Like a giant elevator to the stratosphere
In tropical thunderstorms over the West Pacific air masses and the chemical substances they contain are quickly hurled upward to the edge of the stratosphere. If there are sufficient OH molecules in the atmosphere, the air is extensively cleaned by chemical transformation processes. Where OH concentrations are low, such as those now found in large sections of the tropical West Pacific, the cleaning capacity of the atmosphere is reduced.
Photo: Markus Rex, Alfred Wegener Institute
Photo: Markus Rex, Alfred Wegener Institute
5
April, 2014
Recent
research results show that an atmospheric hole over the tropical
West Pacific is reinforcing ozone depletion in the polar regions and
could have a significant influence on the climate of the
Earth.
Potsdam, 3 April 2014. 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.
Potsdam, 3 April 2014. 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.
At
first Dr. Markus Rex suspected a series of flawed measurements. In
October 2009 the atmospheric physicist from the Alfred Wegener
Institute, Helmholtz Centre for Polar and Marine Research (AWI) was
on board the German research vessel “Sonne” to measure trace
substances in the atmosphere in the tropical West Pacific
|
Tried
and tested a thousand times over, the ozone probes he sent up into
the tropical sky with a research balloon every 400 kilometres
reported – nothing.
Or to be more accurate: almost nothing. The ozone concentrations in his measurements remained nearly constantly below the detection limit of approx. 10 ppbv* in the entire vertical range from the surface of the Earth to an altitude of around 15 kilometres. Normally ozone concentrations in this part of the atmosphere are three to ten times higher.
Although low values at an altitude of around 15 kilometres were known from earlier measurements in the peripheral area of the tropical West Pacific, the complete absence of ozone at all heights was surprising. However, after a short period of doubt and various tests of the instruments it dawned on the worldwide recognised ozone specialist that he might be onto a phenomenon yet unknown to science. A few research years later and after the involvement of other colleagues came confirmation: Markus Rex and his team on board the “Sonne” had tracked down a 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.
Or to be more accurate: almost nothing. The ozone concentrations in his measurements remained nearly constantly below the detection limit of approx. 10 ppbv* in the entire vertical range from the surface of the Earth to an altitude of around 15 kilometres. Normally ozone concentrations in this part of the atmosphere are three to ten times higher.
Although low values at an altitude of around 15 kilometres were known from earlier measurements in the peripheral area of the tropical West Pacific, the complete absence of ozone at all heights was surprising. However, after a short period of doubt and various tests of the instruments it dawned on the worldwide recognised ozone specialist that he might be onto a phenomenon yet unknown to science. A few research years later and after the involvement of other colleagues came confirmation: Markus Rex and his team on board the “Sonne” had tracked down a 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.
Nearly
all chemical substances produced by people, animals, plants,
algae or microorganisms on the ground or in the oceans react
quickly with OH and break down in this process. During
this chemical self-cleaning process substances that are
not easily
water-soluble are transformed into water-soluble products and then washed out by precipitation. Through this mechanism OH molecules remove most substances from the atmosphere. The OH molecule is therefore also called the detergent of the atmosphere. Only extremely long-lived chemical compounds, such as methane or CFCs, also known as "ozone killers", can rise through the OH shield into the stratosphere. Graphics: Markus Rex, Alfred Wegener Institute |
“Even
though the sky appears to be an extensively uniform space for most
people, it is composed of chemically and physically very different
layers,” Markus Rex explains the complex makeup of the
atmosphere.
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, “then they also get through the tropopause and enter the stratosphere.” Tropopause refers to the boundary layer between the troposphere and the next atmospheric layer above it, the stratosphere. Particularly substances that enter the stratosphere unfold a global impact. The reason for this is that 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 (“laughing gas”), 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.
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, “then they also get through the tropopause and enter the stratosphere.” Tropopause refers to the boundary layer between the troposphere and the next atmospheric layer above it, the stratosphere. Particularly substances that enter the stratosphere unfold a global impact. The reason for this is that 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 (“laughing gas”), 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.
Location
and extent of low ozone concentrations and thus of the
OH hole over the West Pacific. Fig. (a) shows the region
of origin of the air in the stratosphere, Fig. (b) ozone
sonde measurements (dots) and satellite measurements (coloured map) of the total amount of ozone in the tropospheric column of air and Fig. (c) the total amount of OH in the tropospheric column of air calculated with a model. Graphics: Markus Rex, Alfred Wegener Institute. |
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,” Markus 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.”
And at the same time discovered a phenomenon that raises a number of new questions for climate policy.
Researchers are now tackling these questions in a new research project funded by the EU with around 9 million euros, i.e. “StratoClim”, which is coordinated by the Alfred Wegener Institute. Within this project a new monitoring station will be established in the tropical Westpacific, together with the Institute of Environmental Physics at the University of Bremen.
“We have to realise,” reminds the Potsdam atmospheric physicist, “that chemical compounds which enter the stratosphere always have a global impact.” Thanks to the OH hole that the researchers discovered over the tropical Pacific, 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. Since scientists identified this phenomenon and took it into account in the modelling of stratospheric ozone depletion, their models have corresponded excellently with the actually measured data.
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 Markus Rex using an apt comparison. 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. To put it simply, 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,” Markus Rex explains a problem given little attention to date. “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.”
“Nevertheless measured ozone depletion rates were often quite a bit larger than theoretically calculated in our models,” Markus 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.”
And at the same time discovered a phenomenon that raises a number of new questions for climate policy.
Researchers are now tackling these questions in a new research project funded by the EU with around 9 million euros, i.e. “StratoClim”, which is coordinated by the Alfred Wegener Institute. Within this project a new monitoring station will be established in the tropical Westpacific, together with the Institute of Environmental Physics at the University of Bremen.
“We have to realise,” reminds the Potsdam atmospheric physicist, “that chemical compounds which enter the stratosphere always have a global impact.” Thanks to the OH hole that the researchers discovered over the tropical Pacific, 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. Since scientists identified this phenomenon and took it into account in the modelling of stratospheric ozone depletion, their models have corresponded excellently with the actually measured data.
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 Markus Rex using an apt comparison. 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. To put it simply, 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,” Markus Rex explains a problem given little attention to date. “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.”
This
is how air reaches the stratosphere. Through the rapid upward
transport in tropical thunderstorms they reach an area of
slow large-scale ascent and rise from there through
the tropopause into the stratosphere over the course of
weeks. This process is most pronounced during northern
hemispheric winter. Model calculations show that, during
this season, this process mainly takes place over the
tropical West Pacific. Due to the formation of cirrus (=
ice) clouds in the extremely cold tropical tropopause, a
large portion of the water-soluble chemical substances
is removed from the air and cannot reach the
stratosphere. OH molecules transform water-insoluble
into water-soluble compounds. Hence, if the
concentration of OH molecules along the dotted transport
pathways shown above is high only few chemical compounds
make it into the stratosphere. Conversely, the lower the
OH concentration is along the transport pathways, the
more chemical compounds enter the
stratosphere.
Graphics: Yves Nowak, Alfred Wegener Institute. |
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. 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,” Markus Rex vigorously shakes his head. “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.”
Background:
Why is there an OH hole over the West Pacific?
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,” Markus Rex vigorously shakes his head. “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.”
Background:
Why is there an OH hole over the West Pacific?
The air in the tropical West Pacific is extremely clean. Air masses in this area were transported across the expanse of the huge Pacific with the trade winds and for a long time no longer had contact with forests or other land ecosystems that produce innumerable short-lived hydrocarbons and release them into the air. Under these clean air conditions OH is formed from ozone through chemical transformation to a great degree. If there is hardly any ozone in the lower atmosphere (= troposphere), as is the case in the West Pacific, only little OH can be formed. The result is an OH hole.
The
graph shows ozone profiles measured in three different marine
regions: the tropical Atlantic, the tropical West Pacific and
the West Pacific outside the tropics. The red curve
clearly shows that ozone is consistently very low up to
an altitude of 15 kilometres over the tropical West
Pacific. In the other regions the ozone concentrations
are in a range typical
for the troposphere. Graphics: Markus Rex, Alfred Wegener Institute |
Ozone,
in turn, forms in the lower atmosphere only if there are sufficient
nitrogen oxides there. Large amounts of nitrogen oxide compounds are
produced in particular by intensive lightning over land.
However, the air masses in the tropical West Pacific were not exposed to any continental tropical storms for a very long time during their transport across the giant ocean. And the lightning activity in storms over the ocean is relatively small. At the same time the lifetime of atmospheric ozone is short due to the exceptionally warm and moist conditions in the tropical West Pacific. In this South Sea region the surface temperatures of the ocean are higher than anywhere else on our planet, which makes the air not only quite warm, but also quite moist.
The ozone is thus quickly lost, especially directly above the water. And due to the lack of nitrogen oxide compounds little ozone is subsequently formed. Rapid vertical mixing in the convection areas that exist everywhere over the warm ocean and in which the warm air rises takes care of the rest. Finally, there is no more ozone in the entire column of air in the troposphere. And without ozone (see above) the formation of OH is suppressed.
What impact does the OH hole over the West Pacific have?
However, the air masses in the tropical West Pacific were not exposed to any continental tropical storms for a very long time during their transport across the giant ocean. And the lightning activity in storms over the ocean is relatively small. At the same time the lifetime of atmospheric ozone is short due to the exceptionally warm and moist conditions in the tropical West Pacific. In this South Sea region the surface temperatures of the ocean are higher than anywhere else on our planet, which makes the air not only quite warm, but also quite moist.
The ozone is thus quickly lost, especially directly above the water. And due to the lack of nitrogen oxide compounds little ozone is subsequently formed. Rapid vertical mixing in the convection areas that exist everywhere over the warm ocean and in which the warm air rises takes care of the rest. Finally, there is no more ozone in the entire column of air in the troposphere. And without ozone (see above) the formation of OH is suppressed.
What impact does the OH hole over the West Pacific have?
The
illustration shows the average lifetime of sulphur dioxide and
some brominated hydrocarbons for normal conditions over
the tropical Atlantic and for conditions of
reduced
OH-concentrations over the tropical West Pacific. Graphics: Markus Rex, Alfred Wegener Institute |
The
OH molecule is also called the detergent of the atmosphere. Nearly
all of the thousands of different chemical substances produced by
people, animals, plants, fungi, algae or microorganisms on the
ground or in the oceans react quickly with OH and break down in this
process. Therefore, virtually none of these substances rises into
the stratosphere. In the area of the OH hole, however, a larger
portion of this varied chemical mix can enter the stratosphere.
And local emissions may unfold a global impact, especially if they make it to the stratosphere. There they spread globally and can influence the composition of the air for many years – with far-reaching consequences for ozone chemistry, aerosol formation and climate.
Why wasn’t the OH hole discovered earlier?
And local emissions may unfold a global impact, especially if they make it to the stratosphere. There they spread globally and can influence the composition of the air for many years – with far-reaching consequences for ozone chemistry, aerosol formation and climate.
Why wasn’t the OH hole discovered earlier?
The tropical West Pacific is one of the most remote regions on our planet. That is why extensive measurements of the air composition have yet to take place in this area. There is also a considerable gap in the otherwise dense network of global ozone measurement stations here. Even in the past measurements from the peripheral sections of the now investigated region showed minimal ozone values in the area of the upper troposphere, but not the consistently low values that have now been found across the entire depth of the troposphere. The newly discovered phenomenon reveals itself in its full scope only through the measurements that were conducted to such an extensive degree for the first time and was thus not able to be grasped at all previously.
*One part of ozone per billion by volume (ppbv) means there is one ozone molecule for every billion air molecules.
Press
release from the Alfred
Wegener Institute,
which conducts research in the Arctic, Antarctic and oceans of the
high and mid-latitudes. It coordinates polar research in Germany and
provides major infrastructure to the international scientific
community, such as the research icebreaker Polarstern and stations
in the Arctic and Antarctica. The Alfred Wegener Institute is one of
the 18 research centres of the Helmholtz Association, the largest
scientific organisation in Germany.
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