosphere through the stratosphere-troposphere exchange, and this effect is detectable by the current instrumentation. The stratosphere-troposphere exchange not only modifies the level of heavy CO2 in the lower troposphere, but also influences its seasonal cycle. Implications for the use of the isotopic composition of CO2 to constrain the gross carbon flux between the atmosphere and terrestrial biosphere [Luz et al., 1999] and the dynamics in the remote mesosphere are discussed.