Ozon

[Presentasi Power Point]

Penipisan Lapisan Ozon

The name ozone comes from the Greek Ozon meaning smell. At atmospheric temperatures, ozone is a colorless gas with an odor similar to chlorine that can usually be detected at a level of about 0.01 parts per million.

High in the atmosphere, ozone plays an important protective role by diminishing the amount of potentially damaging ultraviolet radiation reaching Earth. In sufficient concentration, however, ozone is a poison that at lower atmospheric levels, is a pollutant that can be damaging to health. Ozone is also a strong oxidizing agent used in many industrial processes for bleaching and sterilization. Although ozone is often used in water treatment, the largest commercial application of ozone is in the production of pharmaceuticals, synthetic lubricants, and other commercially useful organic compounds.

In the atmosphere, ozone is formed predominantly by electric discharges (e.g., lightning). In the laboratory, ozone can be extracted form a mixture of oxygen and ozone by fractionation.

Ozone can also be formed by ultraviolet light. Ultraviolet light is energetic, and when it strikes the atmosphere it can break down some oxygen molecules producing highly energized oxygen atoms (free radicals). These free radicals can then react with molecular oxygen to produce ozone. The absorption of energetic light radiation also triggers the decomposition of ozone. As a result, ozone is an unstable molecule that exists in a dynamic equilibrium of formation and destruction. Consequently, the protective ozone layer is also in dynamic equilibrium.

The area where ozone is formed at the fastest rate is in the atmosphere at a height of approximately 164,042 ft (50 km). At this height, the number of free radicals made by ultraviolet light and electric discharge is balanced by the concentration of diatomic oxygen, which is sufficiently high to ensure that reactive collisions occur.

The protective ozone layer is found in the upper reaches of the atmosphere (between 98,000–295,000 ft [30–90 km]) where it absorbs ultraviolet radiation that, in excess, can be harmful to biological organisms. The potential detrimental effects of increased exposure to ultraviolet light due to a lessening of atmospheric ozone are of great concern. Holes in the ozone layer, or a global breakdown of stratospheric ozone would lead to increasing doses of ultraviolet radiation at Earth's surface. Scientists fear that significant increases exposure to ultraviolet light will increase risks of cancer in animal skin, eyes, and immune systems. Studies have shown that high ultraviolet radiation doses can supply the needed energy for chemical reactions that produce highly reactive radicals that have the potential to damage DNA and other cell regulating chemicals and structures.

There are several atmospheric trace elements, including ozone, that are important in the regulation of the global climate. Although the atmosphere consists of mainly of nitrogen and oxygen, approximately one percent of Earth's atmosphere is made of small amounts of other gases. Trace gases include water vapor, carbon dioxide, nitrous oxide, methane, chlorofluorocarbons (CFCs), and ozone. Because the amount of trace gases in the atmosphere is small, human activities can significantly affect the proportions of atmospheric trace gases.

Chloroflourocarbons (CFCs) easily react with ozone, which has the effect of breaking down an already unstable molecule. Until recently, CFCs were commonly used in refrigeration and in aerosol propellants (a pressurized gas used to propel substances out of a container). After evidence indicating that the use of CFCs was tipping the ozone equilibrium toward overall ozone layer depletion, many industrialized countries opted to enforce restrictions on the use of CFCs. Consumer aerosol products in the United States have not used ozone-depleting substances such as CFCs since the late 1970s. Federal regulations, including the Clean Air Act and Environmental Protection Agency (EPA) regulations restrict the use of ozone-depleting substances.

Ozone played a critical role in the development of life on Earth. Once primitive plants evolved, oxygen started to accumulate in the atmosphere. Some of this oxygen was converted into ozone and the developing ozone layer gave needed protection from disruptively energetic ultraviolet radiation. As a consequence, complex organic molecules which would otherwise have been destroyed began to accumulate.

As well as being found high in the atmosphere, ozone can be found at ground level. At these locations it is regarded as a pollutant. Ozone at ground level can be manufactured as part of photochemical smog. This is brought about by the disassociation of oxides of nitrogen that produce oxygen free radicals. These free radicals can react with diatomic oxygen to produce ozone. Pollutant ozone can also be a by-product of the action of photocopiers and computer printers. Low level ozone is usually found at a concentration of less than 0.01 parts per million, whereas in photochemical smog, it can be encountered at levels as high as 0.5 parts per million. Levels of ozone exposure between 0.1 and 1 part per million cause headaches, burning eyes, and irritation to the respiratory passages in humans. Elderly people, asthma sufferers, and those exercising in photochemical smog suffer the greatest adverse effects.

Some plant species (e.g., the tobacco plant) are particularly sensitive to low-lying ozone. The presence of excessive ozone causes a characteristic spotting of the leaves. High ozone levels are also known to damage structural material such as rubber.

Replacing more dangerous chlorine gas, ozone is used in many waste treatment facilities to purify water. Ozone is responsible for disinfecting the water and the efficient removal of trace elements such as pestisida. Ozone kills bacteria and other small life forms and it reacts with organic compounds. During the process, the ozone is transformed to molecular oxygen.

Ozone Layer and Ozone Hole Dynamics

In 1985, atmospheric scientists discovered that stratospheric ozone over Antarctica had been reduced to half its natural level. This local loss, termed the Antarctic ozone hole, was traced to destruction of stratospheric ozone by human-made chemicals, especially chlorofluorocarbons (CFCs; artificial compounds consisting of chlorine, fluorine, and carbon and widely used as refrigerants and aerosol spray propellants). Other evidence indicates that ozone levels potentially declining over other regions, though nowhere as drastically as over Antarctica.

The ozone hole covers an area over the Antarctic continent, the surrounding ocean, the southern tip of South America in which stratospheric ozone begins to diminish every August (at the beginning of the Southern hemisphere's spring season), reaches a minimum of less than 50% of its natural value in October, and returns to normal levels by the beginning of December.

Essential to the formation of the Antarctic ozone hole is the polar vortex, which forms every winter over the South Pole. The pole is in 24-hour darkness in midwinter, so the air above it becomes very cold. Cooling air lowers its pressure. Air nearer the equator, warmer and therefore at higher pressure, is sucked toward the pole by the low pressure there. As this warm air moves southward it is twirled into a circular wind by the spin of the earth. This circular wind, the polar vortex, sits over the South Pole like a halo, isolating the air over the pole and allowing it to become even colder. Intermittently, the stratosphere over the pole becomes cold enough to form clouds. The droplets and ice crystals in these stratospheric clouds accelerate the breakdown of ozone by chlorine, essentially eliminating ozone from the lower stratosphere and allowing twice the usual amount of UV-B to reach the surface.

No ozone hole forms at the North Pole because the north-polar winter vortex is smaller and warmer than the southern one. There is nevertheless a 30% decline in north-polar ozone every March. Ozone levels have also declined by 3–6% over the inhabited (middle) latitudes, allowing more UV-B to reach the surface and increasing skin cancer rates.

The ozone layer protects the earth by absorbing UV-B, which can cause skin cancer and eye damage. Low-altitude ozone, however, blocks little UV-B and is toxic to plant and animal life.

Ozone (O3) is a trace ingredient of the atmosphere that stops most solar radiation in the 280–315-nm ultraviolet (UV-B) band from reaching the ground. Ozone is produced in the stratosphere by the breakup of molecular oxygen (O2) by solar radiation. It is also produced artificially in the lower atmosphere (troposphere) by the burning of coal and gasoline. Ninety percent of the atmosphere's ozone is concentrated in the lower stratosphere, about 6–30 miles (10–50 km) up; this concentration of ozone is the ozonosphere or ozone layer.

Ozone is formed in the stratosphere when an O2 molecule is split by a photon in the 175–242-nm ultraviolet band (1 nm[nanometer] = 10−9 m.) Each O then joins with an O2 to form an O3 (ozone) molecule. Ozone converts the energy it gains from absorbing ultraviolet (and infrared) photons into heat, supplying an average 15 watts of power to every square meter of the stratosphere. This ozone-driven heating defines the temperature-versus-altitude structure of the stratosphere.

Because ozone is created by sunlight it forms more rapidly over the tropics, where there is more sunlight per square meter. Some ozone created at tropical latitudes circulates through the upper stratosphere to the polar regions, but natural polar ozone levels remain lower than tropical levels. This contributes to the greater vulnerability of the polar regions to ozone depletion by CFCs and other chemicals, discussed below.

Ozone is destroyed primarily by the ClO (chlorine oxide) radical that is produced by the breakdown by sunlight of more complex chlorine-bearing molecules. ClO facilitates the reaction, participating as a catalyst. ClO radicals are free to facilitate reactions again and again. This catalytic persistence explains how minute concentrations of a human-made substance can alter the chemistry of an entire layer of the atmosphere: ozone is a million or so times more abundant in the stratosphere than ClO, but each ClO radical destroys thousands of ozone molecules.

Not all chlorine-containing compounds threaten the ozone layer, because not all are capable of reaching the stratosphere. Only non-water-soluble compounds such as CFCs, carbon tetrachloride (CCl4), and methyl chloroform (CH3CCl3) can evade water capture in the troposphere and eventually circulate to the ozone layer. There they last anywhere from 5 years (methyl chloroform) to 100 years (CFC-12). CFC-F11 (CCl3F), the primary contributor to stratospheric chlorine and therefore to ozone loss, has a lifetime of 45 years in the stratosphere.

CFCs are not the only compounds that affect stratospheric ozone; nitrous oxide (N2O), the bromine-containing compounds termed halons, and methane (CH4), also do so. Sulfur dioxide (SO2) injected into the stratosphere by violent volcanic eruptions, such as that of Mt. Pinatubo in 1991, can cause significant, albeit temporary, drops in global stratospheric ozone.

In 1987, over 100 nations signed an international agreement to reduce emissions of CFCs and other ozone-depleting chemicals, the Montreal Protocol. Later amendments to the Protocol greatly increased its effectiveness, and today scientists estimate that with strict observance of the Protocol, and barring unforeseen side effects of global climate change, stratospheric ozone will cease to decline at some point in the next 10–20 years and recover to 1980 levels by about 2050.

Ozone Layer Depletion

The ozone layer is a part of the atmosphere between 18.6 mi and 55.8 mi (30 and 90 km) above the ground. The ozone present is responsible for blocking potentially harmful ultraviolet radiation reaching the surface of the earth. During the last twenty years, evidence has accumulated that human activity may be the cause of a generalized depletion of the ozone layer. This phenomena is global and distinct from the natural factors that induce annual ozone layer hole formation over Antarctica.

Ozone is constantly created and destroyed in natural processes (manufactured by the action of lightning on oxygen and destroyed by the action of ultraviolet radiation), however the amounts normally balance each other out so there is no net increase or decrease due to natural processes. In 1970, Paul Crutzen showed that naturally occurring oxides of nitrogen can catalytically destroy ozone. In 1974, F. Sherwood Rowland and Mario Molina demonstrated that chlorofluorcarbons (CFCs) could also destroy ozone. In 1995, all three were jointly awarded the Nobel Prize for chemistry.

The CFCs that were observed as being damaging included Freon 11 (CFCl3) and Freon 12 (CF2Cl2). These chemicals are widely used in industry and the home. They have uses as propellants in aerosol spray cans, refrigerant gases, and foaming agents for blown plastics. One problem associated with these gases is their relative lack of reactivity. When released there is very little that will break them down and, as they are not soluble in water, they are not removed from the atmosphere by rain. As a consequence, once released they tend to concentrate in the upper regions of the atmosphere. It is estimated that some several million tons of CFCs are present in the atmosphere.

Once in the upper atmosphere the CFCs are exposed to high energy radiation that can cause disassociation of the molecule, producing free chlorine atoms. This atomic chlorine reacts readily with ozone to produce chlorine monoxide and molecular oxygen. The chlorine monoxide can further react to produce molecular oxygen and more atomic chlorine. This all accelerates the destruction of ozone beyond its natural ability to regenerate. Overall, there is a net reduction in the amount of ozone present in the upper atmosphere. This has led to a thinning of the ozone layer. The majority of this loss is at an altitude between 7.44 mi and 18.6 mi (12 and 30 km) and in the late 1990s evidence was seen that suggested losses were also occurring at other altitudes. In addition to the annual holes in the ozone layer now detected over Antarctica, in the late 1990s, holes were detected over Australia and atmospheric sampling indicated a dramatic thinning of the ozone layer in the Northern Hemisphere during the winter months. In the Northern Hemisphere losses of some 30% have been recorded at an altitude of 12.4 mi (20 km).

In 1987, the Montreal Protocol was signed with the appropriate countries agreeing to reduce CFC production. By 1996, more than 100 countries agreed to cease widespread commercial use of CFCs and to stop or curtail production of CFCs.

In the absence of the ozone layer, harmful ultraviolet radiation is able to reach the surface of the earth in higher doses. This can lead to increases in skin cancers.

Ultraviolet Rays and Radiation

Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. On the spectrum, ultraviolet light lies between violet light and x rays, with wavelengths ranging from four to 400 nanometers. Although it is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light, for it is this radiation that causes tanning, sunburn, and can lead to skin cancer.

The man credited with the discovery of ultraviolet light is the German physicist Johann Ritter. Ritter had been experimenting with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum—blue, indigo, violet—was a much more efficient catalyst for this reaction. Experimenting further, he discovered that silver chloride broke down most efficiently when exposed to radiation just beyond the blues, radiation that was invisible to the eye. He called this new type of radiation ultraviolet, meaning "beyond the violet." While ultraviolet radiation in large doses is hazardous to humans, a certain amount is required by the body. As it strikes the skin, it activates the chemical processes that produce Vitamin D. In areas that lack adequate sunshine, children are sometimes plagued by rickets. In order to treat these cases, or to supplement natural light in sun-starved communities, ultraviolet lamps are often used in place of natural sources.

There are three varieties of ultraviolet lamps, each producing ultraviolet light of a different intensity. Near-ultraviolet lamps are fluorescent lights whose visible light has been blocked, releasing ultraviolet radiation just beyond the visible spectrum. These lamps are also known as black lights, and are primarily used to make fluorescent paints and dyes "glow" in the dark. This effect is often seen in entertainment, but can also be used by industry to detect flaws in machine parts.

Middle-ultraviolet lamps produce radiation of a slightly shorter wavelength. They generally employ an excited arc of mercury vapor and a specially designed glass bulb. Because middle-ultraviolet radiation is very similar to that produced by the Sun, these lamps are frequently used as sunlamps and are often found in tanning salons and greenhouses. Photochemical lamps generating middle-ultraviolet light are also used in industry, as well as by chemists to induce certain chemical reactions.

Far-ultraviolet lamps produce high-energy, short-wavelength ultraviolet light. Like middle-ultraviolet lamps, they use mercury-vapor tubes; however, far-ultraviolet radiation is easily absorbed by glass, and so the lamp's bulb must be constructed from quartz. Far-ultraviolet light has been found to destroy living organisms such as germs and bacteria; for this reason, these lamps are used to sterilize hospital air and equipment. Far-ultraviolet radiation has also been used to kill bacteria in food and milk, giving perishables a much longer shelf life.

A more passive application of ultraviolet light is in astronomy. Much of the light emitted by stars, particularly very young stars, is in the ultraviolet range. By observing the output of ultraviolet light, astronomers can determine the temperature and composition of stars and interstellar gas, as well as gain insights into the evolution of galaxies. However, most of the ultraviolet light from distant sources is unable to penetrate the Earth's atmosphere; therefore, ultraviolet observations must be made from Earth's orbit by sounding rockets, space probes, or astronomical satellites.