Seasonal cycle of C16O16O, C16O17O, and C16O18O in the middle atmosphere

Abstract. The mass-independent nature of the isotopic composition of ozone is caused mainly by non-statistical partitioning in the formation chem-istry of ozone, resulting in δ49O3 ≈ δ50O3 ≈ 100 relative to atmospheric O2. This isotopic anomaly is large compared with common biogeochemical processes, and can be transferred to other trace molecules such as CO2 through the exchange reactions with O(1D), a photolytic product of O3. Higher in the mesosphere, Lyman-α photolysis of O2 provides a more important source of heavy O(1D) than O3 photolysis. Here, we present a two-dimensional sim-ulation of the isotopic composition of CO2 from the surface to an altitude of ∼130 km. The model can adequately reproduce the observed seasonal cy-cle of CO2 in the upper troposphere and the age of air in the stratosphere. Our model suggests that stratospheric-tropospheric exchange not only mod-ifies the level of heavy CO2 in the lower 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 through stratosphere