Ozone depletion
The ozone hole is a strong thinning of the ozone layer over Antarctica in particular. Cause of ozone depletion are mainly chlorine atoms from chlorofluorocarbons (CFCs ). The weakened ozone layer allows more of the UV -B component of solar radiation by the ground. This has negative consequences for many living things.
The ozone hole occurs since the early 1980s on an annual basis. Within a few weeks after the sunrise breaks the ozone concentration and recovered completely within a few months. Cause of this dynamic is the storage and conversion of pollutants on the ice crystals stratospheric clouds that evaporate after the long, cold polar night.
The depth of the slump developed within a few years from a few percent to more than fifty percent. It affects the entire polar vortex, an area of several million square kilometers, as the then incipient Remote Sensing illustrated impressively. The dramatic development and the unequivocal scientific evidence of causes led quickly to a global ban of CFCs, agreed in 1987 by the Montreal Protocol, a highlight of the environmental movement.
As early as 1974 warned the physical chemist Mario J. Molina and Frank Sherwood Rowland, the enrichment of poorly degradable CFCs in the atmosphere would eventually lead to a significant decrease in ozone concentration, worldwide and throughout the year - the ozone hole nobody had foreseen. They got together with the atmospheric chemist Paul Crutzen the Nobel Prize for Chemistry in 1995 for the elucidation of the mechanism of the ozone hole.
The photolysis of CFC molecules by UV -C not only releases the harmful halogen atoms, but also initiates the complete degradation of the CFC molecule. However, the lifetime of most CFC compounds is by this process for many decades, because the UV-C radiation from the sun does not reach most of the atmosphere. Therefore, the ozone hole will close until the second half of the 21st century.
Research history of ozone depletion
The fact that more ozone in the upper atmosphere must be present as was chemically demonstrated on the ground, joined Walter Noel Hartley in 1881 from the comparison of the absorption spectrum of ozone in the UV -B range ( Hartley band) there with the steep spectrum of the sun. Quantitative measurements were made around 1930 with the Dobson spectrophotometer routine. At the same time Sydney Chapman developed a first reaction mechanism that explained the relative stability of the ozone layer, the ozone - oxygen cycle.
After that stratospheric ozone is formed by ultraviolet radiation from the sun as well as split. The cleaved oxygen atom usually reconnects with an oxygen molecule to form ozone. Here, a loss occurs due to occasional reactions between oxygen atoms and ozone, which limits the increase in the ozone concentration.
This model predicted correctly predicted that in the height range of the UV-C absorption (30-60 km ), the ozone concentration decreases with air density, which was confirmed in the early 1970s with sounding rockets. The reason is that the reaction of the oxygen atom, which leads back to ozone,
Actually an intermolecular reaction is: There is a third collision partner M needed, which removes the binding energy as kinetic energy:
The smaller ( the height) of the concentration of M is, the longer is the oxygen atom for the loss reactions.
The obvious flaw of this model in 1930 was that after him the ozone layer should be thicker by a factor of 3 than it is. Searched for other loss processes. First HOx (HO, HO2) was blamed. Therein comes the H atom from compounds that are degraded slowly enough in the troposphere that they go up to the stratosphere: H2, H2O and CH4. In the lower stratosphere where UV -C is not sufficient, they are by singlet oxygen, O ( 1D) cracked, eg
O (1D ), the electronically excited oxygen atom that results in the photolysis of ozone,
But the runs at each impact the risk of being de-energized to an O atom in the electronic ground state, eg,
The excitation energy is converted into kinetic energy of the reaction products, thus to heat.
In the upper stratosphere, these compounds are of UV-C to be cleaved, for example,
And then, in rapid succession,
The resulting hydroperoxyl radical, HOO •, start from now on O atoms that would otherwise form ozone:
Without consuming this is because HO • is under further ozone depletion again HOO •:
This catalytic cycle explains a good part of the lower observed ozone column heights.
Another substance that can enter the stratosphere, where it forms radicals, is nitrous oxide, N2O. He, too, is photolyzed by UV -C or reacts with O ( 1D). They form the nitrogen oxides NO and NO2, are in equilibrium with each other:
This reaction pair is effectively a version of the photolysis of ozone is accelerated, since the visible light from the sun is much more intense than that of UV-B, which is necessary for photolysis of ozone. Nitrogen oxides contribute to ozone depletion indirectly, since the O atoms are not fully reversible ozone, see above.
Crutzen warned in 1970, before the entry of nitrous oxides by flying in the lower stratosphere supersonic aircraft, similar to the Concorde, which were then planned in large numbers.
The chemical process of ozone depletion
Chlorine atoms and other radicals X • result in additional losses, mostly according to the following catalytic cycle:
The second reaction is rate-limiting, because O atoms are even daytime comparatively scarce. Therefore, there is, for example, Chlorine in the stratosphere predominantly not as Cl •, but as ClO •. React particular, in the twilight, the (possibly different ) radicals XO • one another, eg
Also in this case, the two radicals X • be released again. For some other combinations of XO • this is not as important examples are
The resulting reaction products are called reservoir species, because in them the radicals are only temporarily bound, mainly at night while they are photolyzed during the day more or less quickly. During the long twilight of the polar night is more likely the longer lasting chlorine nitrate ClONO2, because it is UV -C necessary to divide it, whereas chlorine Cl2 enough the sun is low in much more intense UV -A.
For the strong ozone hole just in the spring of the nightly Polar Stratospheric Clouds formation of (PSC, polar stratospheric clouds ) is responsible. Usually there are no clouds in the stratosphere, since it is too dry. At the very low temperatures of the polar night, especially the southern, to below -80 ° C, but can freeze leftovers of water vapor H2O with nitric acid HNO3. The nitric acid is produced from nitrates, chlorine nitrate as above:
Where halogen compounds, as here, the hypochlorous acid HClO, remain in the gas phase and are cleaved at sunrise quickly. Suddenly there are a lot of ozone -destroying radicals. Only gradually evaporate the PSC and bring the nitrogen compounds back into the air, which, together with the chlorine radical reservoir species and thus attenuate the ozone depletion.
Appearance at the South Pole and North Pole
The strong and stable polar vortex over Antarctica is the reason for the very low temperatures in the center of the vortex, to below 190 K ( about -80 ° C). The northern polar vortex is usually not cold enough for stratospheric clouds, so that there is no significant ozone hole. The adjacent four figures above for each of the northern hemisphere, below the southern hemisphere, left the color-coded annual mean ozone column height for 1979. Before the discovery of the ozone hole, Right for 2007
According to analyzes of measurements from the year 2012 we can speak of a reversal of the ozone trends at the South Pole for the first time. The head of the meteorological observatory of the research station Neumayer III led this circumstance back among other things, the success of the worldwide ban on CFCs.
Natural halogen compounds
Sea salt (NaCl) is water soluble and will wash out of the atmosphere before reaching the stratosphere.
Plants, however, provide a measurable contribution to ozone-depleting compounds. Cruciferous plants produce methyl bromide. But the rape produces 6600 tonnes a year, this is a share of 15 percent of what is still produced industrially. Evergreen trees and potatoes, however, synthesize methylene chloride.
Also during volcanic eruptions escape halogen compounds: While hydrochloric mostly as sea salt is washed out, bromine compounds may affect the ozone layer, at least locally. During an eruption of a super volcano, it is a massive damage to the ozone layer. The last one took place 74,000 years ago.
Based on research results of environmental physicist at the University of Heidelberg, scientists suspect that the ozone layer is possibly damaged by natural chlorine, bromine and iodine compounds may be formed mainly in the coastal areas of the oceans of water plants and microorganisms. A research project is to investigate the natural sources of halogenated hydrocarbons and atmospheric transport and degradation of these ozone-depleting compounds. Meanwhile hardened studies in coastal waters and in the atmosphere above the waters in the context of a research project, these guesses.
Nitrous oxide ( " laughing gas " ) has now replaced the chlorofluorocarbons (CFCs ) as the most important source of ozone- damaging emissions of the 21st century.