Interhemispheric asymmetry in OH abundance inferred from measurements of atmospheric ¹⁴CO

Abstract

THE hydroxyl radical, OH, is the chief oxidizing agent in the atmosphere, and is responsible for removing many natural and anthropogenic trace gases¹. At present, OH cannot be measured directly with sufficient accuracy over the spatial and temporal scales needed for global models of atmospheric chemistry². Consequently, estimates of atmospheric OH abundance rely on a combination of models incorporating OH chemistry and observations of trace gases sensitive to OH. ¹⁴CO is an important diagnostic of OH abundance^3–6. It is produced in the atmosphere mainly by the immediate oxidation of ¹⁴C produced by cosmic radiation, and it is subsequently removed more slowly through oxidation to ¹⁴CO₂ by hydroxyl radicals⁷. The mean lifetime of¹⁴CO in clean air during summer in the middle and low latitudes is about one month, which makes ¹⁴CO a more sensitive indicator of OH than the longer-lived trace gases commonly used. Until now, only a few Northern Hemisphere ¹⁴CO determinations have been published^3,8. Using accelerator mass spectrometry we present here an extensive set of ¹⁴CO data in New Zealand and several new Northern Hemisphere results. We find that Southern Hemisphere¹⁴CO concentrations are ∼40% lower than at comparable latitudes in the Northern Hemisphere. Such a large difference is surprising because the dominant sources and sinks are believed to be similar in both hemispheres. Although there are several complicating factors, from our results we suggest that OH abundances may be significantly higher in the Southern Hemisphere than in the Northern Hemisphere, in contrast to predictions using current photochemical models.