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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).