Stratospheric δ 13 CO 2 observed over Japan and its governing processes

Le résumé fourni par la source

Due to very few reports of δ 13 CO 2 (the stable carbon isotopic ratio of CO 2 ) observations in the stratosphere, its variations are not well understood. In order to elucidate stratospheric δ 13 CO 2 variations and their governing mechanisms, we have collected stratospheric air samples using balloon-borne cryogenic samplers over Japan since 1985 and analyzed them for δ 13 CO 2 . To obtain precise δ 13 CO 2 values, we incorporated the mass-independent fractionation of 17 O and 18 O in the δ 13 CO 2 calculation. δ 13 CO 2 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 δ 13 CO 2 values did not show a time delay compared to the tropical tropospheric values. This could be explained by the production of CO 2 by CH 4 oxidation and the gravitational separation of 13 CO 2 and 12 CO 2 . To confirm this hypothesis, we used a two-dimensional model to simulate the stratospheric δ 13 CO 2 values while accounting for these processes. The results indicate that these two effects strongly impact the vertical distribution of δ 13 CO 2 . We newly defined “stratospheric potential δ 13 C” ( δ 13 C P ) as a quasi-conservative parameter incorporating the kinetic isotope effect of CH 4 oxidation and gravitational separation, and we found that δ 13 C P 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 δ 13 CO 2 variations are governed by the airborne production of 13 C-depleted CO 2 by CH 4 oxidation, the gravitational separation, and the propagation of the decreasing tropospheric δ 13 CO 2 trend into the stratosphere.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.