Observational evidence for interhemispheric hydroxyl-radical parity

Observations of methyl chloroform combined with an atmospheric transport model predict a Northern to Southern Hemisphere hydroxyl ratio of slightly less than 1, whereas commonly used atmospheric chemistry models predict ratios 15–45% higher. The hydroxyl radical is an important atmospheric oxidant, but our knowledge of its global distribution remains imprecise, with estimates for the ratio of Northern Hemisphere to Southern Hemisphere hydroxyl radical concentration varying from 0.85 to 1.4. These authors use a three-dimensional chemistry-transport model that has been well validated for interhemispheric transport using sulphur hexafluoride measurements, to obtain an interhemispheric hydroxyl radical ratio of 0.97±0.12. This information can help improve our understanding of the fate of atmospheric pollutants and greenhouse gases. The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere1,2,3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane4,5,6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004–2011 the model predicts an annual mean NH–SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns11,12,13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.