What is it?The
phenomenon which we call the ozone hole was first discovered in the 1980s.
The ozone hole isn't actually a "hole" in the literal sense, but rather
more of a wearing thin of the normal ozone levels. It is defined by the
scientific community as anywhere in which the amount of column ozone in the
atmosphere is under 220
Dobson Units. The ozone hole now occurs every year in Antarctica
between the months of September and November, which is the South Pole's
springtime. During this time, ozone levels over most of Antarctica
are reduced by about 60% of their usual amount.
Causes of the Ozone Hole
When it was first detected in the 1980s that the minimum
springtime levels of ozone were steadily dropping,
great concern began to arise in the scientific community. Almost
immediately, theories regarding the cause of such a phenomenon
were developed. Some of these theories included a belief that changing
wind conditions in the Antarctic was causing ozone-low tropospheric air to be
blown up into the stratosphere. This was eventually proven false
as scientists observed a lack of other gases that would have
been present in the troposphere had this theory been correct.
Another theory included the idea that ozone was destroyed by
chemical reactions with other trace gases. In the end, it was
this theory that was determined to be correct. The
chemical culprit prompting these reactions was determined to be
chlorine, which was being introduced into the atmosphere largely
in the form of CFCs (see below). Satellite data and observations further
strengthened this idea, as it was found that as ozone levels
dropped, levels of ClO (chlorine monoxide - the product of a
reaction between chlorine and oxygen atoms) increased.
What are CFCs?
CFCs, also known as
chlorofluorocarbons (composed of carbon, fluorine,
and chlorine) were invented in the early 1930s. Over the years, they have
served many functions, including propellants for
aerosols, refrigeration coolants, and electronic
circuit board cleaners. There are different types of CFCs: CFC-11 and 12 for
example are common refrigeration coolants. CFC-11 is also known under the brand
name of Freon.
After determining the cause of the problem, a question naturally arises, if most of
the CFCs are released over major industrial countries such as the United
States and Japan, then why is the ozone hole forming over Antarctica?
The answer lies in at least two reasons.
First, when something (like a CFC molecule) is released into the air,
it does not remain in the atmosphere directly above its origin. Because
CFCs have a life-span of several decades, they remain intact long enough
to make their journey up into the
The key to the long life of CFCs is their non-reactivity. They don't react
with other substances in the
and only break apart in the
stratosphere when they are exposed to high-energy ultraviolet radiation - a
process that could take up several years. Therefore, winds in the troposphere
and stratosphere have sufficient time to distribute CFC molecules around the
Secondly, the weather conditions in the Antarctic are such that they
encourage the creation of so-called polar stratospheric clouds
(PSCs). These clouds form only under persistently cold
conditions, which is why they are usually only found in Antarctica
(PSCs can also be found in the Arctic,
but because the weather is not as persistently cold, they are
less common.) To understand why it is that PSCs contribute to
ozone depletion, more information about the chemistry taking
place in the stratosphere is needed.
As mentioned before, when CFCs enter the stratosphere, they are
exposed to high-energy ultraviolet rays from the sun, which
cause the chlorine (chemical symbol Cl) to break apart from the CFC molecule. One
chlorine atom is capable of fragmenting over 1000 ozone
molecules (for example via the reaction Cl + O3 ->
ClO +O2) before it is trapped again in stable
molecules (sometimes called reservoir substances), such as
chlorine nitrate (ClONO2). This fact is interesting
by itself because it explains some of the ozone decrease
observed world-wide. Yet it does not explain the ozone hole,
which only forms when chlorine is again released from the
This is were the PSCs come into play. At the surface of these icy
clouds, the reservoir substances are again transformed into more
active forms of chlorine. For example, ClONO2 reacts
with hydrochloride acid (HCl) to form chlorine gas (Cl2)
and HNO3. During the period of complete darkness
during the polar night large quantities of Cl2 can
accumulate, but still only little ozone decrease is observed.
The massive destruction of ozone that finally leads to the ozone
hole takes place only when the first rays of sunlight are
striking the Antarctic atmosphere after the polar night,
splitting Cl2 into two atoms of chlorine
(Cl2-> 2 Cl). Now ozone destruction can start again via the
reaction Cl + O3 -> ClO +O2 (It
goes without saying that the complete chemistry that is going
on is far more complex).
As there is so much chlorine in an active form at the end of the
polar night (September in Antarctica) the ozone
hole can grow to a size larger than the United States. At
the South Pole, ozone levels below 100 Dobson Units
are now frequently observed in late September and early October.
Before the ozone hole existed, typical ozone values were 300
Relevance to the Rest of the World
Because the location of the ozone hole is far separated from most of the
human population, it would be easy to dismiss it as a phenomenon irrelevant
to the rest of the world. It does, however, have a significant impact on the
rest of society. First, the oceans around Antarctica are rich in life, which
is threatened by increased ultraviolet radiation under the ozone hole. Secondly,
when the polar vortex,
which keeps ozone depleted air trapped over Antarctica during
the spring, breaks up, ozone-poor air is dispersed over nearby human-inhabited areas.
Such areas include Australia, New Zealand, Southern Argentina, and
Southern Chile. And thirdly, there are concerns that the
stratosphere over the North Pole could cool down during the next
decades. If that were the case, PCS would become more frequent in
the northern hemisphere, causing severe ozone depletion over Alaska,
Canada, Northern Europe and Siberia. Here human settlements are far more
frequent than in the Southern Hemisphere.
What is being done about it?
In light of the increasing evidence that man-made products were partly
responsible for the newly observed ozone hole, the world agreed that measures
needed to be taken. As a result, 24 nations convened in September of 1987 and
designed what we know as the
Montreal Protocol, a treaty intended to limit the
production of CFCs and other ozone depleting substances (95 chemicals in all),
to the eventual goal of ending their manufacture completely. As outlined in the
original protocol, developed countries would be required to phase out CFC
production completely by 1996. Developing countries were also acknowledged in
the agreement, and were allotted more time to end production - they were given
until 2010 to perform the same task. However, the Montreal Protocol was
designed in such a way that revisions were easily accomplished, and have been.
Since 1987, there have been a number of amendments to the original document,
resulting in a more rapid decline of ozone depleting substances. These
additions include the London, Vienna, Copenhagen, Montreal, and Beijing
amendments. More information on the Montreal Protocol is given
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