By ANTHONY MATHIAS KAYJI
OZONE LAYER DEPLETION
- Meaning of ozone…………………………………………………………2
- Formation of ozone……………………………………………………….3
- Ozone layer………………………………………………………….…………….4
- Meaning of ozone layer …………………………………..………..……..5
- Distribution or location of ozone layer ………………………………..….6
- Importance of ozone layer……………………………………….………..6
- Ozone layer depletion…………………………………………………….……….7
- Meaning of ozone layer depletion…………………………………..….….7
- Ozone hole…………………………………………………….………..…9
- Consequences of ozone layer depletion or hole………………………..…………11
- Public Policy in response to the ozone hole (depletion)…………………………13
- Concern of Tanzania on ozone layer depletion………………………….……….15
Ozone layer depletion is an issue of concerned to the whole world because it touches life of human beings and other creatures found on our planet. In other word it’s a global challenge simply because no one nation which is free from the detrimental effects of ozone layer depletion. The only solution is to join our efforts together and combat with sources which lead into the destruction of the ozone in the atmosphere.
Meaning of ozone
Ozone is a molecule containing three oxygen atoms (O3). It is blue in colour and has a strong odour.
While normal oxygen (O2) we breathe has two oxygen atoms and colourless and odourless. The ozone is much less common than normal oxygen. It is estimated that for each 10 million air molecules 2 million are normal oxygen but only 3 are ozone.
In order for scientists to evaluate how much ozone is in the layer, a unit of measurement called the Dobson Unit is employed. A Dobson Unit is a measurement of how thick a specific portion of the ozone layer would be if it were compressed into a single layer at zero degrees Celsius with one unit of atmospheric pressure acting on it (standard temperature and pressure – STP). Thus, one Dobson Unit (DU) is defined as 0.01 mm thickness at standard temperature and pressure.
Formation of ozone
Ozone is formed through photochemical processes. The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sidney Chapman in1930
There are three forms (or allotropes) of oxygen which are involved in the ozone-oxygen cycle: 1. oxygen atoms (O or atomic oxygen), 2.oxygen gas (O2 or diatomic oxygen), and 3. Ozone gas (O3 or diatomic oxygen).
Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom “recombines” with an ozone molecule to make two O2 molecules: Stages are as follows;
Oxygen molecules are photolysed, yielding two oxygen atoms (slow)
UV light + O2 → O. + O. ———————– (1)
The atomic oxygen then combines with unbroken oxygen molecule (O2)
O. + O2 → O3——————————— (2)
The ozone molecule is also unstable (although in the stratosphere, long-lived) when ultraviolet light hits ozone it splits into a molecule of O2 and an atomic oxygen (faster)
UV light + O3 → O2 + O. —————– (3)
Ozone is also destroyed by the following reaction (too slow)
O. + O3 → 2O2—————————– (4)
This is a continuous process called the ozone-oxygen cycle.
Meaning of ozone layer is a portion of earth’s atmosphere that contains high levels of ozone.
The atmosphere is divided into five layers:
The troposphere is the layer closest to earth and is where all weather happenings occur and starts from zero to 10 Km
The stratosphere is located directly above the troposphere, about 10-50 kilometers above the planet, and houses the ozone layer at an altitude of 20-30 kilometers.
The mesosphere is located approximately 50-80 kilometers above the earth.
The thermosphere rests at an altitude of approximately 100-200 kilometers above the earth’s surface.
Finally, the boundary of the outermost layer, the exosphere, extends roughly to 960-1000 kilometers above the earth.
Figure 1: Earth’s atmosphere is divided into layers, which have various characteristics.
Source: British Antarctic Survey (BAS), 1985
Our concern will be limited to the only two layers, in which ozone layer is found; the layers are as shown in figure 1 below.
Figure 2: Earth’s atmosphere is divided into layers, which have various characteristics.
Source: NOAA Aeronomy Laboratory, 1998
Location of ozone layer; most atmospheric ozone is concentrated in a layer in the stratosphere about 15-30 Km above the Earth’s surface, it is approximate to 90% of ozone. About ten percent (10%) of atmospheric ozone is found in troposphere region.
Importance of ozone layer;
Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet UV (radiation) emitted from the sun.
Categories of UV-radiation based on their wavelengths.
- UV-A (400-315) nm; most reaches the surface; but has significantly less harmful, although it can potentially cause genetic damage.
- UV-B (315-280) nm; this can be harmful to the skin and is the main cause of sunburn, excessive exposure can cause genetic damage, resulting in problems such as skin cancer. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at Earth’s surface is 350 billion times weaker than at the top of the atmosphere. Nevertheless, some UV-B reaches the surface
- UV-C (280-100) nm; Ultraviolet C would be very harmful to humans, is entirely screened out by ozone at around 35 Km altitude.
The ozone layer is essential for human life. It is able to absorb much harmful ultraviolet radiation, preventing penetration to the earth’s surface. Ultraviolet radiation (UV) is defined as radiation with wavelengths between 290-320 nm, which are harmful to life because this radiation can enter cells and destroy the deoxyribonucleic acid (DNA) of many life forms on planet earth. In a sense, the ozone layer can be thought of as a UV filter or our planet’s built in sunscreen (Geocities.com, 1998). Without the ozone layer, UV radiation would not be filtered as it reached the surface of the earth. If this happened, cancer would break out and all of the living civilizations and all species on earth would be in jeopardy (Geocities.com, 1998). Thus, the ozone layer essentially allows life, as we know it, to exist.
Ozone layer depletion
Meaning of ozone layer depletion
Refers to destruction of the upper atmospheric layer of ozone gas caused by substances formed from breakdown of ozone depleting substances. (Retrieved from http:/www.businessdictionary.com/definition)
There are three main contributors to the ozone problem: one is human activity, second is natural sources, and volcanic eruptions
Figure 3: Human cause more damage to the ozone layer than any other source.
Source: Geocities.com, 1998
Ozone can be destroyed by a number of free radicals catalysts, the most important of which are;
- Hydroxyl (OH.)
- The nitric oxide radical (NO.)
- Atomic chlorine (Cl.)
- Atomic bromine (Br.)
All these have both natural and anthropogenic (manmade) sources. At the present time most of the OH. and NO. in the stratosphere is of natural origin, but human activities has dramatically increased the levels of chorine and bromine. These elements are found in certain stable organic compound know as organohalogen compound e.g. chlorofluorocarbons (CFCs) and bromofluorocarbons which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light (E = hv)
CFCl3 + hν → CFCl2 + Cl.
The Cl and Br atoms then destroy ozone molecules through a variety of catalytic cycles. For instance a Cl atom reacts with an ozone molecule, taking an oxygen atom with it and leaving a normal oxygen molecule
Cl. + O3 → ClO + O2
The chloromonoxide can react with a second molecule of ozone to yield another chlorine atom and two molecules of oxygen.
ClO + O3 → Cl. + 2 O2
The overall effect is a decline or decrease in the amount of ozone.
NOTE: Cl and Br are the common radicals, each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth’s stratosphere, fluorine atoms react rapidly with water and methane to form strongly-bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities
For the first time ozone hole was discovered in 1956 by G.M.B Dobson
Ozone hole is a large area of the stratosphere with extremely low amount of ozone. Is really a reduction in concentration of ozone high above the earth in the stratosphere. Also ozone defined geographically as the area where in the total ozone amount is less than 220 Dobson Units.
Why this hole is pronounced in poles?
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 (A polar vortex is a swirling mass of very cold, stagnant air surrounded by strong westerly winds Roan and Sharon, 1990); over 50% of the lower 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. 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 (Type I PSC) or ice (Type II PSC). 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 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 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, which occurs 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 closes.
Consequences of ozone layer depletion or hole
Effects of ozone layer depletion on humans
UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.
1. Basal and Squamous Cell Carcinomas: The most common forms of skin cancer in humans, basal and squamous cell carcinomas have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood that the absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of Squamous cell carcinoma sometimes requires extensive reconstructive surgery.
2. Malignant Melanoma: Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15% – 20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiation whereas experiments on opossums suggest a larger role for UVB.
3. Cortical Cataracts: Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity .In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date.
4. Increased Tropospheric Ozone: Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.
Effects of ozone layer depletion 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 (Sinha et al. 1999)
Effects of ozone layer depletion 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).
International Policy in response to the ozone hole (depletion)
After a 1976 report by the U.S. National Academy of Sciences concluded that credible scientific evidence supported the ozone depletion hypothesis, a few countries, including the United States, Canada, Sweden, and Norway, agreed to eliminate the use of CFCs in aerosol spray cans. This was widely regarded as a first step towards a more comprehensive regulation policy.
The European Community rejected proposals to ban CFCs in aerosol sprays while even in the U.S. CFCs continued to be used as refrigerants and for cleaning circuit boards. Worldwide CFC production fell sharply after the U.S. aerosol ban, but by 1986 had returned nearly to its 1976 level.
In 1985 20 nations, including most of the major CFC producers, signed the Vienna Convention which established a framework for negotiating international regulations on ozone-depleting substances. That same year, the discovery of the Antarctic ozone hole was announced, causing a revival in public attention to the issue.
In 1987, representatives from 43 nations signed the Montreal Protocol. Meanwhile, the halocarbon industry shifted its position and started supporting a protocol to limit CFC production. The reasons for this were in part explained by “Dr. Mostafa Tolba, former head of the UN Environment Programme, who was quoted in the 30 June 1990 edition of The New Scientist, “…the chemical industry supported the Montreal Protocol in 1987 because it set up a worldwide schedule for phasing out CFCs, which [were] no longer protected by patents. This provided companies with an equal opportunity to market new, more profitable compounds.”
At Montreal, the participants agreed to stop production of CFCs at 1986 levels and to reduce production by 50% by 1999. After a series of scientific expeditions to the Antarctic produced convincing evidence that the ozone hole was indeed caused by chlorine and bromine from manmade organohalogen, the Montreal Protocol was strengthened at a 1990 meeting in London. The participants agreed to phase out CFCs and halons entirely (aside from a very small amount marked for certain “essential” uses, such as asthma inhalers) by 2000. At a 1992 meeting in Copenhagen, the phase out date was moved up to 1996.
To some extent, CFCs have been replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although concerns remain regarding HCFCs also. In some applications, hydro-fluoro-carbons (HFCs) have been used to replace CFCs. HFCs, which contain no chlorine or bromine, do not contribute at all to ozone depletion although they are potent greenhouse gases. The best known of these compounds is probably HFC-134a (R-134a), which in the United States has largely replaced CFC-12 (R-12) in automobile air conditioners. In laboratory analytics the ozone depleting substances can be replaced with various other solvents.
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 km² out of 25 (Newman et al., 2004); 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.
Since 1981, the United Nations Environmental Programme has sponsored a series of reports on scientific assessment of ozone depletion. The most recent is from 2007 where satellite measurement has shown the hole in the ozone layer is recovering and is now the smallest it has been for about a decade (Climate change 2001.The scientific Basis).
Concern of Tanzania on ozone layer depletion
Presentation on the International Day for the Preservation of the Ozone Layer celebrated on September 16 every year. This was held on 16/September/2008.
It was said Tanzania is a signatory in The Montreal Protocol for the protection of the ozone, which, is designed to protect the ozone layer by phasing out the production of a number of substances believed to be responsible for ozone depletion. Also Tanzania is among 193 countries in the world that have ratified the 1986 Montreal Protocol for chemicals which destroy the ozone layer which scientists believe is eroded by an average of 5 percent per decade.
It was stated that the harmful chemicals which deplete the ozone layer include; Chlorofluorocarbons (CFCs) and halons which are used in some second-hand technologies such as used refrigerant which are being imported into the country.
Chlorofluorocarbons are a class of compounds of carbon, hydrogen, chlorine, and fluorine, gases used chiefly in refrigerators and aerosol propellants while halons are uncreative gaseous compounds of carbon with bromine and other halogens, used in fire extinguishers, but now also known to damage the ozone layers. .
“These chemicals that are being used in the second-hand refrigerators are not advisable and instead we should use new refrigerators with new technology that use non-toxic R 34 gases” says Mbarak Abdulwakil, Deputy Permanent Secretary to the Vice President’s Office (Environment).
At a news conference on the day Abdulwakil said chlorofluorocarbons (CFCs) and halons chemicals are also used in production of other industrial products such as mattresses, air conditioners and also during iron melting. There are also health products with these including air-fresheners and inhalers which are used by asthma patients.
When the ozone layer is destroyed, Ultra-violet B radiation (UVB) reaches the earth and as a result causes, among other things, skin cancers. .
The radiation also affects eyes and reduces the body’s immune system to fight diseases.
However Abdulwakil said compliance by Tanzania to the Ozone protection protocol has reduced production of harmful chemicals for about 95 per cent as alternative chemicals and technologies are being developed.
On her part the Minister of State in the Vice President’s Office (Environment) Dr. Batilda Buriani called on industries and businesses importing the harmful chemicals to use alternative chemicals in production of their products. The move, she said, will enable Tanzania to avoid the use of chemicals which deplete the ozone layer, which have been banned by the government but are imported by some businesses in defiance of the ban.
On his part the Principal Environmental Management Officer in the Vice President’s Office (Environment), Issaria Mangalili said the government has set a deadline for importation and distribution of items containing ozone-depleting substances (ODS), such as reconditioned refrigerators and air-conditioners. .
“We want importers, suppliers and the general public to understand that such items that contain chemicals, gases and other related ozone layer depleting substances will no longer be allowed here from 1 January 2010. Whoever defies the ban will be taken to task” the Principal Environmental Management Officer said at the press conference on International Ozone Day. “He said the Government had already put in place the Environmental Management (Control of Ozone Depleting Substances) Regulations of 2007 which requires among other things, ban of importation of items which emit and produce chemicals such as CFCs and HCFs that scientists say have proven to deplete the ozone layer. .
Mr. Mangalili said the reconditioned refrigerators and air conditioners, for example, contain gases commonly known as CFCs(R 12) which had proven to contribute to depletion of the ozone layer. .
Another environmental expert, Mrs. Kemilembe Mutasa said public awareness campaigns were being conducted throughout the country to educate various stakeholders on anticipated changes and the alternative use of gases which are not a threat to the ozone layer. (Source: GUARDIAN 2008-09-17 09:33:20 by CHRISTOPHER MAGOLA)
Naturally the universe existed in such away that it can regulate itself so as to keep every system constant or at equilibrium. But due to increased human being activities the natural systems can no longer regulate themselves. Hence there is need for a purposeful efforts to be done so that to preserve and protect the natural systems to remain intact for the betterment of human life and other living things at large. The world should be alert with the threatening phenomena and find agent solutions. Phenomena like ozone layer depletion, global warming, environmental pollution, water crisis, energy scarcity and food shortage. These seem to threaten life on the earth in the near future, if the solution won’t be sought now. Now, it’s a challenge to both politicians and scientists to scrutinize their brain so as to come up with plausible solutions.
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