The Ozone Layer

The Ozone Layer

"The Ozone layer" refers to the ozone within stratosphere, where over 90% of the earth's ozone resides. Ozone is an irritating, corrosive, colorless gas with a smell something like burning electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graff generators, Tesla coils, arc welders). Each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation. The ozone layer absorbs 97-99% of the sun's high frequency Ultraviolet light , light which is potentially damaging to life on earth. Every 1% decrease in the earths ozone shield is projected to increases the amount of UV light exposure to the lower atmosphere by 2%. Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much of UV light would actually reach the earth’s surface. Recent UV measurements from around the northern hemisphere indicate small UV increases in rural areas and almost no increase in areas near large cities.

The overall amount of ozone is essentially stable in a natural cycle. This has been true for millions of years. Since some decades, according to atmospheric measurements, ozone layer is getting thinner. Ozone depletion has been most severe at the poles, especially over Antarctica, where a seasonal ozone layer "hole" appears. The Antarctic ozone hole was discovered in 1985 by British scientists: it is not technically a “hole” where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August-October). An endlessly circling whirlpool of stratospheric winds called the "polar vortex" isolates the air over Antarctica in winter. The ozone hole grows throughout the early spring until temperatures warm and the polar vortex weakens, ending the isolation of the air. As air from the surrounding latitudes mixes into the polar region, the ozone layer stabilizes until the following spring.

According to scientists, certain man-made chemicals are major contributors to the problem. These chemicals are called Ozone-Depleting Substances (ODS) and include many gases containing chlorine and bromine, such as: chlorofluorocarbons (CFCs, substances containing chlorine, fluorine and carbon) used in refrigerators and blowing agents for foams; the "Halons", used for fire fighting; methyl bromide, used in agriculture.

Since the Second World War, CFCs have been widely employed, mainly because they are chemically inert and, as a consequence, non toxic and extremely stable.
CFCs do not dissolve in rain: after several years, carried by the winds, they reach the stratosphere without being modified. Here their molecules are broken down by the intense UV light, and free chlorine atoms are created by this degradation.
Each chlorine atom can destroy several thousands of ozone molecules before being removed from the atmosphere: chlorine is a catalyst for the ozone depletion. Bromine (e.g. from methyl bromide, used by farmers as a fumigant) is even more effective than chlorine.

Effects of the reduction of the ozone layer

Increase in Ultraviolet Radiation

The reduction of ozone layer will cause an increase of Ultra Violet (UV) radiation at earth level. An excess of UV rays has been linked to skin burns, skin cancer, cataracts, and harm to certain crops and marine organisms.

Since the ozone layer absorbs Ultraviolet light-B (UVB) from the Sun. UVB affects the outer layer of the skin, the epidermis and it’s the primary agent responsible for sunburns. It is the most intense between the hours of 10:00am and 2:00pm when the sunlight is brightest. It is also more intense in the summer months accounting for 70% of a person yearly UVB dose. Ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly due to the fact that UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.

Ozone, while a minority constituent in the earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface. Increases in surface UVB due to the ozone hole can be partially inferred by radioactive transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand).

Because it is this same UV radiation that creates ozone in the ozone layer from O₂ (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.

Biological effects of Increased UV and microwave radiation from a depleted ozone layer

The main public concern regarding the ozone hole has been the effects of surface UV on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown as large as to reach southern parts of Australia and New Zealand, environmentalists have been concerned that the increase in surface UV could be significant.

Effects of Ozone layer degredation on Humans

Ultra Violet B (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 troposphere ozone, which is a health risk to humans. The increased surface UV also represents an increase in the vitamin D synthetic capacity of the sunlight.

The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of increased Ultra Violet (UV) irradiance may outweigh the burden.

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 absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimmers, 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. By combining epidemiological data with results of animal studies, scientists have estimated that a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.

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. Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in no melanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

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. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen. Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalences among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.

4. Increase in Troposphere Ozone: Increased surface UV leads to increased troposphere 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 on Crops & Plankton

An increase of Ultra Violet (UV) radiation would be expected to affect crops and planktons. A number of economically important species of plants, such as rice, depend on cyan bacteria residing on their roots for the retention of nitrogen. Cyan bacteria are sensitive to UV light and they would be affected by its increase. 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).

In stopping Ozone layer degradation, CFCs and other Ozone Depleting Substances should be replaced with environmentally safe substances. Researches are going on for identifying the best alternative substances; presently HCFCs (hydro chlorofluorocarbons, substances containing hydrogen, chlorine, fluorine, carbon) are replacing CFCs, being much less harmful for the ozone layer. In the future, HCFCs will be phased out, too. The "Montreal Protocol" is the 1987 international treaty governing the protection of stratospheric ozone agreement to phase out the Ozone Depleting Substances. According to the Montreal Protocol (and successive amendments) usage of the CFCs and most Halons have been reduced or phased out; other ODS, like HCFCs, will be phased out in the future. Anyway, even if the consumption of all ODS gases would be completely discontinued; it will take a lot of years before complete recovering of the ozone layer, due to their persistence in the atmosphere.

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