ozone layer protection - ulisboa · ozone layer protection antónio gonçalves henriques 2 • the...

EnvironmentalPolicies 2015-2016

OZONE LAYER DEPLETION

António Gonçalves Henriques 1


OZONE LAYER PROTECTION


• 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 (O3) relative to other

parts of the atmosphere, a lthough still very small relative to other gases in the stratosphere.

• About 90% of the ozone in our atmosphere i s conta ined in the s tratosphere. Ozone concentrations are greatest between about 20

and 40 ki lometres, 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

mi l lion.

• If a l l of the ozone were compressed to the

pressure of the air at sea level, i t would be only 3 mil limeters thick.

WHAT IS 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 s tratospheric ozone from the ground. Between 1928 and 1958, Dobson established a worldwide network of ozone monitoring s tations, which continue to operate to this day.

• The photochemical mechanisms that give rise to the ozone layer were discovered by the Bri ti sh physicist Sydney Chapman in 1930.

• The production of ozone in the s tratosphere results primarily from the breaking of the chemical bonds within oxygen molecules (O2) by high-energy solar photons. This process, ca l led 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:

32

2

OOO

OOO UV

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 i t is harmful to health and promotes Smog Photochemical.





• Ris ing atmospheric oxygen concentrations some two thousand million years ago a llowed

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 faci litating the migration of l ife-forms from the oceans to land.

• The thickness of the ozone layer—that i s, 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

compl icated, involving atmospheric ci rculation 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 di fferent: most of the ozone is found in the mid-to-high 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.



• This puzzle is explained by the prevailing

s tratospheric wind patterns, known as the Brewer-Dobson ci rculation.

• While most of the ozone is indeed created over the tropics, the s tratospheric circulation then transports i t poleward and downward to the lower stratosphere of the high latitudes.

• The Brewer-Dobson ci rculation moves very s lowly. The time needed to lift an air parcel by 1 km in the lower tropical s tratosphere is about 2 months (18 m per day).


• However, horizontal poleward transport in the lower s tratosphere is much faster and

amounts to approximately 100 km per day in the northern hemisphere whilst i t is only hal f as much in the southern hemisphere (~51 km per day).

• Even though ozone in the lower tropical stratosphere i s produced at a very s low rate, the l i fting ci rculation is so slow that ozone can build up to relatively high levels by the time i t reaches 26 km.





ABSORTION OF UV RADIATION IN THE ATMOSPHERE

• 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.

• 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 (10–100 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 (400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm).


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





MONITORING THE OZONE LAYER

The Dobson Spectrophotometer measures column ozone by the technique of differential absorption of ultraviolet (UV) l ight with the sun (or moon) as a light source.

By comparing the UV light intensity at wavelengths that are s trongly absorbed and weakly absorbed by ozone, the column ozone content of the atmosphere is accurately determined.

Ground Based Measurements



Airborne Measurements

Airborne measurements of ozone provide a

di rect (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.

Bal loons have been used almost as long as ground devices to measure ozone. They can measure the change in ozone concentration with a ltitude as high as 40 km and provide several days of continuous coverage.






Satellite Measurements

Satellites measure ozone over the entire globe every day, providing comprehensive data. In orbi t, satellites are capable of observing the atmosphere in a ll 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 2006.

MetOp-B was launched on 17 September 2012 and operates in tandem with MetOp-A.

MetOp-C will be launched in 2016.


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 i tems 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 s tratosphere 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 s tratosphere.

Mol ina and Rowland explained that CFCs gradually rise up into the ozone layer, where ultraviolet l ight 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.





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.

This forms a chlorine monoxide molecule (ClO) and an oxygen molecule (O2)

A free oxygen atom attacks the chlorine monoxide,

The chlorine is free to repeat the process of destroying more ozone molecules

UV light and releases a free chlorine atom and forming an oxygen molecule

2

23

OClOClO

OClOOCl


DESTRUIÇÃO DO OZONO NA ESTRATOSFERA

2014-11-11





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 s table, 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.

• Al l of these compounds have atmospheric l ifetimes long enough, years and even decades, to

al low 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).


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 a lso 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 ra in out of the troposphere very quickly. In contrast, CFCs are very s table and do not dissolve in ra in. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. Over time, winds drive the CFCs into the s tratosphere.

• 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 i t is naturally created.

• Large fi res and certain types of marine l ife produce one stable form of chlorine that reaches the s tratosphere. However, numerous experiments have shown that CFCs and other widely-used chemicals produce roughly 84% of the chlorine in the s tratosphere, while natural sources contribute only 16%.





FORMATION OF THE OZONE HOLES • Ozone depletion, the global decrease

in s tratospheric ozone observed since the 1970s , i s most pronounced in polar regions, and it i s well correlated with

the increase of chlorine and bromine in the s tratosphere.

• 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 km2 on a consistent basis since 1992—appears annually over Antarctica between September and November.


• 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 (ClNO3), 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 s tore most atmospheric chlorine (hydrochloric acid and chlorine nitrate) are s table.

• But in the long months of polar darkness over Antarctica in the winter, atmospheric conditions are unusual. An endlessly ci rcling whirlpool of s tratospheric winds called the polar vortex isolates the a ir in the center.

• Because i t is completely dark, the air in the vortex gets so cold that clouds form, even though the Antarctic a ir 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 s tratospheric cloud particles, which may be water, i ce, or nitric acid, depending on the temperature.

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 (Cl2).


• When the sunlight returns to the South Pole in October, UV l ight 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 cata lytic 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 ozone-destroying forms of chlorine disperse. The ozone layer s tabilizes until the following spring.



Air in the upper troposphere and





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 --> Cel lular 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.


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 15-29. While melanoma accounts for about 3% of skin cancer cases, i t causes more than 75% of skin cancer deaths. UV exposure and sunburns, particularly during childhood, are ri sk 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 i f left untreated, causing disfigurement and more serious health problems.





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 i t occurs gradually, often manifesting i tself 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 vis ible skin changes commonly attributed to aging are caused by the sun.


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. Al though 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).

Al l of these problems can be lessened with proper eye protection.

Iris

Cornea

Lens

Vitreous

humour

Retina





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 vi tamin D are associated with increased mortality, the human body has mechanisms that prevent sunlight to produce excess vi tamin D.

• In chi ldren, vi tamin 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.


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 vi tamin D are associated with increased mortality, the human body has mechanisms that prevent sunlight to produce excess vi tamin D.

• In chi ldren, vi tamin 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.






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.





DAMAGE TO MARINE LIFE • Phytoplankton form the foundation of aquatic food webs. Phytoplankton productivi ty is

l imited 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 productivi ty 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 fi sh, 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 l imiting factor, and small increases in UVB exposure could result in s ignificant 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.


OTHER EFFECTS OF INCREASED UV RADIATION

Effects on Biogeochemical Cycles

• Increases in solar UV radiation could affect terrestrial and aquatic biogeochemical cycles, thus a ltering both sources and sinks of greenhouse and chemically-important trace gases e.g., carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulfide (COS) and possibly other gases, including ozone. These potential changes would contribute to biosphere-atmosphere 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, l imiting the length of time for which they are useful outdoors.

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