The Atmospheric Ozone Layer Essay Research Paper

The Atmospheric Ozone Layer Essay, Research Paper

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.

Anthropogenic influences on the stratospheric ozone layer

Figure 2, establishes the basic natural formation and destruction processes

associated with stratospheric ozone. However, several other gases which have

long lifetimes in the troposphere, eventually arrive in the stratosphere through

normal atmospheric circulation patterns and may interfere with or destroy the

natural ozone cycle. The trace gases of most importance are hydrogen species

(particularly OH and CH4), nitrogen species (NO, N2O and NO2) and chlorine

species. The gases not only react directly with ozone or odd oxygen atoms, but

also may combine in several different ways in chain processes to interfere with

the ozone cycle. Figure 2, presents examples of these reactions. The lifetime of

these trace gases is crucial to the chemistry of the stratospheric ozone

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