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Berlin Environmental Atlas

03.06 Near Ground Ozone (Edition 1996)

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Map Description

Map 03.06.1: Generation - Decomposition

The natural balance in the stratosphere

As presented schematically in Figure 1, the ozone concentration greatly increases above 12 km altitude in the stratosphere and reaches 20 to 30-fold the values in ground proximity. The reason for this is the generation of ozone induced by the effects of energy-rich solar radiation from space (wavelengths < 240 nm) on the uppermost layers of the atmosphere. The plentiful molecular oxygen O2 available there is split into both its atoms which join subsequently with a still intact O2 molecule to form ozone (O3) (c.f. Fig. 3).

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Fig. 3 : Schematic Display of Ozone Formation and Decomposition in a Pure Oxygen Atmosphere and of Catalytic Ozone Destruction (from left after to right; source: German Bundestag 1990)

This ozone production stands in balance with the natural ozone decomposition which is depends on the absorption of less energy-rich radiation (wavelengths by 200 to about 300 nm and weaker). Since ozone has a slighter bonding energy than oxygen, O2 and a single oxygen atom are created when these molecules decompose (c.f. Fig. 3). This atom can bond itself again with an oxygen molecule to form O3, so that in the balance the ozone loss remains first of all slight. If one calculates the global ozone distribution alone under consideration these reactions discovered by Chapman in 1930, the actually observed density of the ozone layer in the stratosphere must have about 50 % more ozone and a incorrect vertical distribution. Thus there must still be other ozone destroying reactions about which more will be said below.

At altitudes over 30 km, photochemical balance prevails and the atmospheric transport plays hardly a role for the ozone distribution. The highest ozone values are found in the area with the highest amount of radiation. That means at the equatorial regions and decreasing toward the poles.

In layers between 15 and 30 km high the ozone distribution is clearly influenced by the horizontal and vertical transport processes. The average global distribution of the total ozone, which is determined to over 70 % by the stratospheric ozone in this layer, shows a minimum of around 250 Dobson units (DU) in the equator region and an increase toward the poles.

Since in the tropics through the heavy weather activity an ascending air movement prevails, there low-ozone air climbs from below into the stratosphere. There it is transported in a meridianal direction to the poles and there it sinks again. Because of the high UV-radiation the largest ozone production occurs in the tropical and subtropical stratosphere, so that polewards because of the transport process the ozone values rise to over 400 DU. Since the meridianal air movement is most predominant in the late winter and spring of the respective hemisphere, the ozone maximums are to be observed in the higher latitudes of both hemispheres respectively around this season. In the respective summer, the total ozone values sink and reach their minimum in the late fall. As an example for the annual cycle in our latitudes the average course of the total ozone values (and their standard deviation) over many years at the observatory in Potsdam is presented in Figure 2.

For several years, the spring ozone maximum in the Antarctic, with on average over 340 DU has exhibited a dramatic decline to less than half. In the Antarctic spring 1993 (September/October), the total ozone quantity sank there over a wide area to even under 100 DU, an effect which occurred again in 1994 and 1995. In Figure 4, the penetration of the ozone layer at the end of September can be seen from the seasonal variations in the vertical ozone distribution. The ozone concentrations found in the ozone layer, as measured by the ozone sensors of the German Antarctic station, located at 70 °S, vary from 140 nbar in winter to a drastic reduction sometimes lower than 10 nbar at the end of October. At the beginning of the summer the ozone layer thickness rose again nearly to normal values. This can be traced back to the movement of ozone-rich air from lower latitudes. This is practically suppressed in the winter and up to the beginning of the spring because of the stable wintry polar vortex over the Antarctica. The cause of the rapid ozone decomposition is still to be discussed.

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Fig. 4: Ozone Vertical Distribution in 1995 at the Georg von Neumayer Station (from Alfred Wegener Institute for Polar and Marine Research; from the Internet)

The phenomenon known as the "ozone hole" has no counterpart in the northern hemisphere. There the meridianal transport processes start earlier because the wintry polar vortex over the north pole disappears sooner. Indeed a gradual decomposition of the ozone layer thickness is also to be observed in the northern hemisphere. With 7 to 9 % in the winters 1992 and 1993, it had increased - with reference to the value 10 years before, end of the 80s in winter over North America and Europe about 3 % (c.f. German Bundestag 1990). In the summer, the decline is significantly lower here as well as in the equatorial regions. Nonetheless, the 1995 ozone layer thickness (c.f. Fig. 2), as measured at the Meteorological Observatory in Potsdam, has declined in the summer even compared with the 30 year average. The deviation was approximately minus 8 % as in summer 1994 and 1996. On account of the higher solar position and subsequently higher UV-B background this is of greater significance than the occasionally heavier variation in winter.

The anthropogenic destruction of the ozone in the stratosphere

Since 1974 when the two American scientists Molina and Rowland alarmed the world public with their thesis of the destruction of the ozone layer by human-induced trace gas emissions, it has become even more clear that the complex aerochemical balance in the atmosphere can be easily disturbed through anthropogenic activities. Also trace materials, which due to their chemical inertia endure the long transport from the ground to the stratosphere or are introduced there directly by airplanes and volcanic eruptions, contribute to this effect. Like the atmospheric oxygen, they are also broken down by the energy-rich solar radiation into their elements of which some massively react on the chemical balance at the expense of the ozone.

The best known human-induced trace gases are the chlorofluorocarbons (CFC) and the related halons. These are hydrocarbon compounds in which one or several hydrogen atoms have been replaced by fluorine and chlorine and/or bromine. On account of their chemical inertia, these are industrially versatile materials (coolant, solvent, propellant etc.) Their worldwide output has reached the considerable quantity of 1 mil. t/a. The only known compound from this class of materials with a natural source is methyl chloride, which is emitted by the oceans into the atmosphere. It contributes however only 10 to 20 % of the chlorine content of the stratosphere, which is responsible for the ozone decomposition located there.

A further important material class with both natural as well as anthropogenic sources are the nitrogen oxides. They play a role in form of laughing gas (N2O), which is introduced both as consequence of bacteriological processes in the ground as well as through the increased discharge of nitrogen fertilizer in the atmosphere. Also NO as an element of airplane exhaust fumes counts as an ozone-decomposing element.

Water plays likewise a role in the stratosphere as an ozone-decomposing substance. Due to the extremely low temperatures at the lower edge of the stratosphere considerable water quantities arrive however only through the air traffic and volcanic eruptions in the higher atmosphere layer.

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