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OMTEX CLASSES
2010
II. Ozone depletion: Who is responsible?
It is important to recognize the sources of ozone depletion before one can
fullyunderstand the problem. There are three main contributors to the ozone problem:
humanactivity, natural sources, and volcanic eruptions (See Figure 3).
Figure 3: Humans cause more damage to the ozone layer than any other source.
Source: Geocities.com, 1998
Human activity is by far the most prevalent and destructive source of ozone
depletion, while threatening volcanic eruptions are less common. Human activity, such
as their lease of various compounds containing chlorine or bromine, accounts for
approximately 75 to 85 percent of ozone damage. Perhaps the most evident and
destructive molecule of this description is chlorofluorocarbon (CFC). CFCs were first
used to clean electronic circuit boards, and as time progressed, were used in aerosols
and coolants, such as refrigerators and air conditioners. When CFCs from these
products are released into the atmosphere, the destruction begins. As CFCs are
emitted, the molecules float toward the ozone rich stratosphere. Then, when UV
radiation contacts the CFC molecule, this causes one chlorine atom to liberate. This
free chlorine then reacts with an ozone (O3) molecule to form chlorine monoxide (ClO)
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and a single oxygen molecule (O2). This reaction can be illustrated by the following
chemical equation: Cl + O3--> O2 + ClO. Then, a single oxygen atom reacts with a
chlorine monoxide molecule, causing the formation of an oxygen molecule (O2) and a
single chlorine atom (O + ClO --> Cl +O2). This threatening chlorine atom then
continues the cycle and results in further destruction of the ozone layer (See Figure 4).
Measures have been taken to reduce the amount of CFC emission, but since CFCs
have a life span of 20-100 years, previously emitted CFCs will do damage for years to
come.
Natural sources also contribute to the depletion of the ozone layer, but not nearly as
much as human activity. Natural sources can be blamed for approximately 15 to 20
percent of ozone damage. A common natural source of ozone damage is naturally
occurring chlorine. Naturally occurring chlorine, like the chlorine released from the
reaction between a CFC molecule and UV radiation, also has detrimental effects and
poses danger to the earth.
Finally, volcanic eruptions are a small contributor to ozone damage, accounting for one
to five percent. During large volcanic eruptions, chlorine, as a component of
hydrochloric acid (HCl), is released directly into the stratosphere, along with sulfur
dioxide. In this case, sulfur dioxide is more harmful than chlorine because it is converted
into sulfuric acid aerosols. These aerosols accelerate damaging chemical reactions,
which cause chlorine to destroy ozone.
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The decrease of strastospheric ozone was first reported in 1974 and its decrease was
linked to the presence of manmade compounds in the atmosphere the most damaging
of which is the class of compounds know as Chloroflurocarbons or CFCs.
CFCs are the major category of man-made halocarbons. Halocarbons are formed when
halogen gases such as fluorine, chlorine and bromine become attached to carbon. The
smaller halocarbons turn into a gas quite easily and are the prime suspects in ozone
depletion.
CFCs are used in industry in a variety of ways. They were discovered in the 1930s by
American chemist Thomas Midgley, and came to be used in refrigerators, homeinsulation, plastic foam, and throwaway food containers.
The non-reactivity of CFC's, so desirable to industry, allows them to drift for years in the
environment until they eventually reach the stratosphere. High in the stratosphere,
intense UV solar radiation splits the chlorine molecules off the CFC's. These then attract
one of the three oxygen atoms in the ozone molecule (O3) destroying the ozone by
turning it into oxygen. A single chlorine atom can destroy over 100 000 molecules of
ozone in this way.
CFCs vary widely in their stability and in how effective they are at destroying ozone.
Unfortunately, most of them will persist in the atmosphere for many years (from 50 to
over 200 years). Although many countries have moved to reduce the use of CFCs, this
long life of CFCs means that the impact of chlorofluorocarbons currently in the
atmosphere will continue well into the next century.
The change in the atmospheric ODS concentrations is the most important factor in the
ozone layer changes that have
occurred over the past half a century and also in the predicted return of the ozone layer
to levels that existed prior to 1980.
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However, many other aspects of the Earth system are also changing. These include
changes in climate and tropospheric
composition.
Climate change influences the stratosphere in many ways. The primary influence is acooling of the mid- to
upper stratosphere due to increases in carbon dioxide (CO2
) via radiation to space, which is a well-understood process.
This cooling has been clearly seen in measured temperatures. The cooling influences
the ozone loss rates in the stratosphereincreasing it in the lower stratosphere and
decreasing it in upper stratosphere. At the same time the warming in
the troposphere accelerates processes of ozone formation. Further, climate change
has an effect on transport between the
stratosphere and the troposphere and within the stratosphere, and in turn, climate will
influence the recovery of ozone layer
from the effects of ODSs.
Tropospheric changes also influence stratospheric ozone levels. For example, an
increased abundance of methane
(CH4
) in the troposphere will result in more methane being transported to the stratosphere,
where methane interacts with
chlorine compounds, converting active chlorine that destroys ozone to inactive
hydrogen chloride (HCl) that does not
destroy ozone. Changes in methane also lead to changes in water vapor in the
stratosphere, with important consequences.
Similarly, changes in nitrous oxide (N2
O) also influence ozone destruction. Other tropospheric changes of interest include
processes leading to increases in sulfur in the stratosphere. In some cases, changes of
these tropospheric processes may
be related to climate change. For instance, climate change may affect biogeochemical
cycles and cause an increase in
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tropospheric concentrations of certain species as well as the transport rate between the
troposphere and the stratosphere.
The latter may be particularly important for the very short-lived species.
The timeline of the ozone evolution from the pre-ODS era to roughly 2100 waspresented in the 2006 Assessment to
facilitate discussion on recognition and attribution of the recovery of the ozone layer.
This approach provided a pathway for
interim conclusions on this issue, but many issues remained unresolved. They include:
How should recovery be defined?
What time period is appropriate as a baseline against which we can measure recovery?
How do we separate ozone changes due to ODSs from those due to changes in climate
and tropospheric composition? How do we describe and attribute future
changes in levels of ozone? Given the natural variability, at which point will one be
confident of the recovery from ODS
effects? This Assessment addresses some of these issues and concepts (see Prologue
Box 2 on Recovery Issues).