The Atmospheric Ozone Layer

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The Atmospheric Ozone Layer

The stratospheric ozone layer exists at altitudes between about 10 and 40km depending on latitude, just above the tropopause. Its existence is crucial for life on earth as we know it, because the ozone layer controls the absorption of a portion of the deadly ultraviolet (UV) rays from the sun. UV-A rays, including wavelengths between 320 and 400nm, are not affected by ozone. UV-C rays between 200 and 280nm, are absorbed by the other atmospheric constituents besides ozone. It is the UV-B rays, between 280 and 320nm, absorbed only by ozone, that are of the greatest concern. Any loss or destruction of the stratospheric ozone layer could mean greater amount of UV-B radiation would reach the earth, creating among other problems, an increase in skin cancer (melanoma) in humans. As UV-B rays increase, the possibility of interferences with the normal life cycles of animals and plants would become more of a reality, with the eventual possibility of death.

Stratospheric ozone has been used for several decades as a tracer for stratospheric circulation. Initial measurements were made by ozonesondes attached to high altitude balloons, by chemical-sondes or optical devices, which measured ozone concentrations through the depletion of UV light.

However, the need to measure ozone concentrations from the surface at regular intervals, led to the development of the Dobson spectrophotometer in the 1960s. The British Antarctic Survey has the responsibility to routinely monitor stratospheric ozone levels over the Antarctic stations at Halley Bay (76°S 27°W) and at Argentine Islands (65°S 64°W). Analysis of ozone measurements in 1984 by a team led by John Farnam, made the startling discovery that spring values of total ozone during the 1980-1984 period had fallen dramatically compared to the earlier period between 1957-73. This decrease had only occurred for about six weeks in the Southern Hemisphere spring and had begun in the spring of 1979. This discovery placed the British scientists into the limelight of world publicity, for it revived a somewhat sagging public interest in the potential destruction of the stratospheric ozone layer by anthropogenic trace gases, particularly nitrogen species and chlorofluorocarbons.

Ozone concentrations peak around an altitude of 30km in the tropics and around 15-20km over the polar regions. The ozone formed over the tropics is distributed poleward through the stratospheric circulation, particularly in the upper stratosphere where the airflow is the strongest and most meridional. Since the level of peak ozone is considerably higher in altitude in the tropics, ozone descends as it moves toward the poles, where because of very low photochemical destruction, it accumulates, particularly in the winter hemisphere (see fig.1). Some ozone eventually enters the troposphere over the poles.

Seasonal variations are much stronger in the polar regions reaching 50% of the annual mean in the Arctic. In spring, Northern Hemisphere transport of ozone toward the poles builds to a maximum (40-80°N), associated with the maximum altitude difference in the major ozone regions of the tropics and the poles. The polar flux of ozone ceases as the westerly circulation dominant in winter is replaced by easterlies over the tropics. In the Southern Hemisphere the spring maximum occurs near 60°S, one to two months after the maximum in the subtropics. Throughout the summer, photochemical reactions reach a maximum in the lower tropical stratosphere and ozone concentrations fall. Autumn circulations are the weakest, with the latitudinal gradient between the poles and the equator virtually disappearing. Ozone concentrations throughout most of the stratosphere reach a minimum. As the circumpolar vortex expands for winter, the strength of circulation increases rapidly, ozone transport from the tropics also increases strongly, and meridional circulation and variability peak in the winter months....
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