Principal factors determining the O-18/O-16 ratio of atmospheric CO as derived from observations in the southern hemispheric troposphere and lowermost stratosphere

Measurement of the O-18/O-16 ratio in atmospheric CO based on mass spectrometry after its oxidation to CO2, which is applied to separate it from air, is evaluated. A convenient method to correct for the isotope effect of the addition of an oxygen atom by the oxidant to the original CO molecule is given, results of which confirm earlier data. Measurement of CO extracted from air containing varying amounts of CO derived from transportation and other man-made pollution sources reveals that the O-18/O-16 ratios of CO from these sources approach that of atmospheric oxygen. Analysis of upper tropospheric samples collected using aircraft flights between New Zealand and Antarctica allows a determination of the O-18/O-16 ratio of biomass burning derived CO after its transportation into the extratropical regions. The inferred delta(18)O value of 4.5 parts per thousand would indicate that the kinetic fractionation in the removal of CO by OH during transport has decreased the O-18 content by as much as 10 parts per thousand. Evaluation of the delta(18)O values for samples collected in the stratosphere allows a determination of this fractionation, and a value for k(16)/k(18) of 0.992 is derived, close to laboratory results. Further, by using a southern hemisphere CO budget it is possible to estimate that delta(18)O of CO produced via the reaction CH4 + OH has a value probably as low as 0 parts per thousand.