A large (CO)-C-13 deficit in the lower Antarctic stratosphere due to ''ozone hole'' chemistry .1. Observations

Isotope and concentration measurements are reported for CO and CH4 in air collected in the upper troposphere and lower stratosphere between New Zealand and Antarctica in October 1993. The C-13/C-12 ratio of CO for the stratospheric samples, which are identified using calculated potential vorticity and coherent isotope data, like the abundance of (CO)-C-14 molecules, are much lower than all previously reported atmospheric values. The measurements manifest a very steep decrease in delta(13)C with declining CO, with one sample reaching a delta(13)C value relative to V-PDB of -43 parts per thousand at 20 ppbv CO. This large isotope shift is caused by the local production of several ppbv of extremely depleted CO. Not only is CH4 itself a C-13 depleted precursor of CO, it is specifically the recently discovered large fractionation in Cl + CH4, and the availability of free Cl during ozone hole conditions, which causes the effect.