OZONE LAYER DEPLETION
INTRODUCTION
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone (O3).The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone (O3).
This layer absorbs 97-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on Earth. Over 90% of ozone in earth's atmosphere is present here. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 km to 35 km above Earth's surface, though the thickness varies seasonally and geographically. The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations which continues to operate today. The "Dobson unit", a convenient measure of the total amount of ozone in a column overhead, is named in his honor
Ozone Depleting Chemicals
The most commonly known ozone depletion chemicals (ODCs) are the CFCs or chlorofluorocarbons. Over the last 30 years man-made CFCs have been the main cause of stratospheric ozone depletion. Fortunately, CFCs, once used for example in refrigeration, air conditioning, and fire extinguishers, have been largely phased out in the aftermath of the Montreal Protocol.
CFCs however, are not the only ozone-depleting chemicals. Other ODCs include the methylhalides, carbon tetrachloride (CCl4), carbon tetra fluoride (CF4), and the halons which contain bromine instead of chlorine. Such compounds are called halocarbons.
Carbon tetrachloride (CCl4), despite its toxicity, was first used in the early 1900s as a fire extinguishant, and more recently as an industrial solvent, an agricultural fumigant, and in many other industrial processes including petrochemical refining, and pesticide and pharmaceuticals production. Recently it has also been used in the production of CFC-11 and CFC-12. It has accounted for less than 8% of total ozone depletion. The use of carbon tetrachloride in developed countries however, has been prohibited since the beginning of 1996 under the Montreal Protocol.
Methyl chloroform, also known as 1,1,1 trichloroethane is a versatile, all-purpose industrial solvent used primarily to clean metal and electronic parts. It was introduced in the 1950s as a substitute for carbon tetrachloride. Methyl chloroform has accounted for roughly 5% of total ozone depletion. The use of methyl chloroform in developed countries has been prohibited since the beginning of 1996 under the Montreal Protocol.
Halons, unlike CFCs, contain bromine, which also destroys ozone in the stratosphere. Halons are used primarily in fire extinguishers. Halon-1301 has an ozone depleting potential 10 times that of CFC-11. Although the use of halons in developed countries has been phased out since 1996, the atmospheric concentration of these potent, ozone destroyers is still rising because of their long atmospheric lifetimes. To date halons have accounted for about 5% of global ozone depletion.
A Giant Sunshade
ORIGIN OF OZONE
The photochemical mechanisms that give rise to the ozone layer were worked out by the British physicist Sidney Chapman in 1930. Ozone in the earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere, the region from about 10 to 50 km (32,000 to 164,000 feet) above Earth's surface.
About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 15 and 40 km, where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick.
What Impact Does Ozone Depletion Have?
Some of the environmental, social and economic effects of ozone depletion are listed below to illustrate the issue’s broader significance for sustainability.
Plant growth and productivity. UV-B radiation can affect plant growth and productivity. Plants have the ability to handle a single stress such as increased UV-B levels, however, when this is combined with another stress such as warming as a consequence of the enhanced greenhouse effect, as many as 25% of plants may be affected (DELM, 1993).
Marine food chain. Phytoplankton are microscopic plants that form the basis of the marine food chain. These are particularly susceptible to increases in UV-B radiation. Reduced phytoplankton numbers would significantly effect other marine species, including commercial fish stocks (DELM, 1993).
Human health. At high exposure levels, the UV-B component of UV radiation can weaken the human immune system and cause skin cancer, cataracts and eye cancer. Increased levels of UV radiation will contribute to rising incidences of skin cancer. Australia has high levels of UV radiation and the highest per capita rate of melanoma in the world (AIHW, 2001). The economic cost to the Australian community of skin cancer alone is estimated at approximately $300 million per year
Deterioration of materials. Increased UV-B radiation can accelerate the deterioration of plastics, wood, paper, cotton and wool.
What is Being Done About It?
The Montreal Protocol is an international response to the problem of ozone depletion that was agreed to in September 1987 following intergovernmental negotiations stretching back to 1981. The Protocol came into force on 1 January 1989.
As scientific knowledge has improved and the true extent of the ozone depletion problem has become apparent, several revisions of the Protocol have been made to accelerate the phase-out schedule.
It is believed that without the Protocol, by the year 2050 ozone depletion would have risen to around 50% in the Northern Hemisphere and 70% in the southern mid-latitudes. This would have resulted in a doubling of UV-B reaching earth in the northern mid-latitudes and a quadrupling in the south.
Australia’s obligations under the Montreal Protocol are implemented via complementary legislation and policy developed by Commonwealth, State and Territory Governments. Environment Australia is responsible for coordinating national ozone protection measures and administering relevant legislation.