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 layer.
Figure 3 illustrates the photochemical lifetime of the major trace gases
affecting the ozone layer according to altitude. Many of these major gases have
lifetimes of less than a month in the stratosphere compared to more than 100
years in the troposphere.

Hydrogen species

The influence of OH,