Fate of long-lived trace species near the Northern Hemispheric tropopause 2. Isotopic composition of carbon dioxide ((CO2)-C-13, (CO2)-C-14, and (COO)-O-18-O-16)

CO2 samples collected by aircraft near the winter midlatitude and high-lati tude 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 v arying CO2 and delta(13)C(CO2) values spanning 21 ppm and 1.1 parts per tho usand, respectively, as well as the low 13C/12C ratio of the source/sink sy stem responsible for these variations (inferred by applying a simple "Keeli ng relationship") point to frequent transport of polluted air masses to the tropopause. This hypothesis is supported by the often depleted 14C/12C rat ios in CO2, marking contributions of up to 9 ppm (C-14-free) fossil fuel co mbustion 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), wh ich demonstrates that even the delta(18)O(CO2) data can, as a first approac h, be interpreted in terms of a Keeling relation. However, the apparent del ta(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 is otopically exchanges with O-18-depleted soil water as is net released by th e entire biosphere. A vertical delta(18)O(CO2) gradient of 0.5 parts per th ousand km(-1) was observed above the tropopause. This delta 1(8O)(CO2) incr ease in the stratosphere is most likely due to oxygen isotope exchange betw een CO2 and electronically excited oxygen O(D-1), the isotope composition o f which is controlled by that of O-3, in the stratosphere known to be stron gly enriched in the heavy oxygen isotopes. The typical delta(18)O(CO2) grad ient is assumed to be lower compared to measured because our high-altitude samples were affected by chemically disturbed polar vortex air.