ozone depleation

8/3/2019 Ozone Depleation

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Ozone Layer Depletion

ShareINTRODUCTION

The ozone layer protects the Earth from the ultraviolet rays sent down by the

sun. If the ozone layer is depleted by human action, the effectson the planet could

be catastrophic.

Ozone is present in the stratosphere. The stratosphere reaches 30 miles above the

Earth, and at the very top it contains ozone. The suns rays are absorbed by the

ozone in the stratosphere and thus do not reach the Earth.

Ozone is a bluish gas that is formed by three atoms of oxygen. The form of

oxygen that humans breathe in consists of two oxygen atoms, O2. When found onthe surface of the planet, ozone is considered a dangerous pollutant and is one

substance responsible for producingthe greenhouse effect.

The highest regions of the stratosphere contain about 90% of all ozone.

In recent years, the ozone layer has been the subject of much discussion. And

rightly so, because the ozone layer protects both plant and animal life on the

planet.

The fact that the ozone layer was being depleted was discovered in the mid-1980s. The main cause of this is the release of CFCs, chlorofluorocarbons.

Antarctica was an early victim of ozone destruction. A massive hole in the ozone

layer right above Antarctica now threatens not only that continent, but many

others that could be the victims of Antarctica's melting icecaps. In the future, the

ozone problem will have to be solved so that the protective layer can be

conserved.

OZONE DEPLETION & GLOBAL WARMING

Although they are often interlinked in the mass media, the connection between

global warming and ozone depletion is not strong. There are four areas of

linkage:

The same CO2 radiative forcing that produces near-surface global warming is

expected to cool the stratosphere. This cooling, in turn, is expected to produce a
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relative increase in polar ozone (O3) depletionand the frequency of ozone holes.

Conversely, ozone depletion represents a radiative forcing of the climate system.

There are two opposing effects: Reduced ozone causes the stratosphere to absorb

less solar radiation, thus cooling the stratosphere while warming the

troposphere; the resulting colderstratosphere emits less long-wave radiation

downward, thus cooling the troposphere. Overall, the cooling dominates; the

IPCC concludes that "observed stratospheric O3 losses over the past two decades

have caused a negative forcing of the surface-troposphere system" of about 0.15

0.10 watts per square meter (W/m).

One of the strongest predictions of the greenhouse effect is that

thestratosphere will cool. Although this cooling has been observed, it is not trivial

to separate the effects of changes in the concentration of greenhouse gases and

ozone depletion since both will lead to cooling. However, this can be done by

numerical stratospheric modeling. Results from the National Oceanic and

Atmospheric Administration's Geophysical Fluid Dynamics Laboratory show

that above 20 km (12.4 miles), the greenhouse gases dominate the cooling.

Ozone depleting chemicals are also greenhouse gases. The increases in

concentrations of these chemicals have produced 0.34 0.03 W/m of radiative

forcing, corresponding to about 14% of the total radiative forcing from increases

in the concentrations of well-mixed greenhouse gases.

The long term modeling of the process, its measurement, study, design of theoriesand testing take decades to both document, gain wide acceptance, and ultimately

become the dominant paradigm. Several theories about the destruction of ozone,

were hypothesized in the 1980s, published in the late 1990s, and are currently

being proven. Dr Drew Schindell, and Dr Paul Newman, NASA Goddard,

proposed a theory in the late 1990s, using a SGI Origin 2000 supercomputer, that

modeled ozone destruction, accounted for 78% of the ozone destroyed. Further

refinement of that model, accounted for 89% of the ozone destroyed, but pushed

back the estimated recovery of the ozone hole from 75 years to 150 years.

OZONE HOLE AND ITS CAUSES

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the

recent ozone levels have dropped to as low as 33% of their pre-1975 values. The

ozone hole occurs during the Antarctic spring, from September to early

December, as strong westerly winds start to circulate around the continent and

create an atmospheric container. Within this polar vortex, over 50% of the lower


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stratospheric ozone is destroyed during the Antarctic spring.

As explained above, the overall cause of ozone depletion is the presence of

chlorine-containing source gases (primarily CFCs and related halocarbons). In

the presence of UV light, these gases dissociate, releasing chlorine atoms, which

then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can

take place in the gas phase, but it is dramatically enhanced in the presence of

polar stratospheric clouds (PSCs).

These polar stratospheric clouds form during winter, in the extreme cold. Polar

winters are dark, consisting of 3 months without solar radiation (sunlight). Not

only lack of sunlight contributes to a decrease in temperature but also the polar

vortex traps and chills air. Temperatures hover around or below -80 C. These

low temperatures form cloud particles and are composed of either nitric acid or

ice . Both types provide surfaces for chemical reactions that lead to ozone

destruction.

The photochemical processes involved are complex but well understood. The key

observation is that, ordinarily, most of the chlorine in the stratosphere resides in

stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine

nitrate (ClONO2). During the Antarctic winter and spring, however, reactions

on the surface of the polar stratospheric cloud particles convert these

"reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can

also remove NO2 from the atmosphere by converting it to nitric acid, which

prevents the newly formed ClO from being converted back into ClONO2.

The role of sunlight in ozone depletion is the reason why the Antarctic

ozone depletion is greatest during spring. During winter, even though PSCs are

at their most abundant, there is no light over the pole to drive the chemical

reactions. During the spring, however, the sun comes out, providing energy to

drive photochemical reactions, and melt the polar stratospheric clouds, releasing

the trapped compounds.

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the

much smaller ozone depletion through homogeneous gas phase reactions, whichoccurs primarily in the upper stratosphere.

Warming temperatures near the end of spring break up the vortex around mid-

December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are

destroyed, the ozone depletion process shuts down, and the ozone hole heals.


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CAUSES OF OZONE DEPLETION

Ozone depletion occurs when the natural balance between the production and

destruction of stratospheric ozone is tipped in favour of destruction. Although

natural phenomena can cause temporary ozone loss, chlorine and bromine

released from man-made compounds such as CFCs are now accepted as the main

cause of this depletion.

It was first suggested by Drs. M. Molina and S. Rowland in 1974 that a man-

made group of compounds known as the chlorofluorocarbons (CFCs) were likely

to be the main source of ozone depletion. However, this idea was not taken

seriously until the discovery of the ozone hole over Antarctica in 1985 by the

British Antarctic Survey.

Chlorofluorocarbons are not "washed" back to Earth by rain or destroyed in

reactions with other chemicals. They simply do not break down in the lower

atmosphere and they can remain in the atmosphere from 20 to 120 years or

more. As a consequence of their relative stability, CFCs are instead transported

into the stratosphere where they are eventually broken down by ultraviolet (UV)

rays from the Sun, releasing free chlorine. The chlorine becomes actively

involved in the process of destruction of ozone. The net result is that two

molecules of ozone are replaced by three of molecular oxygen, leaving the

chlorine free to repeat the process:

Cl + O3 ClO + O2

ClO + O Cl + O2

Ozone is converted to oxygen, leaving the chlorine atom free to repeat the

process up to 100,000 times, resulting in a reduced level of ozone. Bromine

compounds, or halons, can also destroy stratospheric ozone. Compounds

containing chlorine and bromine from man-made compounds are known as

industrial halocarbons.

Emissions of CFCs have accounted for roughly 80% of total stratospheric ozonedepletion. Thankfully, the developed world has phased out the use of CFCs in

response to international agreements to protect the ozone layer. However,

because CFCs remain in the atmosphere so long, the ozone layer will not fully

repair itself until at least the middle of the 21st century. Naturally occurring

chlorine has the same effect on the ozone layer, but has a shorter life span in the

atmosphere..


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Every time 1% of the ozone layer is depleted, 2% more UV-B is able to reach the

surface of the planet. UV-B increase is one of the most harmful consequences of

ozone depletion because it can cause skin cancer.

The increased cancer levels caused by exposure to this ultraviolet light could be

enormous. The EPA estimates that 60 million Americans born by the year 2075

will get skin cancer because of ozone depletion. About one million of these people

will die.

The environment will also be negatively affected by ozone depletion. The life

cycles of plants will change, disrupting the food chain. Effects on animals will

also be severe, and are very difficult to foresee.

Oceans will be hit hard as well. The most basic microscopic organisms such as

plankton may not be able to survive. If that happened, it would mean that all of

the other animals that are above plankton in the food chain would also die out.

Other ecosystems such as forests and deserts will also be harmed.

The planet's climate could also be affected by depletion of the ozone layer. Wind

patterns could change, resulting in climatic changes throughout the world.

HEALTH AND ENVIRONMENTAL EFFECTS OF OZONE DEPLETION

Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layerdepletion is expected to increase surface UVB levels, which could lead to damage,

including increases in skin cancer. This was the reason for the Montreal

Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and

there are good theoretical reasons to believe that decreases in ozone will lead to

increases in surface UVB, there is no direct observational evidence linking ozone

depletion to higher incidence of skin cancer in human beings. This is partly due

to the fact that UVA, which has also been implicated in some forms of skin

cancer, is not absorbed by ozone, and it is nearly impossible to control statistics

for lifestyle changes in the populace.

(1) Increased UV

Ozone, while a minority constituent in the earth's atmosphere, is responsible for

most of the absorption of UVB radiation. The amount of UVB radiation that

penetrates through the ozone layer decreases exponentially with the slant-path

thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone


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is expected to give rise to significantly increased levels of UVB near the surface.

Increases in surface UVB due to the ozone hole can be partially inferred by

radiative transfer model calculations, but cannot be calculated from direct

measurements because of the lack of reliable historical (pre-ozone-hole) surface

UV data, although more recent surface UV observation measurement

programmes exist (e.g. at Lauder, New Zealand).

Because it is this same UV radiation that creates ozone in the ozone layer from

O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would

actually tend to increase photochemical production of ozone at lower levels (in

the troposphere), although the overall observed trends in total column ozone still

show a decrease, largely because ozone produced lower down has a naturally

shorter photochemical lifetime, so it is destroyed before the concentrations could

reach a level which would compensate for the ozone reduction higher up.

Biological effects of increased UV and microwave radiation from a depleted

ozone layer

The main public concern regarding the ozone hole has been the effects of surface

UV on human health. So far, ozone depletion in most locations has been typically

a few percent and, as noted above, no direct evidence of health damage is

available in most latitudes. Were the high levels of depletion seen in the ozone

hole ever to be common across the globe, the effects could be substantially more

dramatic. As the ozone hole over Antarctica has in some instances grown so largeas to reach southern parts of Australia and New Zealand, environmentalists have

been concerned that the increase in surface UV could be significant.

(2) Effects on Human Health

Laboratory and epidemiological studies demonstrate that UVB causes

nonmelanoma skin cancer and plays a major role in malignant melanoma

development. In addition, UVB has been linked to cataracts. All sunlight

contains some UVB, even with normal ozone levels. It is always important to

limit exposure to the sun. However, ozone depletion will increase the amount ofUVB and the risk of health effects.

(3) Effects on Plants

Physiological and developmental processes of plants are affected by UVB

radiation, even by the amount of UVB in present-day sunlight. Despite


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mechanisms to reduce or repair these effects and a limited ability to adapt to

increased levels of UVB, plant growth can be directly affected by UVB radiation.

Indirect changes caused by UVB (such as changes in plant form, how nutrients

are distributed within the plant, timing of developmental phases and secondary

metabolism) may be equally, or sometimes more, important than damaging

effects of UVB. These changes can have important implications for plant

competitive balance, herbivory, plant diseases, and biogeochemical cycles.

(4) Effects on Marine Ecosystems

Phytoplankton form the foundation of aquatic food webs. Phytoplankton

productivity is limited to the euphotic zone, the upper layer of the water column

in which there is sufficient sunlight to support net productivity. The position of

the organisms in the euphotic zone is influenced by the action of wind and waves.

In addition, many phytoplankton are capable of active movements that enhance

their productivity and, therefore, their survival. Exposure to solar UVB

radiation has been shown to affect both orientation mechanisms and motility in

phytoplankton, resulting in reduced survival rates for these organisms. Scientists

have demonstrated a direct reduction in phytoplankton production due to ozone

depletion-related increases in UVB. One study has indicated a 6-12% reduction

in the marginal ice zone.

Solar UVB radiation has been found to cause damage to early developmental

stages of fish, shrimp, crab, amphibians and other animals. The most severeeffects are decreased reproductive capacity and impaired larval development.

Even at current levels, solar UVB radiation is a limiting factor, and small

increases in UVB exposure could result in significant reduction in the size of the

population of animals that eat these smaller creatures.

(5) Effects on Biogeochemical Cycles

Increases in solar UV radiation could affect terrestrial and aquatic

biogeochemical cycles, thus altering both sources and sinks of greenhouse and

chemically-important trace gases e.g., carbon dioxide (CO2), carbon monoxide(CO), carbonyl sulfide (COS) and possibly other gases, including ozone. These

potential changes would contribute to biosphere-atmosphere feedbacks that

attenuate or reinforce the atmospheric buildup of these gases.

(6) Effects on Materials


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Synthetic polymers, naturally occurring biopolymers, as well as some other

materials of commercial interest are adversely affected by solar UV radiation.

Today's materials are somewhat protected from UVB by special additives.

Therefore, any increase in solar UVB levels will therefore accelerate their

breakdown, limiting the length of time for which they are useful outdoors.

(7) Effects on crops

An increase of UV radiation would be expected to affect crops. A number of

economically important species of plants, such as rice, depend on cyanobacteria

residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive

to UV light and they would be affected by its increase.

(8) Effects on plankton

Research has shown a widespread extinction of plankton 2 million years ago that

coincided with a nearby supernova. There is a difference in the orientation and

motility of planktons when excess of UV rays reach earth. Researchers speculate

that the extinction was caused by a significant weakening of the ozone layer at

that time when the radiation from the supernova produced nitrogen oxides that

catalyzed the destruction of ozone (plankton are particularly susceptible to

effects of UV light, and are vitally important to marine food webs).

CURRENT EVENTS AND FUTURE PROSPECTS OF OZONE DEPLETION

Since the adoption and strengthening of the Montreal Protocol has led to

reductions in the emissions of CFCs, atmospheric concentrations of the most

significant compounds have been declining. These substances are being gradually

removed from the atmosphere. By 2015, the Antarctic ozone hole would have

reduced by only 1 million km out of 25 complete recovery of the Antarctic ozone

layer will not occur until the year 2050 or later. Work has suggested that a

detectable recovery will not occur until around 2024, with ozone levels

recovering to 1980 levels by around 2068.

A 2005 IPCC summary of ozone issues observed that observations and modelcalculations suggest that the global average amount of ozone depletion has now

approximately stabilized. Although considerable variability in ozone is expected

from year to year, including in polar regions where depletion is largest, the ozone

layer is expected to begin to recover in coming decades due to declining ozone-

depleting substance concentrations, assuming full compliance with the Montreal

Protocol.


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Temperatures during the Arctic winter of 2006 stayed fairly close to the long-

term average until late January, with minimum readings frequently cold enough

to produce PSCs. During the last week of January, however, a major warming

event sent temperatures well above normalmuch too warm to support PSCs.

By the time temperatures dropped back to near normal in March, the seasonal

norm was well above the PSC threshold. Preliminary satellite instrument-

generated ozone maps show seasonal ozone buildup slightly below the long-term

means for the Northern Hemisphere as a whole, although some high ozone events

have occurred. During March 2006, the Arctic stratosphere poleward of 60

degrees North Latitude was free of anomalously low ozone areas except during

the three-day period from March 17 to 19 when the total ozone cover fell below

300 DU over part of the North Atlantic region from Greenland to Scandinavia.

The area where total column ozone is less than 220 DU (the accepted definition of

the boundary of the ozone hole) was relatively small until around 20 August

2006. Since then the ozone hole area increased rapidly, peaking at 29 million km

September 24. In October 2006, NASA reported that the year's ozone hole set a

new area record with a daily average of 26 million km between 7 September and

13 October 2006; total ozone thicknesses fell as low as 85 DU on October 8. The

two factors combined, 2006 sees the worst level of depletion in recorded ozone

history. The depletion is attributed to the temperatures above the Antarctic

reaching the lowest recording since comprehensive records began in 1979.

The Antarctic ozone hole is expected to continue for decades. Ozoneconcentrations in the lower stratosphere over Antarctica will increase by 5%

10% by 2020 and return to pre-1980 levels by about 20602075, 1025 years later

than predicted in earlier assessments. This is because of revised estimates of

atmospheric concentrations of Ozone Depleting Substancesand a larger

predicted future usage in developing countries. Another factor which may

aggravate ozone depletion is the draw-down of nitrogen oxides from above the

stratosphere due to changing wind patterns.

THE WORLD'S REACTION

The initial concern about the ozone layer in the 1970s led to a ban on the use of

CFCs as aerosol propellants in several countries, including the U.S. However,

production of CFCs and other ozone-depleting substances grew rapidly

afterward as new uses were discovered.

Through the 1980s, other uses expanded and the world's nations became


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increasingly concerned that these chemicals would further harm the ozone layer.

In 1985, the Vienna Convention was adopted to formalize international

cooperation on this issue. Additional efforts resulted in the signing of the

Montreal Protocol in 1987. The original protocol would have reduced the

production of CFCs by half by 1998.

After the original Protocol was signed, new measurements showed worse damage

to the ozone layer than was originally expected. In 1992, reacting to the latest

scientific assessment of the ozone layer, the Parties to the Protocol decided to

completely end production of halons by the beginning of 1994 and of CFCs by

the beginning of 1996 in developed countries.

Because of measures taken under the Montreal Protocol, emissions of ozone-

depleting substances are already falling. Levels of total inorganic chlorine in the

stratosphere peaked in 1997 and 1998. The good news is that the natural ozone

production process will heal the ozone layer in about 50 years.

CONCLUSION

The ozone layer is the earth's protection against harmful ultra-violet rays that

cause cancer. The ozone is a region of the earth's atmosphere that is 12 to 30 mi

above the earth's surface. (www.encarta.msn.com) The layer of the Earth's

atmosphere that surrounds us is called the troposphere. The stratosphere, the

next higher layer, extends about 1050 kilometers above the Earth's surface.

Stratospheric ozone is a naturally occurring gas that filters the sun's ultraviolet(UV) radiation.

The ozone's job is to absorb much of the radiation that the sun gives off. There

are two forms of this radiation that come from the sun: visible light and ultra-

violet light. Visible light waves are the only electromagnetic waves we can see

with are eyes and ultra-violet light is the radiation that humans have to protect

themselves against. Humans on earth are destroying the very thing that protects

us and allow us to continue to live on this earth. Without our ozone all human

and living things on this earth will seize to existence. To protect our home we

need to evaluate the causes, the effects and the future of ozone depletion.

With care, the effects of ozone-depletion will not be too bad. On the other hand, a

mechanism for the disposal and safe destruction of ODSs and equipment

containing ODSs does not reliably exist in many countries and should be

implemented as soon as possible, with qualified and conscientious personnel for

the maintenance.


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ozone depleation

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