Using atmospheric 14CO to provide additional constraints for global OH: first results from a new approach and potential for future measurements

The primary source of ¹⁴C-containing carbon monoxide (¹⁴CO) in the atmosphere is via ¹⁴C production from ¹⁴N by secondary cosmic rays, and the primary sink is removal by hydroxyl radicals (OH). Variations in the global abundance of ¹⁴CO that are not explained by variations in ¹⁴C production are mainly driven by variations in the global abundance of OH. Monitoring OH variability via methyl chloroform is becoming increasingly difficult as methyl chloroform abundance is continuing to decline. Measurements of atmospheric ¹⁴CO have previously been successfully used to infer OH variability. However, these measurements are currently only continuing at one location (Baring Head, New Zealand), which is insufficient to infer global trends. We propose to restart global ¹⁴CO monitoring with the aim of providing an additional constraint on OH variability. A new analytical system for ¹⁴CO sampling and measurements has been developed, allowing for a ten-fold reduction in the required sample air volumes and simplified field logistics. The first ¹⁴CO measurements from Mauna Loa Observatory show good agreement with prior measurements in the same latitude band. Preliminary work with a state-of-the-art chemical transport model is exploring sensitivity of ¹⁴CO at potential sampling locations to changes in production rates and OH.