Stratospheric <i>δ</i> ¹³ CO ₂ observed over Japan and its governing processes

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, 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” (δ13CP) as a quasi-conservative parameter incorporating the kinetic isotope effect of CH4 oxidation and gravitational separation, and we found that δ13CP in the mid-latitude mid-stratosphere decreases over time with about a 5 yr lag relative to the tropical upper troposphere. This fact strongly supports that stratospheric δ13CO2 variations are governed by the airborne production of 13C-depleted CO2 by CH4 oxidation, the gravitational separation, and the propagation of the decreasing tropospheric δ13CO2 trend into the stratosphere.