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

Abstract. The mass-independent nature of the isotopic composition of ozone is caused by non-statistical partitioning in the formation chemistry of ozone, resulting in δ49O3 ≈ δ50O3 ≈ 100 h 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 ex-change 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 simulation of the iso-topic composition of CO2 from the surface to an altitude of ∼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. Our model suggests that stratospheric-tropospheric exchange not only modifies 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 af