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ePaper created 2012-09-28, 09:46:25 | version 1.24.0
112|2012 Research in Jülich Cycle of Radicals Hydroxyl radicals (OH radicals) are formed in sunlight from ozone mol- ecules. Other radicals known as HO2 radicals are formed by the split- ting of formaldehyde. These “detergents of the atmosphere” are short- lived and are present only in minute amounts in the atmosphere. Yet they are a decisive factor in determining the length of time that impor- tant trace gases (hydrocarbons, carbon monoxide, nitrogen oxides) re- main in the lower atmospheric layers. The sooner trace gas molecules are degraded in reactions with radicals, the less they spread. However, the degradation of trace gases produces secondary pollutants such as ozone. On the other hand, measurements by Jülich scientists in China have revealed that there could be other degradation processes in which few ozone molecules are formed. The graphic shows the most impor- tant known chemical processes involved in the degradation of trace gases by OH radicals and HO2 radicals, which are converted into each other during the cycle in a matter of seconds. In each case, a carbon monoxide or a hydrocarbon molecule is oxidized. Depending on the level of nitrogen oxide pollution, an ozone molecule is either created or destroyed. When this cycle has been repeated around ten times, the radicals then disappear forming nitric acid (HNO3) and hydrogen peroxide (H2O2) and are no longer part of the “detergent cycle”. HCHO UV light UV light UV light O2 CH4 OH HO2 HO2 H2 O2 H2 O NO2 O3 HNO3 NO2 NO O3 O3 CO O2 ENGINEERING GENIUS AT LOFTY HEIGHTS To achieve this, all instruments on the ground and on-board must run like clock- work. The engineers and scientists there- fore also keep an eye even on tiny flying objects. “Insects can block the measur- ing nozzles,” says Dahlhoff, explaining the situation. “For this reason, we have installed a camera to monitor the nozzles on the Zeppelin’s top platform.” This is also because the platform is very difficult to reach, even when the Zeppelin has landed and is anchored. It floats around twenty metres above the ground with all of its scientific instruments. Such cir- cumstances made it necessary to devel- op a new calibration tool. In the labora- tory, a device weighing around five kilograms with a height of one metre is used to calibrate the main instruments measuring the OH radicals. The solution for the Zeppelin is much more refined: a slender remote-controlled robot arm swivels across the measuring nozzles and discharges air with a known OH con- centration. “This procedure is performed regularly to ensure that the values of the different measurements remain compa- rable,” says Hofzumahaus. To ensure that the robot arm does not cause any prob- lems during the flights, the calibration source and the robot arm disappear in- to a sort of garage. “The source must first be positioned with an accuracy of 0.1 millimetres above the measuring probe,” says Dahlhoff. And this too is a task that the engineers have mastered. :: OH radicals emit light when excited with a laser. COVER STORY | PEGASOS PEGASOS-Blog Take-Off for PEGASOS