Berlin Environmental Atlas

03.06 Near Ground Ozone (Edition 1993)

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Catalytic ozone decomposition

The significant ozone-decomposing effect of these trace materials is without the influence energy-rich solar radiation independent of the time of day. They stands in contrast to the incredibly low concentrations in which these materials occur in the atmosphere. The catalytic decomposition reactions sketched in Figure 3 can mean, for instance, that one CFC molecule in a billion other air particles is responsible for the destruction of several thousand ozone molecules. The ozone-decomposing elements of the trace materials in Figure 3 substitute for the different substances marked X and lie again in their original form namely at the end of a reaction chain. They can perform their destructive work repeatedly before they are removed from circulation in part only after several years by other chemical reactions.

The catalyst thereby attacks two-fold at the expense of the ozone in the chemical balance: n one hand it transforms an ozone molecule into oxygen O2 and bonds with the remaining oxygen atom. On the other hand, individual free oxygen atoms are consumed at this degeneration of ozone in oxygen, which are no longer available for ozone formation (cf. Fig. 3).

Chlorine and bromine which originate predominantly from CFC and/or the halons, as well as NO and OH radicals, work as catalysts. The latter come from steam, methane and hydrogen and originate from predominantly natural sources.

The simplicity of the current depiction belies the intense complexity and non-linearity of the aerochemical processes in the stratosphere. The different reaction cycles are coupled with each other strongly and in different ways. For instance, BrO, NO or OH can be involved in the reverse process of ClO to Cl instead of an O atom as partner. A further important role is played by heterogeneous reactions in which the ozone-decomposing materials are conveyed in harmless substances or conversely from ozone-neutral reservoir compounds and are activated as catalytic substances again. This explains the sudden significant decomposition of the ozone layer in the spring over the Antarctica.

Heterogeneous reactions

The reasons are chemical reactions in the stratospheric clouds. They are composed of ice and nitric acid particles, on which the chlorine from compounds forms residues which are stored before and withdrawn from the ozone decomposition cycle. The clouds emerge only at temperatures under minus 80 °C and are observed predominantly in the area of the stable wintry polar vortex over Antarctica. Energy-rich solar radiation is also necessary for the reactivation of the chlorine, which is available first at the end the polar night, therefore at the beginning of the Antarctic spring. In this period the chlorine is released in large quantities from the reservoir of substances formed in the winter. Since the supply of ozone-rich air transmitted from the lower latitudes through the polar vortex is suppressed until the spring, the massive ozone loss over the Antarctica takes its course. Through the transport processes which begin again in the summer, the ozone deficit is again largely equalized.

In the northern hemisphere, a comparably large and long existing polar vortex does not exist, so that the low temperatures necessary are reached only rarely or short-term and meridianal air currents occur more frequently. The polar clouds necessary for heterogeneous reactions are available therefore in very much slighter density and the ozone decomposition is less dramatic. The balancing out of thin places in the ozone layer through south-north transports of ozone-rich air is more likely. Therefore decomposition comparable to the Antarctic ozone hole in the ozone layer over the northern polar region is not to be expected. Indeed the question was put lately, whether through the increasing greenhouse effect a cooling of the stratosphere is provided and supports the strengthened formation by stratospheric clouds also in the northern hemisphere. This would lead to a heightened decomposition of the ozone through heterogeneous reactions.

Further influential factors

As mentioned at the outset, volcanic eruptions also influence the thermal structure and the chemistry in the upper atmosphere. Due to the high intensity of the energy released at an outbreak, material and gases are shot into the stratosphere, whose chemical composition draws from that of the early earth's crust and differ therefore very greatly from the present composition of the atmosphere. It is suspected that they directly or indirectly favor the origin of the above described polar stratospheric clouds. Besides the ozone-decomposing chlorine is brought directly in the stratosphere in the form of hydrochloric acid. As measurement results from a large laid out research program 1993 showed, over the northern polar region, an increasing amount of dust particles was found which had originated from the Pinatubo eruption in 1991 and spread out over wide parts of the hemisphere. Indeed there was no spatial coincidence between the incidence of volcano dusts and the stratospheric clouds which appeared in the area of the northern polar vortex in the winter 1992/93. The thinning of the ozone layer at this time can not be explained directly as a result of volcanic eruption. In contrast to the relatively rare emissions through volcanic eruptions, the ozone layer is not in the position from which to recover from the decimation of continuous CFC emissions.

This also applies to the possible increase in the supersonic air traffic in the stratosphere, through which nitrogen oxides and water vapor, which contribute directly and indirectly to ozone decomposition, are directly emitted.

Future development

Which consequences a further increase of the trace gas emissions on the ozone layer will have in future, can only be estimated with the help of mathematical model calculations because of the numerous influence factors. Most of the scenario calculations which have been performed up to now consider only the effect of the catalytic gas phase reactions and with it only a part the ozone-decomposing effects.

The results of these calculations show that the insufficient measures of the 1987 Montreal Conference (only 50 % emission decrease by 1999 with numerous exceptions) allow the chlorine content in the stratosphere to quadruple by the middle of the next century, which would lead to an ozone depletion in the stratosphere of around 30 %. As a consequence, the subsequent penetration of the atmosphere by UV radiation into troposphere is calculated to produce a robust rise of the ozone concentration of around 20 %. Thereby the weakening of the UV-filter effect of the stratospheric ozone layer is partially compensated, so that the model calculations of an increase assumes an increase in the cell damaging UV radiation at the ground of 4 to 10 %. Indeed a generally higher ozone level in the lower atmosphere would allow the frequency of injuriously high ozone concentrations to climb there still further, where because of the photochemical formation of ozone an excessively high burden already prevails.

If unhindered CFC emissions are considered in the long-term consequence assessment along with the most important heterogeneous reactions which lead to creation of the Antarctic ozone hole, then the increase of the UV radiation at the ground in the global average can reach 20 to 25 %.

Damage to the ecosystem

Since the area marked as UV-B short-wave radiation (290 to 330 nm) has a cell damaging effect, a significant increase in intensity has direct negative effects on the animals and flora and also on humans.

Indirectly humans are affected as the last link in a long food chain by a possible decline of plant growth. Under these circumstances an important reciprocally strengthening connection rests also itself between ozone decomposition and global temperature increase. The photosynthesis performance of plants is damaged by a higher UV dose, particularly that of the phytoplanktons in the ocean. It draws as much carbon dioxide from the atmosphere through its metabolism as do all terrestrial plants together. If the increasing UV B radiation would deaden for instance 10 % of the plankton, almost so much less carbon would be removed from the atmosphere annually, as the entire mankind discharges through the fossil fuel combustion. The result would be a further reinforcement of the greenhouse effect and with it a further warming of the earth's atmosphere.

An increasing UV B radiation intensity can become a direct danger since larger radiation quantities, when hitting the skin, induce a carcinogenic effect and can cause over and above this damage to the eye lens (gray star). Since the damage gains with the reduction of the wavelength and with a weakening of the ozone layer the short-wave spectrum of the UV light becomes stronger, a thinning of the ozone layer by 1 % can lead to an increase in skin damage of around 1.7 %. Due to the strong seasonal fluctuations in UV intensity, in the summer four to five-fold the usual winter values with a cloudless sky, the ozone layer thickness in the summer half-year plays a role in the assessment of a possible additional health threat.

Also there is a fair-sized variation of the UV radiation with the geographical latitude because of the different distances for the radiation through the atmosphere. The solar radiation is during summer with cloudless skies around 40 to 100 % more intense in the Tropics than in our latitudes. Therefore the clear rise in skin cancer illnesses can also be traced to the travel fever of an increasing number of persons whose skin is only genetically adjusted to the slighter radiation intensity in higher latitudes and is exposed to the more intense solar radiation in the equatorial vacation areas.

Long-time measurements of the UV B radiation show a significant increase up to now only at stations in New Zealand, South Australia and South America. Presumably the slightly declining filter effect of the stratosphere has been equalized in Europe and the USA through an increase in the ozone and the dust content in the lower atmospheric layers.

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