CO 2 samples collected by aircraft near the winter midlatitude and high‐latitude Northern Hemispheric tropopause were measured for their stable isotope ( 13 C/ 12 C, 18 O/ 16 O) and radioisotope ( 14 C/ 12 C) ratios. The strongly varying CO 2 and δ 13 C(CO 2 ) values spanning 21ppm and 1.1‰, respectively, as well as the low 13 C/ 12 C ratio of the source/sink system responsible for these variations (inferred by applying a simple “Keeling relationship”) point to frequent transport of polluted air masses to the tropopause. This hypothesis is supported by the often depleted 14 C/ 12 C ratios in CO 2 , marking contributions of up to 9ppm ( 14 C‐free) fossil fuel combustion derived CO 2 . The oxygen isotope ratio δ 18 O(CO 2 ) was found to correlate negatively with the CO 2 mixing ratio (R ≈ −0.8), which demonstrates that even the δ 18 O(CO 2 ) data can, as a first approach, be interpreted in terms of a Keeling relation. However, the apparent δ 18 O(CO 2 ) source/sink signature was found to drop from −(11±3)‰ south of the polar front down to −(27±4)‰ north of it. The low Arctic δ 18 O(CO 2 ) values can be explained by the assumption that in the wintertime Arctic about double the amount of CO 2 isotopically exchanges with 18 O‐depleted soil water as is net released by the entire biosphere. A vertical δ 18 O(CO 2 ) gradient of 0.5‰ km −1 was observed above the tropopause. This δ 18 O(CO 2 ) increase in the stratosphere is most likely due to oxygen isotope exchange between CO 2 and electronically excited oxygen O( 1 D), the isotope composition of which is controlled by that of O 3 , in the stratosphere known to be strongly enriched in the heavy oxygen isotopes. The typical δ 18 O(CO 2 ) gradient is assumed to be lower compared to measured because our high‐altitude samples were affected by chemically disturbed polar vortex air.
No takes yet. Share an insight, caveat, or question.
Zahn et al. (2000) studied this question.