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.

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We thank the HIPPO science team and the crew and support staff at the NCAR Research Aviation Facility, and all the laboratory staff working for AGAGE and NOAA measurement networks. This work is partly supported by the Japan Society for the Promotion of Science/Grants-in-Aid for Scientific Research (KAKENHI) Kiban-A (grant no. 22241008) and Ministry of Education, Culture, Sports, Science and Technology (MEXT) Arctic GRENE projects. NCAR is sponsored by the National Science Foundation (NSF). HIPPO was supported by NSF grants ATM-0628575, ATM-0628519, ATM-0628388 ATM-0628452 and ATM-1036399, by NASA award NNX11AF36G, and by NCAR. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF, NOAA or NASA. M.C.K. is supported by EU FP7 project PEGASOS. AGAGE is supported principally by NASA grants to Massachusetts Institute of Technology (NNX11AF17G) and Scripps Institution of Oceanography (NNX11AF16G) and also by NOAA and the CSIRO. Mace Head is supported by the Department of Energy and Climate Change, award GA0201. We thank the CSIRO Oceans and Atmosphere Flagship and the Bureau of Meteorology for Cape Grim project funding. NOAA flask measurements are supported in part by NOAA’s Climate Program Office and its Atmospheric, Chemistry, Carbon Cycle and Climate Program.

Author information


  1. Department of Environmental Geochemical Cycle Research, JAMSTEC, Yokohama 236 0001, Japan

    • P. K. Patra
    • , A. Ghosh
    • , K. Ishijima
    • , K. Miyazaki
    •  & M. Takigawa
  2. CAOS, Graduate School of Studies, Tohoku University, Sendai 980 8578, Japan

    • P. K. Patra
  3. Wageningen University, Droevendaalsesteeg 3a, 6708 PB, The Netherlands

    • M. C. Krol
  4. National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory, Boulder, Colorado 80305, USA

    • S. A. Montzka
    • , J. W. Elkins
    • , E. J. Hintsa
    • , D. F. Hurst
    • , B. R. Miller
    •  & F. L. Moore
  5. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA

    • T. Arnold
    • , J. Mühle
    •  & R. F. Weiss
  6. The Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA

    • E. L. Atlas
  7. Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901, USA

    • B. R. Lintner
  8. National Center for Atmospheric Research (NCAR), Boulder, Colorado 80301, USA

    • B. B. Stephens
  9. School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 02138, USA

    • B. Xiang
    •  & S. C. Wofsy
  10. Centre for Australian Weather and Climate Research, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere Flagship, Aspendale, Victoria 3195, Australia

    • P. J. Fraser
    • , P. B. Krummel
    •  & L. P. Steele
  11. National Institute of Polar Research, 10-3, Midoricho, Tachikawa, Tokyo 190-8518, Japan

    • A. Ghosh
  12. CIRES, University of Colorado, Boulder, Colorado 80309, USA

    • E. J. Hintsa
    • , D. F. Hurst
    • , B. R. Miller
    •  & F. L. Moore
  13. School of Chemistry, University of Bristol, Cantock’s Close, BS8 1TS, UK

    • S. O’Doherty
    •  & D. Young
  14. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • R. G. Prinn
  15. School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • H. J. Wang


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P.K.P., M.K., S.A.M., B.X., B.B.S., B.R.L., T.A. and A.G. designed the model experiments and performed data analysis. T.A., E.L.A., S.A.M., B.B.S., J.W.E., P.J.F., E.J.H., D.F.H., P.B.K., B.R.M., F.L.M., J.M., S.O.D., R.G.P., L.P.S., H.J.W., R.F.W., S.C.W. and D.Y. conducted measurements. All co-authors participated in writing the manuscript and contributed through discussions.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to P. K. Patra.

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