TOPIC: OZONE - DEVIL IN DISGUISE?1.0 INTRODUCTION Ozone (O3) is an allotropic form of oxygen with three atoms in each molecule
CHEMISTRY YEAR 12 SOCIAL RELEVANCE TASK
TOPIC: OZONE - DEVIL IN DISGUISE?
.0
INTRODUCTION
Ozone (O3) is an allotropic form of oxygen with three atoms in each molecule. Ozone is derived from a Greek word 'ozein', meaning 'to smell'. At standard temperature, ozone is pale blue, emits a strong odor and is very poisonous. However, ozone in liquid form is dark blue and strongly magnetic. Though it has three oxygen molecules, ozone does not have a triangular structure due to the spatial arrangement of its atoms where each oxygen atom only forms one bond, while the remaining negative charge is spread throughout the molecule (Stattersfield n.d.).
Figure 1.1: Structure of ozone molecule
2.0 HIGH LEVEL OZONE
2.1 Background
Almost 95% of the ozone is in the stratosphere, 15-30km from the Earth's surface. This ozone layer acts as a shield to protect the Earth's surface by absorbing harmful ultraviolet radiation. Its volume seldom exceeds 10 parts per million and is measured in Dobson units (DU). The ozone layer averages at 300DU, ranging from less than 100DU to more than 500DU globally (Green Nature 2004, Nebel 1987).
Figure 2.1: The global concentration of ozone layer
2.2 Formation and destruction of ozone layer
In the stratosphere, ozone molecules are very reactive and are constantly being formed and destroyed through reactions with ultraviolet (UV) radiations though the total amount remains relatively similar.
Ozone is formed when UV radiation causes oxygen molecules to dissociate into 2 oxygen radicals:
O2 + hv O + O (1)
Oxygen radicals will then react with more oxygen molecules to form ozone:
O + O2O3 (2)
Ozone can be destroyed through the following reactions:
O + O3 2O2 (3)
O3 + hv O2 + O (4)
Reaction 4 is vital as the ozone layer will shield the earth from UV radiation as it absorbs energy from the radiation when destroyed (Stattersfield n.d.).
2.3 Importance of ozone layer
The main role of the ozone is to screen the amount of UV radiation reaching the surface of Earth as it damages living cells, causing serious health and environmental problems. A weak ozone layer also contributes to adverse climatological effects as ozone can absorb infrared radiation reflected from the earth into space (Stattersfield n.d.).
3.0
LOW LEVEL OZONE
3.1 Background
Ozone is also present in the troposphere. Even though ozone is essentially toxic, it is relatively harmless in quantities less than 30 parts per billion. However, human activities have increased the amount of ozone dramatically, resulting in damage to living organisms. High concentration of ozone contributes to acid rain, green house effect and also photochemical smog (Stattersfield n.d.).
3.2 Formation of ozone
Ozone is formed when nitrogen dioxide dissociates after being excited by UV radiation. The oxygen radical released will react with an oxygen molecule to form ozone with the help of a stabilizing molecule, M, usually another nitrogen or oxygen molecule:
NO2 + hv NO + O (1)
O2 + O + M O3 + M (2)
3.3 Acid rain
Ozone helps in forming acid rain by aiding the oxidization of nitrogen dioxide (NO2) to nitric acid (HNO3) and also helps in converting sulfite ions HSO3- (from SO2) to sulfate ions (HSO4-). These products are the main components that lower the pH of rainwater, causing damage to trees, aquatic organisms and buildings (Stattersfield n.d.).
Figure 3.1: Effect of acid rain in lowering pH in certain parts of the world
3.4 Photochemical smog
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NO2 + hv NO + O (1)
O2 + O + M O3 + M (2)
3.3 Acid rain
Ozone helps in forming acid rain by aiding the oxidization of nitrogen dioxide (NO2) to nitric acid (HNO3) and also helps in converting sulfite ions HSO3- (from SO2) to sulfate ions (HSO4-). These products are the main components that lower the pH of rainwater, causing damage to trees, aquatic organisms and buildings (Stattersfield n.d.).
Figure 3.1: Effect of acid rain in lowering pH in certain parts of the world
3.4 Photochemical smog
During combustion of fossil fuels, ozone is produced following a series of photochemical reactions involving nitrogen oxides and hydrocarbons (equations are similar as in 3.2). Ozone then becomes a secondary pollutant in the formation of photochemical smog, a complex form of air pollution. Ozone can further react with hydrocarbons to form other secondary pollutants such as aldehydes, ketones and peroxyacyl nitrates, all of which are strong oxidants that irritate the eyes and throat. Photochemical smog also leads to devastating agricultural loss (Green Nature 2004).
Figure 3.2: The sources of nitrogen oxides
3.5 Other negative effects of low level ozone
As ozone is a toxic, long-term exposure to it can cause various health problems to the respiratory system and exacerbate existing ailments such as emphysema, bronchitis, asthma and heart diseases. The World Health Organisation has set the maximum hourly dose of ozone at 80ppb and some symptoms are seen below:
The physiological symptoms of ozone exposure
Ozone Levels Hourly (ppb)
Symptoms
50
Headaches
50
Eye Irritation
270
Coughs
290
Chest Discomfort
Table 3.1: Symptoms of long-term ozone exposure
Low level ozone also harms plants and causes the gross reduction in crop productions annually. This is because it obstructs photosynthesis and affects plant resistance towards diseases, insects, pollutants and weather. Ozone too destroys the landscape of forests, parks and cities (Stattersfield n.d., US Regulatory Commission 2003).
4.0
OZONE DEPLETION
4.1 Ozone depletion processes
During the early 1980s, scientists discovered that the ozone level in the Antarctic has been decreasing steadily every spring. Ozone holes, areas with very little ozone, appeared there while research shown that ozone depletion also happened in North and South America, Europe, Asia, Australia and Africa. The graph below shows the ozone levels of the Antarctic during the October months from 1955 to 1995 (Green Nature 2003, Stattersfield n.d.).
Figure 4.1: Levels of ozone in the Antarctic for the October months from 1955-1995
Chlorofluorocarbons (CFCs) were found to be the main reason causing ozone depletion. CFCs are non-toxic, unreactive, non-flammable and cheap to produce. They are used as refrigerants, cleaning solvents, blowing agents for plastics and aerosol propellants. As stable molecules, they can last up to 75 years in the atmosphere, during which they are blown to the stratosphere. UV radiation breaks them down into chlorine and hydrogen fluoride. These two products are primarily responsible in the destruction of ozone. The reactions of chlorine radicals are given as below:
Cl + O3O2 + ClO (1)
ClO + OCl + O2 (2)
O + O2O3 (3)
Unlike oxygen atoms, chlorine radicals react 1500 times faster with ozone. Cl atoms are regenerated in Reaction 2, hence they can go on destroying about 100,000 ozone molecules. Other than CFCs, chlorine can also be found in methyl chloroform and carbon tetrachloride. Bromine, another ozone-depleting substance which exists in halons and methyl bromide, plus aerosols from volcanic eruptions accelerate the depletion as they undergo the same processes as chlorine radicals (Green Nature 2003, Stattersfield n.d.).
Figure 4.2: Sources that cause the depletion of the ozone layer
4.2 Health and environmental effects
Ozone depletion allows larger amounts of UVB radiation to reach the earth with disastrous consequences. UVB is strongly linked to non-melanoma skin cancer, promote the development of malignant melanoma and also causes cataract.
UVB also disrupts the physiological and developmental processes of plants, suppressing plant growth. Economical crops suffer fewer yields while aquatic food chains are affected too as the phytoplankton population, an important plant in the aquatic ecosystem, are destroyed by UVB. Many aquatic animals also experience retarded growth, resulting in the change of population sizes.
UV radiation disrupts natural cycles such as the terrestrial and aquatic biogeochemical cycles. The sources and sinks of greenhouse gases and chemically-important trace gases like carbon dioxide, carbon monoxide, carbonyl sulfide and ozone will be altered, leading to the build-up of these gases in the atmosphere.
UVB radiation can damage polymers and other materials such as rubber and plastics. Hence, high UVB levels hasten the breakdown of these materials, reducing their life span (U.S. Environmental Protection Agency 2005).
4.3 Ways to overcome the problem
The discovery of ozone depletion caused several countries including US to ban the use of CFCs as aerosol propellants. However, production of ozone-depleting substances was heightened when alternative uses for them were found. The Vienna Convention was held in 1985 along with various other global efforts, which finally resulted with the Montreal Protocol in 1987.
This protocol seeks to reduce the production of CFCs by 50% by 1998 but in 1992, the aim was changed to fully terminate the production of halons by 1994 and CFCs by 1996 due to new reports of excessive damage to the ozone layer. As a result, the ozone layer began to heal and it is estimated that within 50 years, the ozone layer will be completely restored (Allaby 1986, U.S. Environmental Protection Agency 2005).
5.0
OTHER REACTIONS OF OZONE
Ozone can also react with alkali metal hydroxides to form ozonide, a compound containing O3-. Another reaction involving ozonides is ozonolysis, a reaction between ozone and alkenes. A machine called ozonator is used where ozone is passed through an alkene solution to produce ozonide which is then reduced. A cleavage forms around the C=C bond, with an oxygen atom attached to each of the C atom as shown below:
(Stattersfield n.d.)
6.0
BENEFICIAL USES OF OZONE
6.1 Medical uses
Ozone therapy is a new form of 'medicine' which is highly beneficial in countering health problems. Among the valuable uses of ozone can be seen below:
Year
Achievements
857
A German, Werner von Siemens, created the first ozone generator
870
Ozone is used in blood-purification by C. Lender in Germany
902
J.H. Clark wrote about the use of ozonated water to treat anemia, influenza, diabetes, morphine and strychnine poisoning, canker sores, and whooping cough
904
Uses of ozone expanded to counter tuberculosis, asthma, tinnitus, bronchitis, hay fever, pneumonia, gout and syphilis
912
Ozone used for treatment of gangrene, poison gas effects and wounds during World War I
915
Treatment of colon and cervical cancer by Dr. A. Wolff
926
Cancer's relationship with the lack of oxygen is explained by Dr. Otto Warberg, hence paving the way towards the utilization of ozone in overcoming cancer
983
During the 6th World Ozone Conference held in Washington in May, the following benefits of ozone were disclosed:
- removes dangerous bacteria and viruses from blood
- decreases chances of being infected by contagious diseases such as hepatitis, HIV virus and syphilis
- treats allergy, effective for rheumatoid and arthritic diseases
- eliminates cancerous tumors, leukemia and lymphoma
- helps in cardiovascular and cerebrovascular disease and arteriosclerosis as it improves oxygen circulation
- effective for nerve-related health problems such as multiple sclerosis, Alzheimer's, Parkinson's and senility
- treats a wide array of other diseases including yeast infections, urinary tract infections, liver cirrhosis, AIDS, herpes and also effective for external use in treating burns, acne and wound disinfection
- ozone usage is extremely successful as it is painless, has no side effects and inexpensive
Table 6.1: The brief history of ozone uses in the medical industry
(Clark 2004, Richard n.d.)
6.2 Industrial uses
As a strong oxidizing agent, ozone is able to break down various chemicals that are harmful towards our health and the environment. Ozone is possibly the most powerful antibiotic, deodorant and sanitizer known. The uses of ozone in different areas are shown below:
Area
Action of ozone
House or workplace
* Neutralise the chemicals in cigarette smoke such as tar, soot, vinyl chloride, benzene and hydrocarbons
* Breaks down dangerous gases emitted by carpets, wallpaper and furniture such as formaldehyde
* Useful in eliminating carbon monoxide, solvents from paints, cooking smells, animal odors, garbage stench, bacteria and mildew
Water purification (in factories and swimming pools)
* Kills bacteria, mold and fungus more efficiently than chlorine, hypochlorous acid, hypochlorite or chloramine
* The usage is ozone is convenient, easy and very popular as it also removes odor and taste
Food production (food plant, restaurant, kitchens and canteens)
* Ozone generators help in removing the gas, steam and other odors released during the preparation of food
Table 6.2: The uses of ozone in various areas in the industrial sector
Figure 6.1: A model of an ozone generator used to remove kitchen odors
(Clark 2004, Lenntech 2005)
7.0 CONCLUSION
Ozone can be beneficial and destructive simultaneously, depending on the atmospheric level that it is at. By itself, ozone is relatively safe, and human activities are the main factor behind any negative ozone-induced effects such as pollution and acid rain. The stratospheric ozone layer especially is essential in stabilizing the Earth; hence the problem of ozone depletion demands immediate attention. Fortunately, global co-operation was prompt, thus the issue is easily overcome as ozone-depleting substances are minor in the world's economy and have many ready alternatives. To conclude, ozone is not only vital for maintaining the ecosystem, but also has the potential to be further utilized in many areas for the benefit of mankind.
(1485 words)
REFERENCE LIST
Allaby, M 1986, Ecology facts, Hamlyn Publishing, Middlesex.
Clark, H 2005, 'Ozonization: its purpose and its use in therapy', Dr. Clark Information Center, viewed 24 Jan. 2005, <http://www.drclark.net/info/ozone.htm>.
Green Nature, 2005, 'An introduction to the science of ozone depletion', 'Good and bad ozone', viewed 24 Jan. 2005, <http://greennature.com/article33.html>.
Lenntech, 2005, 'Ozone: odour removal', viewed 21 Jan. 2005, <http://www.lenntech.com/ozone_kitchen.htm>.
Nebel, BJ 1987, Environmental science: The way the world works, 2nd edn, Prentice-Hall, Inc., New Jersey.
Richard, D n.d., 'The miracle of ozone', Spiritual Endeavors, viewed 24 Jan. 2005, <http://www.spiritual-endeavors.org/health/ozonemiracle.htm>.
Stattersfield, E n.d., 'The chemical of ozone', 'High level ozone', 'Low level ozone', University of Bristol, viewed 16 Jan. 2005, <http://www.chm.bris.ac.uk/motm/ozone/Eloise.htm>.
U.S. Environmental Protection Agency, 2004, 'Region 8 - Did you know?', 'The effects of ozone depletion', viewed 24 Jan. 2005, <http://www.epa.gov/region8/didyouknow/071404.html>.
U.S. Nuclear Regulatory Commission, 2003, 'Ozone: Good up high, bad nearby', Almanac of Policy Issues, viewed 24 Jan. 2005, <http://www.policyalmanac.org/environment/archive/ozone.shtml>.
