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Berlin Environmental Atlas

03.06 Near Ground Ozone (Edition 1996)

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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: On 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 (c.f. 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 (c.f. Fig. 4).

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. Nonetheless the question must be raised whether the increase in the greenhouse effect leads to a cooling off of the stratosphere and thereby increased stratospheric cloud formation in the northern hemisphere as well. This would lead to an increased ozone decomposition caused by heterogeneous reactions. In February and March 1996, it was possible to detect for the first time in central European latitudes such stratospheric clouds at altitudes of between 20 and 24 km using the LIDAR device operated by the FU Berlin. At the same time, the Potsdam measurements of the ozone layer decreased to about 250 DU - the long time average lies at about 370 DU. However, the drop lasted only a few days.

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.

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 % (c.f. German Bundestag 1990).

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.

High resolution spectral measurements of UV-B radiation, as needed for assessing pollutant effects, have only been available for a few years due to the recent development of the corresponding measuring technology. For instance, it has only been since July 1993 that all seven measuring stations in Federal Environmental Agency and German Weather Service have been able to generate UV-B spectral measurements of sufficiently high resolution so as to allow the calculation of the "so-called biologically weighted UV-B radiation" on the basis of the frequency of heavily varying pollutant effects.

Comparisons between such new measurement between Germany and a location at a similar latitude in the southern hemisphere in New Zealand showed a 1.4 to 1.8 times greater UV-B radiation level. Here a factor between 1.3 and 1.6 is solely due to the decomposition of the ozone layer in the southern hemisphere (c.f. Seckmeyer and McKenzie 1992). Measurements in Canada between 1989 and 1993 showed an increase in the sunburn-inducing UV-B radiation of five percent in winter and two percent in summer (c.f. Kerr and McElroy 1993). An increase in the DNA-damaging radiation of seven percent per decade for the regions between 45 and 55 °N could be determined based on satellite data from the past fifteen years (c.f. Herman et al. 1996). The increase in the ozone and dust concentrations in the lower atmospheric layers in Europe and the USA could have had an inhibiting effect on the increase in UV-B radiation.

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