Stratospheric δ¹³CO₂ observed over Japan and its governing processes and potential as an air age tracer

Abstract. Due to very few reports of δ¹³CO₂ (the stable carbon isotopic ratio of CO₂) observations in the stratosphere, its variations are not well understood. In order to elucidate stratospheric δ¹³CO₂ variations and their governing mechanisms, and to investigate usefulness of δ¹³CO₂ as an air age tracer, we have collected stratospheric air samples using balloon-borne cryogenic samplers over Japan since 1985 and analyzed them for δ¹³CO₂. To obtain precise δ¹³CO₂ values, we incorporated the mass-independent fractionation of ¹⁷O and ¹⁸O in the δ¹³CO₂ calculation. δ¹³CO₂ has decreased through time in the mid-stratosphere with an average rate of change of −0.026 ± 0.001 ‰ yr⁻¹ for the period 1985–2020, consistent with that in the troposphere. However, mid-stratospheric δ¹³CO₂ values did not show a time delay compared to the tropical tropospheric values. This could be explained by the production of CO₂ by CH₄ oxidation and the gravitational separation of ¹³CO₂ and ¹²CO₂. To confirm this hypothesis, we used a two-dimensional model to simulate the stratospheric δ¹³CO₂ values while accounting for these processes. The results indicate that these two effects strongly impact the vertical distribution of δ¹³CO₂. We newly defined ‘stratospheric potential δ¹³C’ (δ¹³C_P) as a quasi-conservative parameter incorporating the kinetic isotope effect of CH₄ oxidation and gravitational separation, and we used it to estimate the mean age of stratospheric air. Despite large uncertainties, the mean age derived from δ¹³C_P was consistent with that derived from the CO₂ mole fraction, suggesting its usefulness for further investigation of stratospheric transport processes.