- 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)
- 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
- 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
- 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:
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).
- OZONE DEPLETION
- 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
- 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).
- 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.)
- 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:
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:
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.
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