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, 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 75?S. 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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