Ozone
From Thermal-FluidsPedia
A molecule of ozone contains three atoms of oxygen bound together (Figure 1). In the lower atmosphere (troposphere), ozone is formed from the reaction between nitrogen oxides and volatile hydrocarbons in the presence of sunlight. In the upper atmosphere (stratosphere), ozone is produced from the reaction of oxygen with ultraviolet radiation from the sun. Ozone is highly reactive, attacking and oxidizing almost anything it contacts.
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The Good Ozone and the Bad Ozone
The ozone layer may be regarded as the earth’s sunglasses. It serves as a shield, protecting earth from the harmful effects of ultraviolet radiation. Most of the ozone is found at around 15-35 km above the ground (Figure 2). As a result of large-scale meteorological weather systems, the ozone layer somewhat varies in thickness from one day to the next and greatly fluctuates with the seasons. Depending on its frequency, ultraviolet light can be classified as UV-A (320-400 nm), UV-B (290-320 nm), or UV-C (100-290 nm).
UV-A, also known as black light, is responsible for skin tanning and has the lowest energy. It is generally considered to be harmless, although some new studies have linked some damage to the retina with long term exposure to this type of radiation. UV-C has the most energy, but is entirely absorbed by the outer layers of the earth’s atmosphere and, under normal circumstances, is not of concern. Because it is highly effective in killing germs, bacteria, viruses, mold, fungi and spores, UV-C is used in hospitals and some industrial applications. UV-B has a lower energy than UV-C, and most is absorbed by the ozone layer in the upper atmosphere, however, even the small amount that reaches the ground results in long term cataracts, skin cancer (malignant melanoma), and breakdowns of the immune system in a large number of people (Figure 3). A 1% reduction in the thickness of the ozone layer will result in a similar increase in the intensity of UV-B radiation and a 2% increase in the probability of skin cancer. The effect is more severe at higher latitudes (such as in Scandinavian countries) and higher altitudes.
In contrast to stratospheric ozone that is considered “good,” the ozone in the lower atmosphere is “bad.” As discussed in the previous section, it is formed mainly as a result of photochemical reactions between nitric oxides, hydrocarbons, and sunlight. Furthermore, ozone is a strong oxidant and easily attacks metals, rubber, and other materials. At high concentrations, ozone is harmful to plants. It damages crops and slows forest growth. In humans, ozone reduces immunity to various diseases, reduces lung capacity, and causes chest pain, coughing, and asthma.
Ozone Depletion
The reduction in the amount of ozone in the stratosphere is another environmental problem widely attributed to the increased use of fossil fuels. This reduction, commonly referred to as the “ozone hole,” was first reported over Antarctica in 1985. Since then it has been recorded over the Arctic and elsewhere around the globe (Figure 4). Since 1978, in areas where ozone concentration has been routinely measured, there has been a 4-5% loss of stratospheric ozone over the continental United States.
Among the chemicals most threatening to the ozone layer are chlorofluorocarbons (CFCs). CFCs are odorless, non-toxic, non-flammable and inexpensive chemicals originally used as a substitute to ammonia, which was the primary coolant in refrigerators. Marketed under the trade name Freon, it quickly found its way into a large number of other products such as paints, propellants, insulation, seat cushions, solvents, and a foam lowering agent for aerosol cans. In addition to CFCs, methyl bromide (a product mainly used in pest control and fire extinguishers) and nitrogen oxides (produced during combustion and a common component of fertilizers) are also shown to have similar destructive properties. Supersonic passenger aircrafts, rockets, and space shuttles release chlorine gases directly into the atmosphere and thus contribute to the depletion of the ozone layer.
The mechanism responsible for ozone destruction was first investigated by Rowland and Molina, who showed that the depletion is a result of chlorine, fluorine, and bromine compounds transported from the lower to the upper atmosphere. Once in the stratosphere, these chemicals are broken down by high concentrations of ultraviolet rays which, in turn, decompose ozone molecules into molecular and monatomic oxygen. Although vehemently attacked by various manufacturers of CFCs, the experimental data became so convincing that the production and use of CFCs was eventually banned. Rowland and Molina received the 1995 Nobel Prize in chemistry for this pioneering work.
Although ozone depletion is widespread over the entire globe, it is of particular significance in the Antarctic. The severity of depletion in the Antarctic is due to its extreme cold temperature (as low as -90oC in winters) and the presence of ice crystals that hover over Antarctica and act as catalysts to breakup CFCs and release chlorine needed to destroy ozone. There are no such ice clouds around the North Pole and so the ozone hole is relatively smaller. (Fahey, 2002)
The Montreal Protocol
The rapid increase in the rate of ozone depletion led to an international agreement -- known universally as the 1987 United Nations Montreal Protocol-- obliging the signatories to gradually phase out the production of CFCs and other ozone depleting compounds. According to this agreement and the subsequent amendments (London, 1990; Copenhagen, 1992; Vienna, 1995; Montreal, 1997 and Beijing, 1999), a set of interim steps were devised that put a cap on the production of ozone depleting materials. Responsibility for the implementation of the Protocol and subsequent mandates remains within individual countries and no reinforcement authority is assigned. As a first step, it was recommended that CFCs, halon, and other ozone depleting emissions should be substituted with hydrofluorocarbons (HFC) and hydrochlorofluorocarbons (hydrogenated CFCs or HCFCs). HFCs do not have chlorine and are therefore not ozone-depleting, but are not as efficient as a coolant, either. HCFCs have a much shorter lifetime in the atmosphere and are much less harmful than CFCs to the ozone layer. These products are scheduled to be phased out by 2030 and substituted with harmless alternatives.
The preliminary results are encouraging, as the amount of CFCs released by major industrial countries has fallen by 99% (2). It should be noted, however, that despite a substantial decrease in the release of ozone depleting materials into the atmosphere, because of their long lifetimes, these chemicals remain in the atmosphere for many decades so repair to the ozone layer is very slow. It is expected that by 2050, stratospheric ozone concentrations will return to its original levels.
References
(1) Fahey, D. W. et al., “Twenty Questions and Answers about the Ozone Layer,” 2002 Scientific Assessment of Ozone Depletion, Les Diablerets, Switzerland, 24-28 June 2002.
(2) National Renewable Energy Laboratory Fact Sheet, DOE/GO-102000-1048, May 2000.
(3) Rowland, F. S., and Molina, M. J., “Ozone Depletion: 20 years after the Alarm,” Chemistry & Engineering News, August 15, 8-13, 1994.
(4) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Further Reading
Gore, A., An Inconvenient Truth, Penguin Books, 2007.
Roleff, T., Pollution: Opposing viewpoints, Greenhaven Press, 2000.
Walsh, P. J., Dudney, C. S., Copenhave, E. D., Indoor Air Quality, CRC Press, 1984.
Environmental Science and Technology, published by the American Chemical Society.
External Links
Environmental Protection Agency (http://www.epa.gov).
Occupational Safety and Health Administration (OSHA) (http://www.osha.gov).
Intergovernmental Panel on Climate Control (IPCC), (http://www.ipcc..ch).
United Nations Environment Programme (http://www.unep.org).
World Health Organization (WHO) (http://www.who.ch).