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Research | Atmosphere | Ozone and UV radiation
The essential guide to ozone
Ozone is a molecule in the Earth's atmosphere. As much as 90 % of ozone lies at heights of 10-50 km above the Earth's surface. An ozone molecule consists of three oxygen atoms. Despite the fact that ozone forms only a small fraction of all the constituents of the atmosphere, it is still an important factor for the continuity of life on planet Earth.
Ozone either protects or harms organisms on Earth depending on the height at which it is found. In the high stratosphere "good" ozone acts like a shield protecting Earth's flora and fauna against the Sun's ultraviolet (UV) radiation. An excess of UV radiation increases the risk of getting skin cancer or ocular diseases. It may also weaken the natural immunity of human beings and animals.
On the other hand, near the surface, in the air we breathe, ozone is a pollutant, which in large amounts causes damage to man, animals and plants. It makes up about 10 % of all ozone, and is produced, for example, as a consequence of the exhaust fumes from cars. After a thunderstorm one can smell a sharp scent in the air. The same kind of "electric" smell may also occasionally be sensed while travelling on the metro. This is "bad" ozone, which in large amounts irritates the eyes and lungs. In summertime ozone may cause smog in large cities.
During the Earth's past history, natural conditions have varied the amount of ozone in the atmosphere. The amount of ozone depends on the balance between its production and depletion. The ozone balance can be understood by thinking of a leaky bucket. As long as one pours water into the bucket at the same rate as water is dripping out of it, the amount of water in the bucket remains constant. In a similar way, as long as atmospheric ozone is produced as fast as it is depleted, the total amount stays in balance.
The Sun emits short-wave radiation which then enters the Earth's atmosphere. The radiation is divided into UV (ultra-violet, 8 % of the total energy), visible light (39 %) and infrared radiation (53%).
Short-wave UV radiation is divided into three parts according to its wavelength (l): UV-C (200 nm < l < 280 nm), UV-B (280 nm < l < 315 nm) and UV-A (315 nm < l < 400 nm). The oxygen in the atmosphere absorbs the UV-C radiation in the upper layers of the atmosphere, preventing this destructive radiation from entering the lower parts of the atmosphere and reaching the surface of the Earth.
UV-B radiation leads to changes, some of them harmful, in the chemical structure of the DNA of living cells. Atmospheric ozone reduces the UV-B radiation from the Sun in such a way that the radiation power at different latitudes is at a level that is safe for the natural biosphere of the region. The location on the Earth, together with the season and the time of day, essentially determines the power of the UV radiation detected on the ground. UV-B radiation is also reduced by aerosols and clouds in the atmosphere, as well as by ozone. The darker the cloud is, the stronger it absorbs UV-B radiation. A thick rain cloud can absorb UV radiation by as much as 70 %. It is, however, noteworthy that the same cloud absorbs visible light even more, so that UV radiation is less absorbed by clouds than visible light is.
About one half of the UV radiation reaching the Earth's surface is so-called direct radiation, against which one can protect oneself with a hat or a shade. The skin may still get burnt even in the shade, because half of the radiation is composed of so-called diffuse radiation. Diffuse radiation arises from the radiation scattering process caused by atmospherical gases and particles.
UV radiation is at its strongest at noon in mid-summer. Due to the current time system, summertime solar radiation is most powerful in Finland during the afternoon from 1 to 3 o'clock.
The stratosphere lies in a region about 10-50 km above the surface of the Earth. In the stratosphere, UV radiation from the Sun both creates and destroys ozone. When the UV rays encounter an oxygen molecule (O2), the molecule splits into two oxygen atoms (O). After that, the released oxygen atom can link up with an oxygen molecule, and produce an ozone molecule (O3).
Ozone has the ability to absorb UV radiation emitted by the Sun. This feature makes the ozone layer a molecular shield which protects the Earth against UV radiation. On absorbing UV radiation, an ozone molecule divides into an oxygen molecule and an oxygen atom. The free oxygen atom can consequently join up with an oxygen molecule to create a new ozone molecule, or alternatively grab an oxygen atom from an ozone molecule, leading to the formation of two oxygen molecules. These ozone formation and depletion processes caused by UV radiation are known as Chapman reactions.
Consisting of three oxygen atoms, ozone is an extremely unstable molecule. It reacts readily, releasing its extra oxygen atom, which bonds itself to other elements occuring in nature. These elements are naturally released for example from the soil and oceans, and they have also always been present in the stratosphere. Such natural phenomena as seasonality, winds and volcanic eruptions also affect the distribution of the ozone in the atmosphere.
During the last few decades it has been noticed that man's actions have begun to disturb the ozone balance. The ozone concentration in the upper atmosphere has been declining since the 1970's. The cause of the chemical ozone depletion is the presence of chlorine and bromine originating in man-made freons and halogen compounds.
The main reason for the ozone depletion is the stratosphere becoming polluted by compounds containing chlorine (freons) or bromine (halogens). In addition, ozone is dissipated by nitrous oxide. Due to the fact that the compounds destroying ozone are chemically stable while in the lower atmosphere, they do not break up until they have been carried with the airstreams into the upper atmosphere. Dissipation is caused by powerful UV radiation, as a consequence of which highly reactive chlorine and bromine atoms and their compounds are born. During its lifetime, a single atom or compound of this kind breaks down numerous ozone molecules into ordinary two-atom oxygen molecules, thus preventing the natural reformation of ozone molecules.
CFC compounds include chlorine, fluorine and carbon. For half a century they have been used among other things in producing refrigerators, air conditioning devices and insulating materials.
Fortunately, CFC compounds, chlorine and bromine atoms do not remain in the stratosphere for ever. When, say, a chlorine atom reacts with certain gases (with methane, for example), it forms a hydrochloride molecule. Under the influence of gravity, hydrochloride molecules drift from the stratosphere into the lower atmosphere, i.e. the troposphere (extending from a height of 0 to 10 km above the ground), and finally down to the ground.
If man through his own actions can succeed in preventing freons and other ozone-destroying compounds from drifting into the stratosphere, the damage to the ozone layer will gradually be repaired by itself. The ozone layer is estimated to be at its thinnest at the beginning of the next century, after which it is believed the upper atmosphere will slowly recover within 50 years. A prerequisite for this is that the international agreements restricting the use of compounds destructive to ozone are observed. Measurements carried out in the last few years indicate that concentrations of the most serious ozone-destructive compounds in the atmosphere have been declining since the beginning of the 1990's.
Especially clearly ozone depletion has been observed in the middle atmosphere above the Antarctic. Since the 1980's, observations have indicated that ozone depletion has been getting ever stronger and wider above the Antarctic. During the last few years, the ozone layer has been thinned at its greatest by over 60 %. The ozone layer gets depleted in September - October, after which the loss is again made up in November-December. The reason for this is the extreme coldness. In temperatures sinking to as low as -80 degrees Celsius, small cloud crystals are formed in the stratosphere, which release chlorine in a reactive form, binding the nitrogen molecules required to take part in the restoring reactions. If the Sun has already began to radiate the area after the Arctic night, the catalytic ozone loss process will get started, leading to a wide ozone depletion over the course of several weeks.
In 1992-1996, unusually strong periodical ozone depletions were observed in the northern hemisphere in the winter and spring time. In 1993, in the ozone layer above Finland, for example, areas were detected with an ozone depletion as large as 40 %. It is believed that these depletions may have been a consequence of, among other things, the eruption of Mt. Pinatubo in 1991. This eruption spewed a huge number of particles into the air, which then found their way into the stratosphere. Pollutants in the air may have enhanced the ozone loss caused by those particles.
The extent of the observed ozone losses have to be compared with the significant variation naturally occurring in the ozone amount. In Finland, the thickness of the ozone layer is, in a normal year, about 30 % smaller in the autumn than that in the spring. At the equator, there is on average 35 % less ozone than there is at the latitude of Finland. Due to the circulation of the atmosphere, even variations of over 20 % of the average during one day are not unusual.
In the northern hemisphere, the ozone layer has so far escaped Antarctic-like depletion. The reason for this is that in the Arctic the upper atmosphere is in winter warmer than it is in the Antarctic, which prevents the formation of stratospheric clouds. This, in turn, is caused by differences in the land-ocean distribution, and atmospheric flow differences between the hemispheres. Mid-atmospheric cooling and low-atmospheric warming caused by the global climate change can, however, make it possible for an Antarctic-like ozone loss to also form in the northern hemisphere in the near future.
As the ozone layer gets thinner, UV radiation at the surface of the Earth increases. If the ozone amount decreases by 10 % during the spring and summer, the annual UV dose increases by about 12 %. Some indications of increasing UV doses have been noticed during short incidental depletion episodes in Finland during the last years. This had nevertheless only a minor effect on the annual dose, because, according to measurements, the burning UV radiation had increased by over 20 % for only a few days in April and May. People's behaviour in the Sun is still dominating on the total UV exposure they receive.
Early in 1996, the climatological conditions in the mid-atmosphere of the northern polar region were exceptionally favourable for the occurrence of ozone depletion. According to stratospheric temperature observations made in Finland, temperatures lower than -78 degrees Celsius, which are conducive to ozone loss, have so far been occurring more frequently than at any time since the beginning of the 1960's, when measurements were started. Ozone depletion has been at its strongest in early 1996 above the Nordic countries and Greenland, and the loss of ozone has been estimated to stem mainly from chemical factors. At its largest, the ozone depletion has been 40 %. Consequently, the UV radiation has occasionally been stronger than the average for the season, but it has still remained below 1/3 of the summertime radiation intensity in Finland.
The Finnish Centre for Radiation and Nuclear Safety (STU) and the Finnish Meteorological Institute (FMI) are developing accurate methods for the measurement of solar UV radiation. UV radiation has been regularly measured by the Finnish Meteorological Institute since 1991, and at the moment there are UV measurement devices at six different sites ranging from Utö up to Sodankylä. The Finnish Meteorological Institute commenced measurements of total column ozone at Sodankylä in 1987 and at Jokioinen in 1991. Measurements of ozone concentrations at different heights in the atmosphere were started in 1988.
The Finnish Meteorological Institute has developed facilities to monitor ozone and the UV situation in real time, so that the observations may be forwarded daily for public use. Additionally, the behaviour of the ozone layer is being monitored globally with the aid of satellite observations. Announcements regarding ozone and UV observations are made as the need arises. Predictions on the development of UV radiation within the following 1-3 days are made on a routine basis http://www.fmi.fi/uvi.
The most well-known effect of UV radiation is the slight reddening or burning of the skin in sunshine. Tanning will occur when the UV radiation causes a pigment called melanin to form in the pigment cells of the skin. This only demonstates the skin's attempt to protect itself against further damage.
Over a period of years, exposure to radiation originating either from the Sun or solariums causes damages in the skin's connective tissues, so-called photoageing. This shows itself as a thickening of the skin, as wrinkles and decreasing elasticity. Elastine and collagen fibres determining the firmness and elasticity of the skin are damaged. UV radiation increases the risk of getting skin cancer.
UV radiation enhances the dimming of the eye's lens, which means that potential cataracts begin to evolve at earlier ages. Part of the UV radiation reaches the back of the eye, causing cells in the retina to slowly begin to deteriorate. Damage will in time particularly occur to near vision. Radiation is partly absorbed in the lens of an adult eye, but will go right through the lens of a child, reaching the back of the eye. For this reason, children's eyes in particular should be protected against strong sunlight.
Strong UV radiation can also cause inflammation of the cornea, snow blindness. Symptoms of this kind of an infection include the eyes becoming reddish, a sensitivity to light, enhanced excretion of tears, the feeling of having some dirt in one's eye, and pain. The trauma appears 3-12 hours after exposure. Thanks to the quick regeneration of the eye cells, symptoms will normally disappear within a few days. A long-term exposure to UV radiation may cause permanent damage to the cornea.
UV radiation may weaken the immune system taking care of the body's defence against e.g. infection. These effects are not restricted to the part of skin actually subject to exposure, but may also occur on shielded parts of skin and in the whole immune system.
At the present time, the significance of the immune system weakening caused by UV radiation is not properly understood.
UV radiation also benefits health, generating vitamin D production on the skin. The required amount of radiation is, however, quite small: in summer, an exposure of 15 minutes to the hands and face is adequate. There is no need to get a tan for this purpose. Vitamin D is also found in food. People living in Finland and following a normal diet get enough vitamin D in their food, even in winter. In the treatment of some skin diseases such as psoriasis, UV radiation is being effectively exploited. Under a doctor's control, the benefit from the treatment is much greater than any consequential increase in skin cancer risk.
The greatest risks connected with the depletion of ozone in the stratosphere are ecological. Exposure tests made in USA and Australia have showed that over one hundred species are sensitive to changes in UV radiation, most important of these being the soy bean and a particular pine (loblolly pine) growing in USA. The University of Oulu has investigated the impact of UV radiation on coniferous trees. It has been noted that differences between trees growing in different areas are extremely large. For example, a pine brought from the Kola Peninsula began to grow better than before after receiving boosted UV radiation, whereas the needles of a pine from Kittilä began to shrink under the same treatment. Old needles are able to protect themselves by strengthening the wax coating the outermost layer of their needles and by increasing the amount of protective pigment. In contrast to this, young growing needles suffer easily.
On a global scale, half of the carbon annually bound up in biological assimilation is produced by plankton in oceanic ecosystems. In this way, plankton maintains the basic production of the oceanic food chain. The tolerance of plankton to UV radiation has been found to be very variable, depending on species, which may lead to a disturbance of the balance between separate species. At present, research concerning the impact of UV radiation on oceanic ecosystems is still in its infancy.
International research has revealed that some species of rice suffer from even minor increases in UV radiation, while other species capable of tolerating even intense radiation have also been found. With the help of research, as well as the efficient breeding and cultivation of strong species it will be possible to be prepared for years with a considerably decreased prevailing level of ozone level.
UV radiation degrades many materials commonly used outside, like wood and different plastics. It has been estimated that the increase of UV radiation following a depletion of ozone would most seriously damage these materials.
In 1987, 50 countries together decided, by means of the so-called Montreal protocol, to decrease their use of chemical compounds destructive to ozone in the upper atmosphere by 50 % by the year 1999. The agreement was supplemented by agreements made in London in 1990 and in Copenhagen in 1992, by which the same countries promised to stop using CFC and most of the other chemical compounds destructive to ozone by the year 2000.
In most cases it has been fairly easy to develop and introduce compounds and methods to replace CFC compounds and freons. The use of CFC compounds in aerosols and foam plastic packaging has already been abandoned in most countries. On the other hand, compounds capable of replacing CFC compounds in cooling devices and insulating materials are still under development.
Some countries, like China and India, are strongly increasing the use of air conditioning and cooling devices. Using CFC compounds in these devices would be cheaper than using replacement compounds harmless to ozone. An international fund has therefore been set up to help these countries to introduce new and environmentally more friendly technologies and chemicals. The depletion of the ozone layer is a world-wide problem which does not respect the frontiers between different countries. It can only be affected through determined international co-operation.
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