Newly Discovered Stratosphere Hole Sucks Pollutants
An international team of researchers headed by Potsdam scientist Dr. Markus Rex from the Alfred Wegener Institute has discovered a previously unknown atmospheric phenomenon over the South Seas. Over the tropical West Pacific there is a natural, invisible hole extending over several thousand kilometres in a layer that prevents transport of most of the natural and manmade substances into the stratosphere by virtue of its chemical composition. Like in a giant elevator, many chemical compounds emitted at the ground pass thus unfiltered through this so-called “detergent layer” of the atmosphere. Scientists call it the “OH shield”.
The newly discovered phenomenon over the South Seas boosts ozone depletion in the polar regions and could have a significant influence on the future climate of the Earth – also because of rising air pollution in South East Asia.
Rex and his team tracked down the giant natural hole over the tropical South Seas, situated in a special layer of the lower atmosphere known as the OH shield. The research results on the newly discovered OH minimum will be published soon in the journal “Atmospheric Chemistry and Physics”, with the Institute of Environmental Physics of the University of Bremen and other international research institutions as partners.
“Even though the sky appears to be an extensively uniform space for most people, it is composed of chemically and physically very different layers,” Rex explains. The air layers near the ground contain hundreds or even thousands of chemical compounds. This is why winter and spring, mountains and sea, city and forests all have a distinct smell. The great majority of these substances are broken down into water-soluble compounds in the lower kilometres of the atmosphere and are subsequently washed out by rain. Since these processes require the presence of a certain chemical substance, the so called hydroxyl (=OH) radical, this part of the atmosphere is called the OH shield. It acts like a huge atmospheric washing machine in which OH is the detergent.
The OH shield is part of the troposphere, as the lower part of the atmosphere is called. “Only a few, extremely long-lived compounds manage to make their way through the OH shield,” says Rex. These means they can have global impact because once they have reached the stratosphere, their degradation products remain up there for many years and spread over the entire globe.
Extremely long-lived chemical compounds find their way to the stratosphere, even where the OH shield is intact. These include methane, nitrous oxide, halons, methyl bromide and chlorofluorocarbons (CFCs), which are notorious as ozone killers because they play a major role in ozone depletion in the polar regions.
After many years of research scientists now understand the complicated process of stratospheric ozone depletion very well. “Nevertheless measured ozone depletion rates were often quite a bit larger than theoretically calculated in our models,” Rex points out a long unsolved problem of atmospheric research. “Through the discovery of the OH hole over the tropical West Pacific we have now presumably made a contribution to solving this puzzle.”
“We have to realize that chemical compounds which enter the stratosphere always have a global impact,” says Rex. Thanks to the OH hole, greater amounts of brominated hydrocarbons can reach the stratosphere than in other parts of the world. Although their ascent takes place over the tropical West Pacific, these compounds amplify ozone depletion in the polar regions.
However, it is not only brominated hydrocarbons that enter the stratosphere over the tropical West Pacific. “You can imagine this region as a giant elevator to the stratosphere,” states Rex. Other substances, too, rise here to a yet unknown extent while they are intercepted to a larger extent in the OH shield elsewhere on the globe. One example is sulphur dioxide, which has a significant impact on the climate.
Sulphur particles in the stratosphere reflect sunlight and therefore act antagonistically to atmospheric greenhouse gases like CO2, which capture the heat of the sun on the Earth. Whereas greenhouse gases in the atmosphere heat the globe, sulphur particles in the stratosphere have a cooling effect. “South East Asia is developing rapidly in economic terms,” Rex explains. “Contrary to most industrial nations, however, little has been invested in filter technology up to now. That is why sulphur dioxide emissions are increasing substantially in this region at present.”
If one takes into account that sulphur dioxide may also reach the stratosphere via the OH hole over the tropical West Pacific, it quickly becomes obvious that the atmospheric elevator over the South Seas not only boosts ozone depletion, but may influence the climate of the entire Earth, he says. In fact, the aerosol layer in the stratosphere, which is also composed of sulphur particles, seems to have become thicker in recent years. Researchers do not know yet whether there is a connection here.
But wouldn’t it be a stroke of luck if air pollutants from South East Asia were able to mitigate climate warming? “By no means,” says Rex. “The OH hole over the South Seas is, above all, further evidence of how complex climate processes are. And we are still a long way off from being in a position to assess the consequences of increased sulphur input into the stratosphere. Therefore, we should make every effort to understand the processes in the atmosphere as best we can and avoid any form of conscious or unconscious manipulation that would have an unknown outcome.”