In the atmosphere at a height of 10 to 50 km there is a layer with a high concentration of ozone (O3) - ozone layer. The maximum ozone concentration is maintained at a height of approx. 23 km. Since the late 70s - significant decrease of ozone amounts is observed, especially over Antarctica, in the region of the South Pole. The ozone hole is the reduction of the concentration of ozone in the ozone layer.
The ozone hole is formed by the destruction of the ozone layer by chlorofluorocarbons - chemicals called freons or CFCs.
Under the influence of ultraviolet radiation freons undergo photolysis reaction, whereby the chlorine atoms are released. Chlorine reacts with ozone to form active chlorine oxide (ClO) and the usual oxygen (O2). Then, the reaction between two molecules of chlorine monoxide leads to the formation of chlorine dioxide (ClO2) and subsequent release of chlorine, which breaks another ozone molecule. In addition, the chlorine dioxide may decompose to chlorine and diatomic oxygen molecule.
The following chemical equations of reactions that occur during the destruction of ozone:
+ x Cl
Cl + O3
ClO + O2
Cl + O2
The above reactions occur until the complete depletion of ozone molecules, or until the removal chlorine by chemical reactions.
It is estimated that the annual rate of decline in ozone content is below 0.2% at the equator, and from 0.4 to 0.8% in temperate latitudes. But the biggest (and still increasing) rate of decline in stratospheric ozone is observed in the region of the south pole during early autumn (late September and October). In the period 1987-1992 the total content of stratospheric ozone has decreased by more than 50% of the content from the 1970, when the average was October yet 300D (1D [Dobson] - the unit used to determine the concentration of ozone, named after the designer of measuring instruments).
One may ask: why the ozone hole over Antarctica is created (in the southern hemisphere), despite the fact that the majority of the emissions of ozone depleting gases
occurs in the northern hemisphere, in the most developed and industrialized areas. This is the mechanism of formation of the ozone hole over Antarctica:
The air contaminated with CFCs, halons and other gases, as a result of pressure differences is set in motion and is conveyed over a certain distance. Along with large-scale air currents in the atmosphere ( stratospheric winds) mass of polluted air is then dispersed across the globe. Currently, CFCs are present over the entire surface of the globe, even in places as far away from industrialized areas (Europe, USA), as Antarctica. At a time when the northern hemisphere begins the spring season, over Antarctica begins polar night. During this time regular, stable, lasting half a year the vortex is formed, in which air circulates around the South Pole. Antarctic air mass is then completely isolated from the supply of equatorial air, which always contains a high concentration of stratospheric ozone. The reactions of ozone destruction by CFCs run faster than the reactions of ozone formation, so its concentration is significantly reduced. In 1982, there was observed a few days' complete lack of ozone in the lower stratosphere.
The presented above mechanism of formation of the ozone hole clearly shows how very important role for humanity serve rainforests, which through the production of vast amounts of atmospheric oxygen (O2
) allow the formation of ozone (O3
Among the gases that have a devastating impact on the ozone layer the largest share have chlorofluorocarbons, halons and oxides of nitrogen.
Chemically, the chlorofluorocarbons (CFCs) are derived from the halogenated hydrocarbons. The molecule contains chlorine or fluorine, and in some cases bromo. They are formed by treatment with hydrogen fluoride to halogenated methane or ethane in the presence of a catalyst - antimony pentachloride. Short CFCs are characterized by significant vapor pressure at low temperatures and high heat of evaporation. Due to the high thermal capacity they have a significant part in increasing the greenhouse effect. They are odorless or have the smell of ether. They are colorless and non-toxic. They have been applied in the production of cooling and air conditioning and (now increasingly rarely) in the production of cosmetics and deodorants. The best-known and most frequently used Freons are dichlorodifluoromethane (CCl2F2), called freon F-12 and dichlorotetrafluoroethane (C2Cl2F4), called freon F-114.
Halons are halo derivatives of methane and ethane. They are non-toxic gases or liquids. There are non-flamable. They are used for the production of halon extinguishers.
Nitrogen oxides are formed in the ozone layer, mainly due to fuel combustion by the engines of aircraft and rockets. In significant quantities of nitrogen oxides are emitted to the ozone layer as a result of nuclear explosions.
Ozone layer absorbs very dangerous to all living organisms ultraviolet (UV) radiation, having a wavelength below 390 nm. The destruction of the ozone layer leads to a decrease in the efficiency of absorption of UV rays. As a result, the organisms are exposed to increased ultraviolet radiation.
Excess of UV rays can lead to imbalance of entire ecosystems. Ultraviolet radiation penetrates water several meters into (for clean water up to several meters). This causes the dieback of vulnerable plant and animal organisms forming the plankton. The consequences of this are visible in the consecutive links of the trophic chain. This may reduce the occurrence of fish that feeding on plankton and fish prey.
The ultraviolet radiation also adversely affects plant. Among the plants that are reactive to UV, more than two thirds species is vulnerable. It should be noted that these are mostly species of crops and industrial plants.
Increasing the intensity of ultraviolet radiation on Earth will influence the human economy. Reducing the size of fish populations as a result of the disappearance of plankton will lead to much smaller fishing in a given area. Thus, fishing and fisheries will suffer. As a result of the destruction by UV chlorophyll crops (e.g. cereal) will diminish crops, so agriculture will suffer as well.
Ultraviolet radiation can, however, have a negative impact on people directly. By producing pigments in the skin, humans are able to protect themselves only to a small extent.
Excessive UV radiation can weaken the human immune system and thus reduce the resistance to infections and disease. Among the most serious of these diseases are certainly cancers, especially cancers of the skin (e.g. Melanoma). Furthermore, ultraviolet radiation causes conjunctivitis, and thus the occurrence of many ophthalmic diseases, especially cataracts. The UV rays cause the acceleration of the aging process of the skin.
If CFCs and other ozone-depleting gases will continue to be emitted into the environment in the near future, the ozone hole will expand considerably in size and will soon appear over the entire globe. It is no longer the ozone hole, but the total loss of ozone in the atmosphere.
Already in 1971, two chemists noticed and proved devastating effects of CFCs on the ozone layer. They were the prof. Dr. Sherwood Rowland and Mario Molina (laureates of the Nobel Prize in Chemistry 1995). The Commission for the United Nations on Environment drew attention to this phenomenon only in 1976. Since then, chlorofluorocarbons are on the list of hazardous chemicals to the environment. Specific measures to prevent the depletion of the ozone layer over the surface of the globe, however, began to be undertaken only since 1982, when Dr. Joe Farman discovered complete disappearance of the ozone in the atmosphere over west part of Antarctica. In 1987, in order to protect the ozone layer, on the initiative of UNEP (Environment Programme of the United Nations) 31 countries (including Poland) signed the Montreal Protocol. The assumption was the 50% reduction in production of CFCs by the year 2000 compared to the value of 1986. Since 1990 a reduction in the rate of growth of CFCs in the atmosphere is actually seen - from 5% per year to less than 3%. In addition, on October 11, 1990 Poland became a member of the Vienna Convention on the protection of the ozone layer, according to which the production of CFCs and the import of foreign cooling equipment containing CFCs are banned.
Thus, we can observe a significant increase in awareness of the authorities and society, which is comforting phenomenon. In the production of cosmetics and deodorants practically no chlorofluorocarbons are used, and other environmentally friendly gases are used as carriers - propane and butane. These cosmetics are referred as "CFC-free" or "ozone friendly". Also, modern refrigerators and coolers are devices CFC-free.
However, despite all efforts to prevent the further emission of CFCs and halons, over the next few years it will be impossible to rebuild the ozone layer as thick as 20 years ago.