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Hot on the Trail of the "Atmospheric Detergent"

Results of the first long-term measurements on the atmosphere's self-purifying capacity published in "Nature"

[10. Juli 2006]

Jülich, 12 July 2006 - The atmosphere regulates its self-purification much more efficiently than previously believed. This is what scientists at Research Centre Jülich and the German Meteorological Service (DWD) report on in the latest edition of the science journal "Nature". They spent five years determining how much of the most important "detergent" is present in the atmosphere: namely, the hydroxyl radical (OH radical). This is a highly reactive molecule, which sets the degradation of pollutants in motion and is also consumed in the process. Fluctuating amounts of pollutants very rarely have any effect on how much of the "detergent" is contained in the air according to scientists' recent findings. The only influence that can be measured is sunshine.

"The OH radical plays a part in thousands of reactions in the atmosphere, reactions that can either create it or destroy it. We had therefore expected that variations in the amount of pollutants present would strongly determine the OH concentration, i.e. with higher concentrations of pollutants, we expected to measure little OH", says Franz Rohrer from the Jülich Institute of Chemistry and Dynamics of the Geosphere. Researchers were therefore even more surprised to learn that the amount of detergent present is actually dependant on the intensity of the sunshine. "However, this does not mean that the sun is solely responsible for regulating OH concentration", explains Rohrer. "The OH radical itself appears to influence its chemical surroundings to such an extent that we simply don't see the other influences."

In climate models, researchers are currently taking the most varied processes and parameters into account in order to be able to calculate the OH concentration and thus the degradation of pollutants. In principle, every single one of these processes has been carefully examined. However, their hitherto assumed interaction has actually been a distorted picture of reality, as revealed by the new data. "We produced the best results, when we used solar radiation alone to predict the OH concentration", reports Rohrer. "Now we must find out what the complex interaction behind the simple relationship looks like: do the well-known OH-radical reactions influence each other in a different manner than previously assumed, or are there still some processes that we do not yet know of?"

Answers to these questions can be provided by calculation models on supercomputers such as the Jülich JUMP, as well as by further long-term measurements. "We do have data from other regions but the measurements carried out there last only for a maximum of a few weeks", says Harald Berresheim of DWD. In DWD's meteorological observatory in Hohenpeißenberg in South Germany, researchers have been measuring the OH concentration on a continuous basis since 1999. "This is globally unique", contends Berresheim. The difficulty here is that OH radicals react almost immediately with every other molecule: they are created by solar radiation, remain stable for less than one second and then remain present only as minute traces in the air. In order to measure these trace amounts, the scientists built an extremely sensitive analysis system in Hohenpeißenberg, whose reliable operation over such a long period of time was a challenge in itself. Supplementarymeasurements elsewhere should show whether the quantity of OH is dependant to the same extent on sunshine in ecosystems such as the ocean or extensive forest areas. This will be made easier by the modelling of the central molecule in atmospheric chemistry.

Alongside the simple dependency of the OH radical, the long-term measurements also revealed a second outcome. "For years, researchers have been discussing whether the atmosphere has become incapable of withstanding the globally rising air pollution and whether the amount of OH radicals is dropping as a result", says Rohrer. "Fortunately, we have not yet uncovered any evidence to support this".


Meteorological station of the German Meteorological Service in Hohenpeißengberg (Bayern). Here, researchers have been measuring the concentration of the atmospheric "detergent", the OH radical, on a continuous basis since 1999

Foto: Deutscher Wetterdienst

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Dr. Barbara Schunk
Science Journalist
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Dr. Angela Lindner
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Research Centre Jülich
52425 Jülich, Germany
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