The C-13, C-14 and O-18 isotopic composition of CO, CH4, and CO2 in the higher southern latitudes lower stratosphere

Large air samples were collected in the lower stratosphere (10-12 km) from 43 degrees to 85 degrees S in June 1993, using a special compressor system. For the important trace gases CO, CH4 and CO2, concentration and isotopic analyses were carried out and significant correlations were discovered. The (CO)-C-14 isotope is considerably in excess of tropospheric levels with very high values from 40 to 120 (CO)-C-14 molecules/cm(3) STP (corresponding to 12,500 percent modern carbon, at 30 ppbv), and is negatively correlated with CO. The linear relationship is used to estimate OH to be 2.9 x 10(5) cm(-3). The O-18/O-16 ratios for CO are the lowest ever measured and reflect the inverse kinetic isotope effect in the oxidation of CO by OH. The C-13/C-12 ratios for CO are not much different from tropospheric values and confirm that fractionation is small but also that the in situ contribution from CH4 oxidation is minor. For CH4 a correlation between delta(13)C and concentration exists from which a fractionation factor for the sink reaction (k(12)/k(13)) of about 1.012 is calculated, well in excess of results from laboratory experiments for OH + CH4. The most plausible explanation presently is the removal of approximately 9% of CH4 by Cl atoms, which, as laboratory experiments have just confirmed, induces a very large fractionation. We also reveal a linear correlation between (CO)-C-14 and (CO2)-C-14, precursor and product. Finally, an analysis of potential vorticity shows a structure that seems to give an overall agreement with the trace gas variations.