The Atmospheric Ozone Layer

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 (76S 27W)
and at Argentine Islands (65S 64W). 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 (40-80N), 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 60S, 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, HO2 and of CH4 on the stratospheric ozone layer tends to be
less important than the other major trace gases, except in the upper
stratopshere. The major indirect influence of the hydrogen species in the mid to
lower stratosphere is through their catalytic properties, enhancing nitrogen and
chlorine species reactions.
Nitrogen species
There is not much information available about seasonal and annual Nox species in
the stratosphere compared to ozone.

NO and NO2 concentrations in winter are
considerably lower than in summer in both hemispheres. In the early 1970s there
was major concern that Nox emissions from supersonic aircrafts would create a
major depletion of the ozone layer. Considerable ozone reductions (16%) were
expected in the Northern Hemisphere, where most of the supersonic transports
would be flying, but stratospheric circulation patterns would ensure at least an
8% reduction in ozone over the Southern Hemisphere. Fortunately for the globe,
the massive fleets of supersonic transports never eventuated.

The Concorde was
barred from landing at many airports for noise and other environmental reasons
and now flies only limited routes, mainly from Great Britain and France. Concern
over Nox emissions has been overshadowed by the potential problems associated
with the chlorofluorocarbons.
Chlorofluorocarbon species
In 1974, Molina and Rowland first suggested that anthropogenic emissions of
chlorofluorocarbons (CFCs) could be depleting stratospheric ozone through the
removal of odd oxygen by the chlorine atom. CFCs released from aerosol spray
cans, refrigerants, foam insulation and foam packaging containers, increased
concentrations of Cl compounds in the troposphere considerably. CFCs are not
soluble in water and thus are not washed out of the troposphere.

There are no
biological reactions that will allow their removal. The result is very long
tropospheric residence times and the inevitable transport into the stratosphere
through normal atmospheric circulation. The chlorine atom, released from a CFC,
reacts with ozone to form ClO and O2. Since ClO reacts with ozone six times
faster than any of the nitrogen species (Rowland and Molina, 1975), it becomes
the dominant mechanism to destroy stratospheric ozone. As a result, a lone Cl
atom can be responsible for destroying several hundred thousand ozone molecules.Based on recent results, reductions of ozone for 5-9% are possible with
locational changes 4% in the tropics, 9% in the temperate zones and 14% in the
polar regions.

Recent discoveries such as that by Farnam (1985) lead most
experts to believe that important destruction of the stratospheric ozone layer
is not far off.
The Polar "Holes" - The Antarctic
With the help of the Dobson spectrophotometer, Farnam (1985) was able to
establish that the total ozone concentrations over the bases in Antarctica had
been falling during the October-November period since 1979. The trend of ozone
loss during this time varied from year to year, but over the six year period
showed an overall decrease. Verification from other bases in Antarctica came
soon afterward (Table 4-Komlyr, 1988).

Further verification came from the Nimbus
satellite, from which the scientists were able to produce graphic colour-
enhanced photographs of the depletion of ozone over Antarctica. The media began
using the phrase "Antarctic Ozone Hole" to describe this phenomenon and
unfortunately its importance has been expanded out of proportion to the global
total ozone situation. By definition, the "hole" represents a depletion of ozone
concentrations over Antarctica, not an empty space in atmosphere.
Atmospheric scientists were at first puzzled about the cause of the ozone hole.

Three theories were suggested. The first was that there was a connection with
the 11 year sunspot cycle. When a large number of sunspots occur, there is
considerable NOx produced in the upper atmosphere which could interact with the
ozone by reactions shown in table 2. The second was that during the period when
the sun was rising, there could be dynamic interactions between the troposphere
and the stratosphere with an upwelling of ozone-poor air into the stratosphere
from below. Such upwelling should also include many tropospheric trace gases not
normally found in abundance in the stratosphere. Third, the ozone hole could be
caused by chemical reactions, particularly reactive Cl, somehow released from
reservoir molecules which were transported to Antarctica by the stratospheric
circulation from source regions much further North.

Detailed investigations of these theories were made by the United States
National Academy of Sciences (N.A.S.) in 1988. The theory suggesting sunspot
influences was discounted because there was minimal NO2 measured in the upper
stratosphere over Antarctica, and in the main area of expected ozone loss, above
25km, ozone concentrations remained relatively high during the lifetime of the
hole. The second theory, suggesting convective upwelling from the troposphere,
was also eliminated as a possibility, since trace gas concentrations normally
found in the troposphere were not measured in the stratospheric ozone hole.

This
left the third possibility, Cl chemistry, which the N.A.S. report suggested,
occurred under a unique set of meteorological circumstances
At the end of the Southern Hemisphere winter, as the sun is beginning to appear
over Antarctica, the circumpolar vortex circulation in the lower stratosphere is
at its strongest. Extremely stable and durable at this time of year (September
and October), the vortex blocks any incursions of warmer air from the mid-
latitudes and allows an extensive drop in temperature inside, over the continent.

Within the depths of the hole, important chemical reactions which deplete the
ozone concentration are taking place. In order for the chemical reaction theory
to work, there must be an overabundance of ClO in the Antarctic stratosphere
between 12 and 25km and a diminished concentration of NOx series, which might
interfere with Cl attacks on ozone. Concentrations of NOx species decrease
toward the hole centre and ClO concentrations are 100 to 500 times higher than
observed outside the hole.
In 1987, the increases in ClO occurred across a very sharp boundary layer,
fluctuating between about 67 and 75S.

Over a latitude span of about 1, ClO
increased from less than 100 pptv to over 200 pptv, depending on altitude. Ozone
averaged 256DU. This area of steep change marked the chemical boundary of the
hole. Spatial distributions of ClO and ozone showed a marked negative
correlation inside the hole.

Whereas ozone decreased by about 60% crossing the
boundary, ClO increased by greater than a factor of 10. This result provides
strong circumstantial evidence that the link between ozone loss and chlorine
over Antarctica is real.
There is still much to be learned about what causes the Antarctic ozone hole.Questions regarding changes in ClO at various latitudes, changes in
concentrations in molecules from day to night, the progressive deepening of the
ozone hole through the 1980s, and several other details remain unanswered.

Colder stratospheric temperatures within the hole are likely to create thicker,
longer lasting clouds which enhance processes for ozone removal, but details are
not yet clear. Day-to-day variations in ozone within the hole have not yet been
properly explained, and there is some question whether the ozone hole will
continue its depth and persistence in future years.
The Arctic
The discovery of the Antarctic ozone hole raised the possibility that a similar
hole could exist over the Arctic. Early results from a series of measurements in
the winter of 1988-89 suggests that ozone loss over the Arctic exists, but not
to the degree of that over the Antarctic
Trends in global total ozone
The publicity surrounding the discovery of and research activity in the
Antarctic ozone hole has unfortunately tended to obscure a potentially far
greater problem, decreases in total ozone concentrations across the globe. The
loss of ozone above the tropics and mid-latitudes, and the resultant increase in
harmful UV radiation could be disastrous to the earth's population if the
changes were major. Since the late 1970s, there has been a slow but steady
decrease in global total ozone, even if the major losses over Antarctica is not
included.

The trend is on the order of -2.7% per year in all seasons with the
greater losses occurring in the Northern Hemisphere autumn and winter (greater
than 3%) and the least in the Northern Hemisphere summer (1.6%).
Surface impacts and political decisions
The impacts of a depleted ozone layer on surface organisms depend on their
location to increased UV-B radiation. As a rough estimate, many experts suggest
that the percentage increase in UV-B radiation affecting surface organisms would
be about twice the percentage loss in stratospheric ozone from anthropogenic
causes. The most immediate effect on human beings would be an increase in
various skin cancers and skin cancers are increasing.

Increases in the evidence
of cataracts and interference with the human immunity system are other possible
influences. A more serious potential long-term threat is the damage to cell DNA
and the genetic structure in not only human beings but in other animals, plants
and organisms.
With the discovery of the Antarctic ozone hole and the resultant world-wide
interest, publicity and concern, a historic meeting occurred in Montreal, Canada
in September 1987. For the first time ever, 57 countries and organisations met
to make a specific decision to limit the emissions of a series of pollutants
which were likely to create major environmental problems affecting the globe in
the future.

The eventual document adopted on September 16, 1987 and entitled
"The Montreal Protocol", was signed immediately by 24 countries and since has
been ratified by several more.
REFERENCES:
1. Jonathan Weiner, "Plant Earth", New York, Bantam Books, 1986
2. "Atmospheric Ozone, Global Ozone Research and Monitoring Project" (Vol. 16,
Geneva 1985 International Organisation of Meteorology)
3.

Lydia Dotto and Harold Sciff, "The Ozone War", Garden City, N.Y., Doubleday,
1978
4. John Gribbin, "The Hole in the Sky", N.Y., Bantam Books, 1988
5.

James G. Titus, "Effect of Changes in Stratospheric Ozone and Global Climate"
Vol. 2, United Nations Environmental Programme
6. G. Levi, 1988, "Ozone depletion at the Poles", Physics Today
7. P.

Bowman, 1988, "Global trends in total Ozone", Science
8. Hans U. Dutsch, "Vertical Ozone Distribution", International Centre for
Atmospheric Research, Boulder, Colorado
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