OZONE LAYER PROTECTION

Transcription

1 OZONE LAYER PROTECTION António Gonçalves Henriques 1 WHAT IS THE OZONE LAYER? The ozone layer or ozone shield refers to a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet (UV) radiation. It contains high concentrations of ozone (O 3 ) relative to other parts of the atmosphere, although still very small relative to other gases in the stratosphere. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometres, where they range from about 2 to 8 parts per million, though the thickness varies seasonally and geographically. While the average ozone concentration in Earth's atmosphere as a whole is only about 0.3 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimeters thick. António Gonçalves Henriques 2 António Gonçalves Henriques 1

2 HOW OZONE IS FORMED IN THE OZONE LAYER? 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 (the Dobsonmeter) 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 continue to operate to this day. The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman in The production of ozone in the stratosphere results primarily from the breaking of the chemical bonds within oxygen molecules (O2) by high-energy solar photons. This process, called photodissociation, results in the release of single oxygen atoms, which later join with intact oxygen molecules to form ozone. Chemically, this can be described as: O 2 UV O O O 2 O 3 O António Gonçalves Henriques 3 HOW OZONE IS FORMED IN THE OZONE LAYER?. Ozone (O3) is a toxic gas, explosive and a powerful oxidant. It mainly concentrates in the stratosphere, 90%, as a protective veil, which filters out 99% of UV radiation reaching the top of the atmosphere. In the troposphere it is harmful to health and promotes Smog Photochemical. António Gonçalves Henriques 4 António Gonçalves Henriques 2

3 HOW OZONE IS FORMED IN THE OZONE LAYER? Rising atmospheric oxygen concentrations some two thousand million years ago allowed ozone to build up in Earth s atmosphere, a process that gradually led to the formation of the stratosphere. Scientists believe that the formation of the ozone layer played an important role in the development of life on Earth by screening out lethal levels of UV radiation and thus facilitating the migration of life-forms from the oceans to land. The thickness of the ozone layer that is, the total amount of ozone in a column overhead varies by a large factor worldwide, being in general smaller near the equator and larger towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity. Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels over the tropics and the lowest over polar regions. The same argument would lead one to expect the highest ozone levels in the summer and the lowest in the winter. The observed behavior is very different: most of the ozone is found in the mid-tohigh latitudes of the northern and southern hemispheres, and the highest levels are found in the spring, not summer, and the lowest in the autumn, not winter in the northern hemisphere. During winter, the ozone layer actually increases in depth. António Gonçalves Henriques 5 HOW OZONE IS FORMED IN THE OZONE LAYER? This puzzle is explained by the prevailing stratospheric wind patterns, known as the Brewer- Dobson circulation. While most of the ozone is indeed created over the tropics, the stratospheric circulation then transports it poleward and downward to the lower stratosphere of the high latitudes. The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel by 1 km in the lower tropical stratosphere is about 2 months (18 m per day). However, horizontal poleward transport in the lower stratosphere is much faster and amounts to approximately 100 km per day in the northern hemisphere whilst it is only half as much in the southern hemisphere (~51 km per day). Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches 26 km. António Gonçalves Henriques 6 António Gonçalves Henriques 3

4 ABSORTION OF UV RADIATION IN THE ATMOSPHERE The concentration of the ozone in the ozone layer is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the sun. Extremely short UV ( nm) is screened out by nitrogen. UV radiation capable of penetrating nitrogen is divided into three categories, based on its wavelength; these are referred to as UV-A ( nm), UV-B ( nm), and UV-C ( nm). UV-C, which is very harmful to all living things, is entirely screened out by a combination of dioxygen (< 200 nm) and ozone (> about 200 nm) by around 35 km altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn. The ozone layer (which absorbs from about 200 nm to 310 nm with a maximal absorption at about 250 nm) is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface, and is important for the skin's production of vitamin D. Ozone is transparent to most UV-A, so most of this longer-wavelength UV radiation reaches the surface, and it constitutes most of the UV reaching the Earth. António Gonçalves Henriques 7 DOBSON UNITS If all the ozone in a vertical prism over a given area is compressed dow n to 1 atm pressure and 0 C temperature w ould form a layer of 3 mm thickness. This corresponds to 300 Dobson units António Gonçalves Henriques 8 António Gonçalves Henriques 4

5 MONITORING THE OZONE LAYER Ground Based Measurements The Dobson Spectrophotometer measures column ozone by the technique of differential absorption of ultraviolet (UV) light with the sun (or moon) as a light source. By comparing the UV light intensity at wavelengths that are strongly absorbed and weakly absorbed by ozone, the column ozone content of the atmosphere is accurately determined. António Gonçalves Henriques 9 MONITORING THE OZONE LAYER Airborne Measurements Airborne measurements of ozone provide a direct (or in situ) method of determining ozone concentrations in the atmosphere. Balloons, rockets, and aircraft carry instruments into the atmosphere, resulting in the most accurate and detailed methods of measuring ozone. However, the measurements are made only over localized regions and cannot provide a global picture of ozone distribution. Balloons have been used almost as long as ground devices to measure ozone. They can measure the change in ozone concentration with altitude as high as 40 km and provide several days of continuous coverage. António Gonçalves Henriques 10 António Gonçalves Henriques 5

6 MONITORING THE OZONE LAYER Satellite Measurements Satellites measure ozone over the entire globe every day, providing comprehensive data. In orbit, satellites are capable of observing the atmosphere in all types of weather, and over the most remote regions on Earth. They are capable of measuring total ozone levels, ozone profiles, and elements of atmospheric chemistry. MetOp, the Meteorological Operational satellite programme is a European undertaking providing weather data services to monitor the climate and improve weather forecasts. MetOp carries a set of instruments that offer improved remote sensing capabilities to both meteorologists and climatologists. The new instruments will augment the accuracy of temperature humidity measurements, readings of wind speed and direction, and atmospheric ozone profiles. MetOp-A was launched on 19 October MetOp-B was launched on 17 September 2012 and operates in tandem with MetOp-A. MetOp-C will be launched in António Gonçalves Henriques 11 OZONE DEPLETION Chemists Mario Molina and Sherwood Rowland discovered that chemicals known as CFCs (chlorofluorocarbons), which are man-made, can reach the stratosphere and destroy ozone gas. This was a weighty discovery with worldwide implications because several products and appliances that, when manufactured or used, release CFCs into the atmosphere: Aerosol spray cans, Styrofoam, air conditioner units and refrigerators are a few items that make the list. The scientists began claiming that CFCs contribute to the formation of "holes" in the ozone. Chlorine and bromine are both substances that can destroy ozone. It turns out that some natural and man-made chemical compounds containing chlorine and bromine are able to rise up to the stratosphere where the conditions allow them to react with and destroy ozone. The earth's natural production of these substances accounts for 17 percent of the chlorine and 30 percent of the bromine in the stratosphere. Molina and Rowland explained that CFCs gradually rise up into the ozone layer, where ultraviolet light breaks the compounds apart, which releases chlorine. A chlorine atom can steal an oxygen atom from an ozone molecule, creating oxygen gas and chlorine monoxide (ClO), which effectively destroys the ozone molecule. But the chlorine atom isn't done yet; a chlorine atom can break from its oxygen atom and cause the destruction of as many as 10,000 more ozone molecules. From their findings, the chemists projected that after years of unrestrained CFC production, the ozone would deplete significantly. António Gonçalves Henriques 12 António Gonçalves Henriques 6

7 OZONE DESTRUCTION IN STRATOSPHERE UV light breaks off a chlorine atom from a CFC molecule The chlorine atom attacks an ozone molecule, breaking it apart so destroying the ozone. UV light The chlorine is free to repeat the process of destroying more ozone molecules and releases a free chlorine atom and forming an oxygen molecule A free oxygen atom attacks the chlorine monoxide, This forms a chlorine monoxide molecule (ClO) and an oxygen molecule (O2) Cl O António Gonçalves Henriques 13 3 ClO O ClO O Cl O 2 2 DESTRUIÇÃO DO OZONO NA ESTRATOSFERA António Gonçalves Henriques António Gonçalves Henriques 7

8 OZONE DESTRUCTION IN STRATOSPHERE Over the course of several decades human activities substantially altered the ozone layer. For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other applications. Other chlorine-containing compounds include methyl chloroform, a solvent, and carbon tetrachloride, an industrial chemical. Halons, extremely effective fire extinguishing agents, and methyl bromide, an effective produce and soil fumigant, contain bromine. All of these compounds have atmospheric lifetimes long enough, years and even decades, to allow them to be transported by winds into the stratosphere. Because they release chlorine or bromine when they break down, they damage the protective ozone layer. The ozone is destroyed as a result of reversible reactions catalyzed by chemical species, such as Br, Cl, C and H, even with very low concentrations of the order of 0.01 ppm. These reactions are triggered by UV radiation. The discussion of the ozone depletion process below focuses on CFCs, but the basic concepts apply to all of the ozone-depleting substances (ODS). António Gonçalves Henriques 15 OZONE DESTRUCTION IN STRATOSPHERE In the 1970s, researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. They also examined the potential impacts of other chlorine sources. Chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. Chlorine compounds from these sources readily combine with water and repeated measurements show that they rain out of the troposphere very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. Over time, winds drive the CFCs into the stratosphere. The CFCs are so stable that only exposure to strong UV radiation breaks them down. One chlorine atom released from a CFC molecule can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created. Large fires and certain types of marine life produce one stable form of chlorine that reaches the stratosphere. However, numerous experiments have shown that CFCs and other widelyused chemicals produce roughly 84% of the chlorine in the stratosphere, while natural sources contribute only 16%. António Gonçalves Henriques 16 António Gonçalves Henriques 8

9 FORMATION OF THE OZONE HOLES Ozone depletion, the global decrease in stratospheric ozone observed since the 1970s, is most pronounced in polar regions, and it is well correlated with the increase of chlorine and bromine in the stratosphere. Depletion is so extensive that so-called ozone holes (regions of severely reduced ozone coverage) form over the poles during the onset of their respective spring seasons. The largest such hole which has spanned more than 20.7 million km 2 on a consistent basis since 1992 appears annually over Antarctica between September and November. António Gonçalves Henriques 17 FORMATION OF THE OZONE HOLES While the chlorine atoms freed from CFCs do ultimately destroy ozone, the destruction doesn t happen immediately. Most of the roaming chlorine that gets separated from CFCs actually becomes part of two chemicals, hydrochloric acid (HCl), and chlorine nitrate (ClNO 3 ), that under normal atmospheric conditions are so stable that scientists consider them to be long-term reservoirs for chlorine. So how does the chlorine get out of the reservoir each spring? Under normal atmospheric conditions, the two chemicals that store most atmospheric chlorine (hydrochloric acid and chlorine nitrate) are stable. But in the long months of polar darkness over Antarctica in the winter, atmospheric conditions are unusual. An endlessly circling whirlpool of stratospheric winds called the polar vortex isolates the air in the center. Because it is completely dark, the air in the vortex gets so cold that clouds form, even though the Antarctic air is extremely thin and dry. Chemical reactions take place that could not take place anywhere else in the atmosphere. These unusual reactions can occur only on the surface of polar stratospheric cloud particles, which may be water, ice, or nitric acid, depending on the temperature. António Gonçalves Henriques 18 António Gonçalves Henriques 9

10 FORMATION OF THE OZONE HOLES The frozen crytals that make up polar stratospheric clouds provide a surface for the reactions that free chlorine atoms in the Antarctic stratosphere. These reactions convert the inactive chlorine reservoir chemicals into more active forms, especially chlorine gas (Cl 2 ). When the sunlight returns to the South Pole in October, UV light rapidly breaks the bond between the two chlorine atoms, releasing free chlorine into the stratosphere, where it takes part in reactions that destroy ozone molecules while regenerating the chlorine (known as a catalytic reaction). A catalytic reaction allows a single chlorine atom to destroy thousands of ozone molecules. Bromine is involved in a second catalytic reaction with chlorine that contributes a large fraction of ozone loss. The ozone hole grows throughout the early spring until temperatures warm and the polar vortex weakens, ending the isolation of the air in the polar vortex. As air from the surrounding latitudes mixes into the polar region, the ozonedestroying forms of chlorine disperse. The ozone layer stabilizes until the following spring. António Gonçalves Henriques 19 FORMATION OF THE OZONE HOLES Air in the upper troposphere and António Gonçalves Henriques 20 António Gonçalves Henriques 10

11 HEALTH EFFECTS OF UV RADIATION Ozone layer depletion decreases our atmosphere s natural protection from the sun s harmful ultraviolet (UV) radiation. UV exposure in humans is principally via the eyes and skin, with effects occurring as a result of the absorption of solar energy by molecules (termed chromophores) present in the tissues and cells present in these organs. The absorption of light energy leads to changes in these molecules that eventually can result in a biologic effect. The chain of events is: Biological UV absorption --> Biochemical change --> Cellular death/alteration --> Organism response Chromophores absorb light energy from the various wavelengths with differing efficiencies. This pattern of absorption is called an absorption spectrum and is characteristic of the type of the chromophores present in skin and eye tissues that are thought to be important to the biologic effects of UV-B in humans and animals. António Gonçalves Henriques 21 HEALTH EFFECTS OF UV RADIATION The health effects of UV radiation are the following: Skin cancer (melanoma and nonmelanoma) Premature aging and other skin damage Cataracts and other eye damage Immune system suppression Skin cancer Laboratory and epidemiological studies demonstrate that UVB causes nonmelanoma skin cancer and plays a major role in malignant melanoma development. Melanoma, the most serious form of skin cancer, is now one of the most common cancers among adolescents and young adults ages While melanoma accounts for about 3% of skin cancer cases, it causes more than 75% of skin cancer deaths. UV exposure and sunburns, particularly during childhood, are risk factors for the disease. However, not all melanomas are exclusively sun-related other possible influences include genetic factors and immune system deficiencies. Non-melanoma skin cancers are less deadly than melanomas. Nevertheless, they can spread if left untreated, causing disfigurement and more serious health problems. António Gonçalves Henriques 22 António Gonçalves Henriques 11

12 HEALTH EFFECTS OF UV RADIATION Premature Aging and Other Skin Damage Other UV-related skin disorders include actinic keratoses and premature aging of the skin. Actinic keratoses are skin growths that occur on body areas exposed to the sun. The face, hands, forearms, and the V of the neck are especially susceptible to this type of lesion. Although premalignant, actinic keratoses are a risk factor for squamous cell carcinoma. Chronic exposure to the sun also causes premature aging, which over time can make the skin become thick, wrinkled, and leathery. Since it occurs gradually, often manifesting itself many years after the majority of a person s sun exposure, premature aging is often regarded as an unavoidable, normal part of growing older. However, up to 90 percent of the visible skin changes commonly attributed to aging are caused by the sun. António Gonçalves Henriques 23 HEALTH EFFECTS OF UV RADIATION Cataracts and Other Eye Damage Cataracts are a form of eye damage in which a loss of transparency in the lens of the eye clouds vision. If left untreated, cataracts can lead to blindness. Research has shown that UV radiation increases the likelihood of certain cataracts. Although curable with modern eye surgery, cataracts diminish the eyesight and cost billions of dollars in medical care each year. Other kinds of eye damage include pterygium (tissue growth that can block vision), skin cancer around the eyes, and degeneration of the macula (the part of the retina where visual perception is most acute). All of these problems can be Retina Vitreous humour Iris lessened with proper eye protection. Lens Cornea António Gonçalves Henriques 24 António Gonçalves Henriques 12

13 HEALTH EFFECTS OF UV RADIATION Immune Suppression Scientists have found that overexposure to UV radiation may suppress proper functioning of the body s immune system and the skin s natural defenses. For example, the skin normally mounts a defense against foreign invaders such as cancers and infections. But overexposure to UV radiation can weaken the immune system, reducing the skin s ability to protect against these invaders. Increased levels of vitamin D Exposure to UV-B radiation increases levels of vitamin D produced in the skin by the radiation. Although higher levels of vitamin D are associated with increased mortality, the human body has mechanisms that prevent sunlight to produce excess vitamin D. In children, vitamin D deficiency may lead to rickets, a disease that results from inadequate bone mineralization during growth with consequent bone abnormalities. A serious deficiency in adults leads to osteomalacia, a condition characterized by a failure in the mineralization of organic matrix of bone, resulting in bone weak, pressure-sensitive weakness in proximal muscles and increased frequency of fractures. António Gonçalves Henriques 25 HEALTH EFFECTS OF UV RADIATION Increased levels of vitamin D Exposure to UV-B radiation increases levels of vitamin D produced in the skin by the radiation. Although higher levels of vitamin D are associated with increased mortality, the human body has mechanisms that prevent sunlight to produce excess vitamin D. In children, vitamin D deficiency may lead to rickets, a disease that results from inadequate bone mineralization during growth with consequent bone abnormalities. A serious deficiency in adults leads to osteomalacia, a condition characterized by a failure in the mineralization of organic matrix of bone, resulting in bone weak, pressure-sensitive weakness in proximal muscles and increased frequency of fractures. António Gonçalves Henriques 26 António Gonçalves Henriques 13

14 António Gonçalves Henriques 27 ADVERSE IMPACTS ON AGRICULTURE, FORESTRY AND NATURAL ECOSYSTEMS Several of the world's major crop species are particularly vulnerable to increased UV, resulting in reduced growth, photosynthesis and flowering. These species include wheat, rice, barley, oats, corn, soybeans, peas, tomatoes, cucumbers, cauliflower, broccoli and carrots. The effect of ozone depletion on the sector could be significant. Only a few commercially important trees have been tested for UV (UV-B) sensitivity, but early results suggest that plant growth, especially in seedlings, is harmed by more intense UV radiation. António Gonçalves Henriques 28 António Gonçalves Henriques 14

15 DAMAGE TO MARINE LIFE Phytoplankton form the foundation of aquatic food webs. Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column in which there is sufficient sunlight to support net productivity. The position of the organisms in the euphotic zone is influenced by the action of wind and waves. In addition, many phytoplankton are capable of active movements that enhance their productivity and, therefore, their survival. Exposure to solar UVB radiation has been shown to affect both orientation mechanisms and motility in phytoplankton, resulting in reduced survival rates for these organisms. Scientists have demonstrated a direct reduction in phytoplankton production due to ozone depletion-related increases in UVB. Decreases in plankton could disrupt the fresh and saltwater food chains, and lead to a species shift in marine waters. Solar UVB radiation has been found to cause damage to early developmental stages of fish, shrimp, crab, amphibians and other animals. The most severe effects are decreased reproductive capacity and impaired larval development. Even at current levels, solar UVB radiation is a limiting factor, and small increases in UVB exposure could result in significant reduction in the size of the population of animals that eat these smaller creatures. Loss of biodiversity in the oceans, rivers and lakes could reduce fish yields for commercial and sport fisheries. António Gonçalves Henriques 29 OTHER EFFECTS OF INCREASED UV RADIATION Effects on Biogeochemical Cycles Increases in solar UV radiation could affect terrestrial and aquatic biogeochemical cycles, thus altering both sources and sinks of greenhouse and chemically-important trace gases e.g., carbon dioxide (CO 2 ), carbon monoxide (CO), carbonyl sulfide (COS) and possibly other gases, including ozone. These potential changes would contribute to biosphereatmosphere feedbacks that attenuate or reinforce the atmospheric buildup of these gases. Effects on Materials Synthetic polymers, naturally occurring biopolymers, as well as some other materials of commercial interest are adversely affected by solar UV radiation. Today's materials are somewhat protected from UVB by special additives. Therefore, any increase in solar UVB levels will therefore accelerate their breakdown, limiting the length of time for which they are useful outdoors. António Gonçalves Henriques 30 António Gonçalves Henriques 15