The ozone layer

The ozone is a layer of the earth’s atmosphere, the stratosphere, where the highest concentration of ozone ,O3 , is found. It is situated at approximately 10 km to 50 km above the surface of the earth. The Ozone only makes up 0.00006% of the atmosphere. The ozone layer was discovered in 1913 by two french phycisists: Charles Farbry and henry buisson.

This diagram shows the concentrations of ozone measured by the nimbus-7 satellite.
(Author: NASA. Website:http://en.wikipedia.org/wiki/File:Nimbus_ozone_Brewer-Dobson_circulation.jpg)

More about Ozone :
Ozone found at the lower atmosphere acts as a harmful substance. Besides being an air pollutnat, Ozone also causes breathing problems to various animals and burns sensitive plants. Ozone which is found at the upper part of the atmosphere, the ozone layer is beneficial as it prevents ultraviolet rays from reaching the surface of the earth.

Diagram of an ozone molecule.
http://en.wikipedia.org/wiki/File:Ozone-CRC-MW-3D-balls.png

The ozone molecule is made up of three oxygen molecules joined together by weak van der waals forces. it is a powerful oxidizing agent, more powerful than O2.

It can react wiith most metals (excpetions: Gold, Platinum, Silver etc.) into oxides of the metals. For example: 2 Cu+ + 2 H3O+ + O3 → 2 Cu2+ + 3 H2O + O2

Ozone can also react with Nitrogen and Carbon compounds. For example, Ozone can oxidise Nitric Oxide into Nitrogen Dioxide: NO + O3 → NO2 + O2 .

Also, reaction of Ozone with Sulfur compounds into sulfates is also possible. For example, lead (II) Sulfide is oxidised to Lead (II) Sulfate: PbS + 4 O3 → PbSO4 + 4 O2.

Formation of the Ozone Layer:

Ultraviolet rays hit the oxygen molecules in the atmosphere, causing the diatomic oxygen molecules to break into two.
O2 ------ultraviolet rays-----> 2O

Then each individual oxygen atom (aka atomic oxygen) combines with unbroken oxygen molecues to form triatomic molecues known as ozone.
O + O2-----------------------> O3

The Ozone-Oxygen Cycle:

This diagram illustrates the ozone-oxygen cycle.
Created by: NASA derivative work Website found: http://en.wikipedia.org/wiki/File:Ozone_cycle.svg

The Ozone-Oxygen Cycle complements the formation of the ozone layer. It shows how oxygen molecules, which make up about 21% of the atmospheric gases, is being transformed from Oxygen molecules, then into Oxygen atoms and then joined back to form the Ozone molecules and then broken apart again into Oxygen molecules and the cycle repeats itself.

Ozone Depletion

Ozone Depletion

Ozone depletion is the anomalous decrease in "total column ozone" over Earth's surface. This could be caused by variations in sunlight, but its not. It caused by contaminants that are added to the atmosphere in addition to what Nature throws up there. These contaminants include water vapor and chlorofluorocarbons (CFCs). After controlling CFCs, their levels are slowly decreasing. So water vapor, that stuff we spew out the back ends of our jets, and spray into the air from our cooling towers, is the major contaminant we have yet to control.

The ozone hole
What is the ozone hole:
A definition by the The American Heritage® Science DictionaryCopyright © 2002. Published by Houghton Mifflin states that the ozone hole is "A severe depletion of ozone in a region of the ozone layer, particularly over Antarctica and over the Arctic. The depletion is caused by the destruction of ozone by CFCs and by other compounds, such as carbon tetrachloride (CCl4) and carbon tetrafluoride (CF4). The amount of ozone in ozone holes is about 55 to 60 percent of the normal concentration in the ozone layer. Although the full effect of increased ozone depletion is not yet known, the amount of ultraviolet radiation the Earth receives is greatly increased by ozone depletion, creating a heightened risk of skin cancers and likely contributing to global warming. "

An article, “the origin of the ozone hole- Natural or Anthropological” by James A. Marusek, written on 24 February 2005 states that there are 2 theories about what caused the ozone hole
1.Anthropological : the release of man-made material
2.Natural: a weakening of the earth’s magnetic field

Anthropological reasoning:
The depletion of ozone is due to release of man-made chemicals such as chlorofluorocarbon (CFC)compounds and ozone depleting substances such as carbon tetrachloride, methyl bromide and methyl chlorofoam.
As they reach into the stratosphere, ultraviolet raditation causes these sompounds to break apart, releasing chlorine atoms, halons and bromine atoms which would in turn destroy the ozone. (See Free Radical Catalysts)

If this theory were to be correct, then we would expect to see that the ozone hole would appear at the mid- Latitude Nothern Hemisphere zone where the industry and population centers in the U.S, Canada, Europe, India, Asia, Russia, China and Japan. However, as we see it now, the ozone hole appears at the Southern hemisphere, one of the most pristine places left on the planet, Antartica.

Natural theory:
As the earth’s magnetic field weakens, less energetic protons in solar and galactic cosmic rays are now able to enter the stratosphere and thereby producing an increase in the ozone depleting chemical, nitric oxide.

NO + O3 ___>NO2 + O2
O + NO2 ___>NO + O2


On the Earth’s surface, the magnetic field varies from being horizontal and of magnitude ~ 30,000 nanoTesla (nT) near the equator to vertical and ~ 60,000 nT near the poles. Charged particles are deflected by the horizontal component of the Earth’s magnetic field. Therefore the magnetic shielding of charged particles is strongest above the equator and weakest above the poles.

If this theory were to be correct, we would expect the highest concentration of nitric oxide production to occur at the magnetic pole because at this location the field in general has only a vertical component.

Present Situation of Ozone Depletion:
Discovered in 1985 by the British Antarctic Survey (BAS), the south pole has been experiencing a decline of magnetic field of 13 percent during the past century. The antartic ozone hole has reached a maximum size of around 10.5 million square miles.

Causes

CAUSES - Chlorine in the atmosphere

Chlorine in the atmosphere can be contributed in two ways: manmade and natural. Manmade chlorine comes from Chlorofluorocarbons (CFCs) and natural chlorine is released during volcanic eruptions or through the production of sea salt. Majority of CFCs released are manmade (75%<)

CFCs

CFCs are mainly used in our daily lives, ranging from air conditioners to aerosal sprays and styrofoam. CFCs are used for household appliances as they are low in toxicity, nonflammable, non-corrosive and non-reactive with other chemical substances. As such, they serve their purposes as user-safe materials. However, CFCs when released in the stratosphere do not have such desirable chemical properties unlike they have on land. CFC’s take about 15 years to reach the stratosphere, and stay there for about 100 years. In this time they destroy up to 100,000 molecules of ozone.

CFC REACTIONS
In the stratosphere, CFCs come into contact with short wavelength ultraviolet radiation which is able to split off chlorine atoms from the CFC molecules
u.v radiation
CCl3F(g) --------------->CCl2F(g) + Cl(g)
These chlorine atoms destroy the ozone layer

Cl(g) + O3(g) -------------> ClO(g) + O2(g)

ClO(g) + O(g) -------------> O2(g) + Cl(g)
http://www.ausetute.com.au/cfcozone.html

Free-radical catalysts:
These are atoms, molecules and or ions with unpaired electrons; elements which have not achieved the stable octet configuration. As such they are highly reactive. While most of the Nitric Oxide Radical (NO-) and Hydroxyl Radical (OH-) are produced naturally, human activities have caused the amount of Chlorine (Cl-) and Bromine (Br-) to increase. These elements are found in certain stable organic compounds especially in chlorofluorocarbons.

CFCl3 + ultraviolet rays → CFCl2 + Cl-
The liberation of the chloride atoms can destroy the ozone molecules in a number of ways. For example:

Cl + O3 → ClO + O2
When the Cl atoms combine with the ozone molecules, they form chlorine monoxide (CIO) and oxygen. Thereby breaking down oxygen.
Then the chlorine monoxide in turn can react with ozone molecules to form another chloride atom and two oxygen molecules.
ClO + O3 → Cl + 2 O2

This diagram illustrates the prediction by the NASA of the ozone layer in the present and in the future if CFCs are not banned.
Author: NASA
Source: http://earthobservatory.nasa.gov/images/imagerecords/38000/38685/ozone_wa_midlat.jpg
Website taken from: http://en.wikipedia.org/wiki/File:Future_ozone_layer_concentrations.jpg

On a per atom basis, the bromide atom is more efficient in breaking down the ozone. Fortunately, at present, the amount of chloride atom is more than the amount of bromide atoms.
This image shows the global monthly average total ozone amount for the time period 1979 through the end of 2001. The green line shows the results from Nimbus-7 TOMS instrument. The red line shows the results from the Meteor-3 TOMS instrument. The blue line shows the results from the Earth Probe TOMS instrument, which developed a fault in 2002.

Author: NASA
Source: http://toms.gsfc.nasa.gov/multi/ozone_time_series.jpg
Website taken from: http://www.newworldencyclopedia.org/entry/Image:TOMS_Global_Ozone_65N-65S.png

Ultraviolet rays:
The primary source of Ultraviolet rays is the sun.

This diagram shows the different “types” of ultraviolet rays and its affect on the different layers of the atmosphere.

Created by: Center for Global Environmental Research, National Institute for Environmental Studies Japan
Website taken from: http://www.theozonehole.com/uvrays.htm

Based on the length of their wavelength, ultraviolet rays are being classified into 3 different categories

1. UVA wavelengths(320-400 nm). Most UVA radiation are able to reach the earth's surface and can contribute to tanning, skin aging, eye damage, and and many other health issues. It is only affected by the Ozone Layer minimally
2. UVB wavelengths(280-320 nm).The amount of UVB radiation is mostly reflected away by the ozone layer. However, a decrease in stratospheric ozone mean that an increase of UVB radiation can reach the earth's surface, causing sunburns, snow blindness, and a variety of skin problems including skin cancer and premature aging.
3. UVC wavelengths (100-280 nm). Only a little UVC radiation can reach the surface of the earth. This is because the Ozone Layer blocks most of it away.
Reference: http://www.theozonehole.com/uvrays.htm

The ozone layer absorbs most of the ultraviolet rays from the sun.

Effects of ozone depletion

The main concern of ozone depletion is the increase in the amount of UVB rays reaching Earth, which can have many effects on humans, our environment, animals, plants as well as materials.

The most known effect of ozone depletion on humans is skin cancer. Reduction in ozone levels will lead to higher levels of UVB reaching the Earth's surface. Less ozone means less protection for the Earth from the Sun's UV rays and thus, more UV rays will enter the Earth. An increase in exposure to UVB rays will lead to more absorption of UVB radiation. This will in fact cause DNA in humans to produce transcription errors when replicating, which causes nonmelanoma cancer.

There are two kinds of cancer involved, nonmelanoma cancer and malignent melanoma. The first is relatively mild and can be treated but the latter is much less common and much more dangerous, being lethal in about 15%-20% of the cases diagnosed. UVB rays are known to play a significant role in the development of malignent melanoma.

Research has shown that UV radiation increases the likelihood of certain cataracts.
Cataracts are a form of eye damage in which a loss of transparency in the lens of the eye clouds vision. If left untreated, cataracts can lead to blindness.

The increase in the amount of UV radiation reaching the Earth can cause adverse health risks, such as respiratory problems and weaking of the immune system. The body's immune system is its first line of defense against invading germs. Recent research has shown that the some viruses can be activated by increased exposure to UV.

However, ozone depletion has one possible beneficial effect. The inceased UV radiation entering the Earth also represents the increase in Vitamin D capacity in the sunlight reaching Earth. UVB and/or Vitamin D has been found to be associated with reduced risk over a dozen forms of cancer.

Ozone depletion also causes a disruption in the marine ecosystem. Studies have shown that UV rays can affect phytoplankton, the foundation of the marine food chain. The UV rays affect the orientation mechanisms and mobility of microorganisms as such and reduce their survival rates.

Solar UVB radiation has also been found to cause damage to the early developmental stages of fish, shrimp, crab, amphibians and other animals. The most severe effects are a decreased reproductive capacity as well as impaired larval development. Hence, an increase in exposure to UVB rays can cause a significant reduction in the marine population for animals which feed on these small animals.

Ozone depletion also affects plants. A number of economically important species of plants, such as rice, depends on cyanobacteria residing on their roots for them to absorb and utilise nitrogen properly. Cyanobacteria are sensitive to UV light and they would be affected by its increase.

Plant growth will also be affected by the increase in UV rays. Physiological and developmental processes of plants are affected by UVB radiation, despite mechanisms to reduce or repair these effects and a limited ability to adapt to increased levels of UVB.

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

The increasing concern for the causes and effects of ozone depletion led to the adoption of the Montreal Protocol, in the year 1987, in order to reduce and control the industrial emission of chlorofluorocarbons. International agreements have succeeded to a great extent in reducing the emission of these compounds, however, more cooperation and understanding among all the countries of the world is required to mitigate the problem.

This is a simple graph to illustrate the effects of ozone depletion.

source: http://flatplanet.wikispaces.com/file/view/effects_of_ozone.jpg/30605011/effects_of_ozone.jpg

sources: http://www.ehso.com/ehshome/OzoneDepletionEffects.htm

http://en.wikipedia.org/wiki/Ozone_depletion#Effects_on_humans
http://www.buzzle.com/articles/ozone-layer-depletion-effects-and-causes-of-ozone-depletion.html
http://flatplanet.wikispaces.com/Definition+of+Topic+and+it's+Effects?f=print

What can be done to prevent ozone depletion

On a personal level, we can first change the light bulbs at home to compact fluoroscent light bulbs so as to conserve energy use which will slow down global warming and in turn reduce the depletion of the ozone layer.

We can also turn off and plug off all appliances and electronics, such as computers, if we are not using them.

When using trasport, we can carpool, walk or cycle instead of driving cars alone. Taking the bus is also a way of reducing the amount of harmful gases produced. Another way to prevent ozone depletion is to invest in a car that emits lesser chemicals.

We can support environmental organizations and spread the word about the causes and effects of ozone depletion to people around us.

Factories can improve the containment of chemicals to prevent leaks, evaporation and emissions of unitended by-products. They can also reudce the amount of CFCs needed in any particular type of equipment, by increasing the use of ammonia and other alternatves that have a lower global warming potential. Techniques that avoid the use of gases the play a part in ozone depletion or contribute to climate change can also be used.

On a global level, there have been treaties signed such as the Montreal Protocol to phase out the use of substances that contribute to ozone depletion.

source: http://www.unep.org/Documents.Multilingual/Default.asp?DocumentID=430&ArticleID=4768&l=en

Misconceptions

1. CFC molecules are too heavy so they cannot reach the stratosphere
Even though CFC molecules have larger atomic mass than nitrogen or oxygen molecules, the mass of the molecules does no determine the composition of air in the atmosphere. The wind in the troposphere is strong enough to carry the air to the atmosphere. As there are no natural processes which break down CFCs, they remain uniformly distributed vertically and horizontally along the atmosphere.

2. Manmade chlorine is does not cause ozone depletion as much as natural chlorine
Manmade chlorine causes more ozone depletion than natural chlorine. Forms of natural chlorine include hydrogen chloride (HCl) released from volcano eruptions and sea salt which is 50% chlorine.

Despite the large amounts of chlorine which are produced naturally, natural chlorine can be depleted. Even though volcanoes release large amounts of hydrogen chloride during eruption, the gas has only enough energy to reach the troposphere and not the stratosphere. Sea salts are released very low in the atmosphere, as such they take 2 to 5 years to reach the stratosphere. Rain from the troposphere will dissolve the chlorine gas, thus the chlorine gas does not contributes minimally to ozone depletion.

On the other hand, CFCs (manmade chlorine) cannot be dissolved by water, and no chemical method can help to remove CFCs. So, CFCs deplete naturally and will have a long lifespan in the atmosphere.

Pie Chart showing percentages of different sources contributing to ozone depletion

3. Ozone layer depletion occurs only in Antarctica
The ozone hole is above Antarctica. However, ozone depletion does not only occur in Antarctica. Ozone levels vary seasonally and according to latitude. Ozone levels are greater at high latitudes (more depletion), and as such, Artic regions also experience ozone depletion. Antarctic region experiences more ozone depletion because low temperatures due to polar vertex allow polar stratospheric clouds to form. Stronger westerly winds circulate around the region – more chlorine molecules. Since the latitudes at tropical regions are lower, Ozone at the tropics do not deplete as quickly.

Montreal Protocol

What is the Montreal Protocol?
The Montreal Protocol on Substances that Deplete the Ozone Layer is an international treaty designed to protect the ozone layer by phasing out a number of substances believed to be responsible for ozone depletion.

The Montreal Protocol was adopted on 16 September 1987. The Montreal Protocol has been revised five times to tighten the control over the subtstances that are believed to be responsible for ozone depletion and their phase-out schedules.

Terms and Purposes of the Montreal Protocol

The treaty is structured around several groups of halogenated hydrocarbons that have been shown to play a role in ozone depletion. All of these substances contain either chlorine or bromine.

For each group, the treaty provides a timetable on which the production of these substances must be phased out and eventually eliminated.

Chloroflurocarbons (CFCs) Phase-out Management Plan

The stated purpose of the treaty is that the signatory states recognizes that worldwide emissions of certain substances can significantly deplete and otherwise modify the ozone layer in a manner that is likely to result in adverse effects on human health and the environment, are determined to protect the ozone layer by taking precautionary measures to control total global emissions of substances that deplete it, with the ultimate objective of their elimination on the basis of developments in scientific knowledge and acknowledges that special provision is required to meet the needs of developing countries.

The signatory states accept a series of stepped limits on CFC use and production, including the following.

The levels of consumption and production of the controlled substances in Group I of Annex A from 1991 to 1992 do not exceed 150% of its calculated level of consumption of those substances in 1986.

The calculated levels of consumption and production of the controlled substances in Group I of Annex A from 1994 onwards does not exceed 25% of its calculated level of consumption and production in 1986.

The calculated levels of consumption and production of the controlled substances in Group I of Annex A from 1996 onwards does not exceed zero.

The substances in Group I of Annex A are:

• CFCl₃ (CFC-11)
• CF₂Cl₂ (CFC-12)
• C₂F₃Cl₃ (CFC-113)
• C₂F₄Cl₂ (CFC-114)
• C₂F₅Cl (CFC-115)
  

There is a slower phase out of substances such as halon 1211, 1301, 2402 and CFCs 13, 111, 112 etc and some chemicals that get individual attention such as carbon tetrachloride and trichloroethane. They are to be phased out to zero by 2010. The phasing out of the less active HCFCs started out in 1996 and will go on until 2030.

Hydrochloroflurocarbons (CFCs) Phase-out Management Plan

Parties to the Protocol agreed to set the year 2013 as the time to freeze the consumption and production of HCFCs. They also agreed to start reducing its consumption and production in 2015.

The HCFCs are transitional CFCs replacements, used as refrigerants, solvents, blowing agents for plastic foam manufacture and fire extinguishers. In comparison to CFCs that have Ozone Depleting Potential (ODP) of 0.6 – 1.0, HCFCs have a lesser ODP of 0.01 – 0.5. In comparison to CFCs in terms of Global Warming Potential(GWP) that has a GWP of 4,680 – 10,720, HCFCs have a lesser GWP of 76 – 2,270.

There are a few exceptions for uses where no acceptable substitutes have been found. For example, in the metered dose inhalers that are commonly use to treat asthma and other respiratory problems or in Halon fire suppression systems used in submarines and aircraft.

Parties to the Protocol must base their future decisions on the current scientific, environmental, technical, and economic information that is assessed through panels drawn from worldwide expert committees. To provide the input to the decision making process, advances in understanding of these topics were assessed in 1989, 1991, 1994, 1998 and 2002.

Several reports have been published by various governmental and non-governmental organizations to present alternatives to the ozone depleting substances, since the substances have been used in various technical sectors such as refrigerating, agriculture, energy production and laboratory measurements.

Impact of the Montreal Protocol
Since the implementation of the Montreal Protocol, the atmospheric concentrations of the most important chlorofluorocarbons and related chlorinated hydrocarbons have either been levelled off or reduced. Halon concentrations have increased, as the halons present in fire extinguishers are released. Their rate of increase, however, has slowed down and their abundances are expected to decline by 2020. The concentration of HCFCs has also increased drastically partly due to the replacement of CFCs by HCFCs. Recent scientific evaluation in 2006 have shown that there is a clear decrease in the atmospheric burden of ozone depleting substances and there are some early signs of stratospheric ozone recovery.

But HCFCs are now thought to contribute to global warming. These compounds are much more potent greenhouse gases than carbon dioxide. Although the Montreal Protocol calls for a phase-out of HCFCs by 2030, there is no restriction placed on its use. Over time, a steady increase in the use of HCFCs could change the climate.

source: http://en.wikipedia.org/wiki/Montreal_Protocol
http://app.mewr.gov.sg/web/contents/Contents.aspx?Contld=939
http://ozone.unep.org

Statistics

▪ Lowest ozone in 1994
• Arctic ozone depletion is highly variable and difficult to predict, but a future Arctic polar ozone hole similar to that of the Antarctic appears unlikely.
• Ozone concentrations in the lower stratosphere over Antarctica will increase by 5%–10% by 2020 and return to pre-1980 levels by about 2060–2075, 10–25 years later than predicted in earlier assessments.

• Detectable recovery by 2024
• Antarctic hole recover in 2068

Graph Showing Ozone Depleting Substances — and a larger predicted future usage in developing countries

Changes in Ozone-Depleting Compounds
▪ In the troposphere observations show that the total abundance of ozone-depleting compounds continues to decline slowly from the peak that occurred in 1992-1994.
▪ Observations in the stratosphere indicate that the total chlorine abundance , bromine abundances and HCFCs

Changes in the Ozone Layer over the Poles and Globally
▪ Springtime Antarctic ozone depletion due to halogens has been large (40-50%; exceptionally 70%) throughout the last decade.
▪ In some recent cold Arctic winters during the last decade, maximum total column ozone losses due to halogens have reached 30%, but in warmer winters Arctic ozone loss is small.
▪ Ozone remains depleted in the midlatitudes of both hemispheres. The global-average total column ozone amount for the period 1997-2001 was approximately 3% below the pre-1980 average values.
▪ Models capture the observed long-term ozone changes in northern and southern midlatitudes.