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Superwash in the Atmosphere

"Science": Research team find accelerated pollutant degradation in the air over Southern China

[4. Juni 2009]

Jülich, 4 June 2009 - A previously unknown amplification mechanism increases the self-purifying capacity of the atmosphere three- to fivefold. An international research team, including scientists from Jülich headed by Prof. Andreas Wahner, have discovered that hydroxyl radicals - the detergent of the atmosphere - can be recycled without the participation of nitrogen oxide. The results were presented today in an online advance publication in "Science Express".

From ten o'clock in the morning, atmospheric degradation in the extremely polluted air above the Pearl River Delta in Southern China goes into top gear. Together with Chinese and Japanese colleagues and in close cooperation with Peking University, Jülich scientists performed extensive measurements there in July 2006, recording the concentrations of all input and output substances involved in the "washing" process in the course of a day. From this time of day onwards, they regularly found an elevated concentration of the hydroxyl radical (OH radical) known as the "atmospheric detergent". The scientists’ measurements show that a previously unknown amplification mechanism must be responsible for this increased concentration, which - in contrast to the familiar mechanism - generates little ozone.

The first surprise for the researchers after evaluating the measurement series was the relatively high concentration of OH radicals of 15 million molecules per cubic centimetre at about midday and the associated high degradation rate for pollutants. The second surprise was that the obvious assumption that the consumed OH radicals would be immediately recycled by the presence of a correspondingly high concentration of nitrogen oxide, as is usually the case, proved to be false. The measured nitrogen oxide concentration was much too low.

Hydroxyl radicals (OH radicals) are highly reactive compounds consisting of an oxygen and a hydrogen atom. They react with most atmospheric pollutants such as nitrogen oxides, carbon monoxide or hydrocarbons and convert them into a water-soluble form. These substances are then washed out of the atmosphere with the next shower of rain. OH radicals are formed in the lower layer of the atmosphere, amongst other processes by the splitting of oxygen. The necessary energy is provided by sunlight. However, the decisive factor for maintaining a high concentration of OH radicals in the atmosphere is the fact that after they have reacted with a pollutant molecule the radicals are usually recycled in a chemical cycle and are thus available for further "washes". The necessary condition for this is, however, a sufficiently high concentration of nitrogen oxide in the air.

On the basis of the data, the Jülich team headed by Dr. Andreas Hofzumahaus and Dr. Franz Rohrer were able to demonstrate for the first time by direct measurements of the entire OH radical cycle that a previously unknown formation mechanism was responsible for the high OH concentration.Further analysis with the aid of simulations showed that an ingredient must be present in the air above the Pearl River Delta which amplifies the cycle of OH radicals several times over. Inexplicably high concentrations of OH radicals and at the same time low concentrations of nitrogen oxide have also already been found in rural wooded regions in North America and in the tropical rainforest of Suriname. The unknown mechanism will now be explored by experiments in the Jülich atmosphere simulation chamber, SAPHIR, and in further measuring campaigns.

The discovery has an important consequence for researchers with respect to the predictability of ground-level ozone concentrations. According to present knowledge, ozone is formed in the lower layer of the atmosphere as a secondary product of the above-mentioned OH cycle with the participation of nitrogen oxide. It was previously assumed that for each degraded pollutant molecule one or two ozone molecules would be formed. This is not longer acceptable - at least until we have gained an understanding of the newly discovered recycling mechanism.


Amplified trace gas removal in the troposphere
Andreas Hofzumahaus, Franz Rohrer, Keding Lu, Birger Bohn, Theo Brauers, Chih-Chung Chang, Hendrik Fuchs, Frank Holland, Kazuyuki Kita, Yutaka Kondo, Xin Li, Shengrong Lou, Min Shao, Limin Zeng, Andreas Wahner, Yuanhang Zhang

Further Information: More Photos


The preparations for the campaign are under way: the arrival of the Jülich measuring instruments in Back Garden Guangzhou, 50 kilometres northwest of the megacity.


Downtown Guangzhou – source of the pollution

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