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Strong correlation between levels of tropospheric hydroxyl radicals and solar ultraviolet radiation


The most important chemical cleaning agent of the atmosphere is the hydroxyl radical1,2, OH. It determines the oxidizing power of the atmosphere, and thereby controls the removal of nearly all gaseous atmospheric pollutants3,4. The atmospheric supply of OH is limited, however, and could be overcome by consumption due to increasing pollution and climate change4,5,6, with detrimental feedback effects. To date, the high variability of OH concentrations has prevented the use of local observations to monitor possible trends in the concentration of this species. Here we present and analyse long-term measurements of atmospheric OH concentrations, which were taken between 1999 and 2003 at the Meteorological Observatory Hohenpeissenberg in southern Germany. We find that the concentration of OH can be described by a surprisingly linear dependence on solar ultraviolet radiation throughout the measurement period, despite the fact that OH concentrations are influenced by thousands of reactants. A detailed numerical model of atmospheric reactions and measured trace gas concentrations indicates that the observed correlation results from compensations between individual processes affecting OH, but that a full understanding of these interactions may not be possible on the basis of our current knowledge of atmospheric chemistry. As a consequence of the stable relationship between OH concentrations and ultraviolet radiation that we observe, we infer that there is no long-term trend in the level of OH in the Hohenpeissenberg data set.

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Figure 1: Correlation of measured OH concentrations with simultaneously observed ozone photolysis frequencies, J (O 1 D).
Figure 2: Monthly averages of simultaneous observations of OH and J (O 1 D) measured in different years at MOHp.
Figure 3: Variance analysis of the 30-s time-resolved OH data measured at MOHp for 1999–2003.
Figure 4: Bimonthly averages of destruction and production processes of OH at MOHp.


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We thank F. Eisele and D. Tanner for help in setting up the CIMS system at MOHp, T. Elste and G. Stange for OH measurements, and E. Tensing for J (O1D) measurements, D. H. Ehhalt, A. Wahner and C. Plass-Dülmer for discussions, the GAW team at MOHp for the ancillary data, and DWD/BMVBS for financial support.

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Correspondence to Franz Rohrer.

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Reprints and permissions information is available at The authors declare no competing financial interests. Requests for the Hohenpeissenberg data sets should be addressed to H.B. (

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Rohrer, F., Berresheim, H. Strong correlation between levels of tropospheric hydroxyl radicals and solar ultraviolet radiation. Nature 442, 184–187 (2006).

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