Fate of long-lived trace species near the Northern Hemispheric tropopause 2.: Isotopic composition of carbon dioxide (13CO2, 14CO2, and C18O16O)

CO2 samples collected by aircraft near the winter midlatitude and high-latitude Northern Hemispheric tropopause were measured for their stable isotope (C-13/C-12, O-18/O-16) and radioisotope (C-14/C-12) ratios. The strongly varying CO2 and delta(13)C(CO2) values spanning 21 ppm and 1.1 parts per thousand, respectively, as well as the low 13C/12C 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 14C/12C ratios in CO2, marking contributions of up to 9 ppm (C-14-free) fossil fuel combustion derived CO2. The oxygen isotope ratio delta(18)O(CO2) was found to correlate negatively with the CO2 mixing ratio (R approximate to -0.8), which demonstrates that even the delta(18)O(CO2) data can, as a first approach, be interpreted in terms of a Keeling relation. However, the apparent delta(18)O(CO2) source/sink signature was found to drop from -(11 +/- 3)parts per thousand south of the polar front down to -(27 +/- 4)parts per thousand north of it. The low Arctic delta(18)O(CO2) values can be explained by the assumption that in the wintertime Arctic about double the amount of CO2 isotopically exchanges with O-18-depleted soil water as is net released by the entire biosphere. A vertical delta(18)O(CO2) gradient of 0.5 parts per thousand km(-1) was observed above the tropopause. This delta 1(8O)(CO2) increase in the stratosphere is most likely due to oxygen isotope exchange between CO2 and electronically excited oxygen O(D-1), 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 delta(18)O(CO2) gradient is assumed to be lower compared to measured because our high-altitude samples were affected by chemically disturbed polar vortex air.