ATMOSPHERIC (CO)-C-14 OBSERVATIONS AND THEIR USE FOR ESTIMATING CARBON-MONOXIDE REMOVAL RATES

Atmospheric (CO)-C-14 can be used as a tracer of the global tropospheric hydroxyl radical (OH) concentration, Which is responsible for the destruction of many trace gas species. (CO)-C-14 is produced throughout the Earth's atmosphere by a N-14 neutron capture reaction followed by hot atom oxidation. It subsequently follows the pathway of any other CO molecule and is thus removed from the atmosphere by OH oxidation. From the isotopic information it is also possible to elucidate the histories of CO from otherwise identical air masses. Results from global sampling campaigns are presented which establish the northern hemisphere (NH) latitudinal dependence and seasonal extremes of this trace constituent. We used the Lawrence Livermore National Laboratory two-dimensional chemistry/transport model to predict (CO)-C-14 temporal and spatial distributions, keeping in mind the importance of reasonable simulations for validating the accuracy of atmospheric models in general. The measured (CO)-C-14 latitudinal gradient in the NH is consistent with results from previous campaigns in the southern hemisphere, which reveal lower values than predicted; vertical gradients within the troposphere are similarly exaggerated in model simulations. Differences between model output and observations, which are of the order of 25% and the likely sources of error are discussed. We also examine the difference in (CO)-C-14 between the hemispheres and estimate this to be between 15 and 25%, with higher levels in the NH.