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The Ozone Layer
Chapter 15
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Good Ozone• The ozone layer = 15-30 km altitude in stratosphere
– Photochemical smog is ozone in the wrong place
• Earth’s Natural Sunscreen: blocks ultraviolet light
• Ozone is only a small fraction of the gases– N2, O2 are still the majority of the gases in ozone layer
– O3 would make a 3.5 mm thick layer at Earth’s surface
• Ozone is produced at the tropics, flows to poles
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Layers of the Atmosphere
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Antarctic Ozone Hole• Ozone totals are measured in “Dobson Units”
– 350 Dobson Units is typical in temperate regions– Ozone levels have decreased below 150 Dobson Units
over Antarctica
• Began to occur in late 1970’s– Levels are lowest in Antarctica’s Spring (Sept—Nov)– Normal levels regained by June—July– Geographic Region effected is growing each year
• Not really a hole; more like a thinner window
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Seasonal Ozone Fluctuations in the N. Hemisphere
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Average October Ozone Levels over Antarctica
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Extent of the Arctic Ozone Hole
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Cause of the Ozone Hole• Chlorine Pollution has been proven as the cause
• CFC’s = Chlorofluorocarbons – One of the main sources of atmospheric chlorine– Used as refrigerants in air conditioners and refrigerators
• Fairly unreactive, so they make it to ozone layer
• Laws have greatly reduced chlorine pollutions
• We will probably still have ozone hole until ~2050
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Molecules and Light• The spectrum
– Visible light: 400—750 nm (ROYGBIV = rainbow)– Color changes as wavelength changes– Ultraviolet light = UV = 50—400 nm (invisible)
• More energetic than visible light
• Energy increases as wavelength decreases
• Molecules can interact with visible and UV light– Absorption Spectrum = graph of light absorbed vs wavelength
– Light absorption increases the energy of the molecule
– Bonds can be broken or new bonds formed
– O2, as all molecules do, absorbs light selectively
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A portion of the electromagnetic spectrum
O2 absorptionspectrum in UV
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Ozone absorbs UV light• O2 absorbs all UV between 120—220 nm
– Above the stratosphere
• UV between 50—120 nm is absorbed by N2, O2
• O3 vital because it absorbs 220—320 nm UV– UV-C = 220—280 nm O3 + O2 remove all– UV-B = 280—320 nm O3 absorbs 70-90%
• Depends on season, latitude, etc…• Never completely successful• Fraction of UV-B escaping increases as wavelength increases
– UV-A = 320—400 nm is not stopped by any gases• Least harmful type of UV light
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Ozone’s Absorption Spectrum
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Biology and the Ozone Layer• UV-B does escape the ozone layer
– 1% loss in ozone = 2% increase in UV-B on the ground– Causes sunburn, cataracts, and skin cancer– Affects immune system– Affects the growth of plants and animals
• DNA absorbs UV-B and is damaged– Replication of new DNA, cells goes wrong = cancer– Skin cancer is most common, since it gets most sun
• Loss of ozone is predicted to cause increase in skin cancer
• Slow spreading form; can be detected and treated
• 25% of Americans will have some form in their lifetimes
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UV light damage to DNA
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Malignant Melanoma• Fatal form of skin cancer
– 1 in 100 Americans will likely get it– Short, high UV exposure early in life linked to it– Fair skin, fair hair, freckles makes you susceptible– 15-25 year lag time between exposure and melanoma
• Likely only seen the tip of the iceberg
• Skin cancer cases are expected to rise (1970’s + 25 = 2000)
– Incidence of melanoma based on several things• Latitude: Texas and Florida have higher rates
• Lifestyle: Australia has higher rates
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Sunscreens• Substances that block UV light on your skin
– Some only block UV-B, not UV-A• Don’t get burned, but can still damage your skin, DNA
– How they work• Reflect all light = ZnO, TiO2; white, inorganic
• Absorb UV before it reaches your skin = organic molecules
– Must not break down as they absorb light ----> useless
O
O
O
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4-Methylbenzylidene camphor (4-MBC) Octyl methoxycinnamate (OMC)
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Other Effects of UV• Cataracts
– Cornea and Lens filter 99% of UV before it reaches your retina
– Chemical reactions cause opaqueness of the lens– Primarily a UV-B caused problem– Usually long-tern; short-term when severe exposure
• Effects on Animals and Plants– Photosynthesis by plants reduced when UV-B increases– Oceans depend on surface phytoplankton– Amphibian mortality and deformity
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Normal Eye Eye with a cataract
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Ozone Hole Chemistry• Light Energy
– Photon = discrete packet of light having a specific energy– Energy of light is inversely related to the wavelength
• Infrared = wavelength longer than visible is less energetic
• Ultraviolet = wavelength shorter than visible is more energetic
• Energy: UV-C > UV-B > UV- A > Visible > Infrared
• Photochemical Reaction– Requires light energy to start a chemical reaction– Light must be of exact wavelength/energy required– 500 nm light activated reaction won’t work with 400 nm
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Creation of Ozone• Ozone formation occurs in the upper stratosphere
– UV-C not yet filtered out at this altitude
O2 + UV-C ----> 2 O
O + O2 ----> O3 + heat
• Temperature of stratosphere is warmer than those below– Temperature Inversion (usually cools as you move
upward)
– Little mixing of the O3 layer that forms
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Recycling of Ozone• Ozone is destroyed as it filters UV
O3 + UV-B/UV-A ----> O2 + O
O2 + O ----> O3 (mostly)
O3 + O ----> 2 O2 (some)
– NO and other gaseous pollutants speed up O3 destruction
– O3 “lives” about 30 minutes at 30 km altitude
– Formation/Destruction is called the Chapman Cycle
– Ozone Layer: < 10 ppm O3 << O2 and N2
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The Chapman Cycle
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Chlorine and Ozone• Chlorine initiates ozone destruction
Cl + O3 ----> ClO + O2 (chorine monoxide)
2 ClO ----> ClOOCl (dichloroperoxide)
ClOOCl ----> ----> ----> 2 Cl + O2
Total: 2 O3 ----> 3 O2
– Cl is a catalyst (speeds up reaction); not used up
– Each Cl atom can destroy 50 O3 molecules per day
– Bromine (Br) reacts in the same way• Small amount compared to Cl pollution
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Chlorine Activation• Only Cl, ClO are active ozone destroying Cl forms
• Cl is mostly found in inactive form– ClONO2 = chlorine nitrate ClO + NO2 ----> ClONO2
– HCl = hydrogen chloride Cl + CH4 ----> HCl + CH3
• Antarctica’s weather favors active forms– Dark/cold: PCS = polar stratospheric clouds (ice crystals)– Air pressure drops = vortex = whirling cold air is isolated
– HCl + ClONO2 + PCS ----> Cl2 + HNO3 (dark)
– Cl2 + light ----> 2 Cl (beginning of spring)
– Ozone decreases 2% per day until PCS clouds melt
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Antarctic Seasons and Ozone Levels
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An Arctic Ozone Hole?• Arctic winters are not as cold as Antarctic ones
– Vortex breaks up before light is present to activate Cl
• Temperatures are expected to drop in Arctic– 1996 record cold year– Observable “hole” in Arctic Ozone occurred
• Chlorine pollution is dropping worldwide– Enough in atmosphere for Arctic Hole for 10-20 years
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Ozone Decreases in Non-Polar Areas• Worldwide ozone amounts fell about 3% since 1980
– Besides poles, mid-latitudes experienced greatest loss– Decreases usually occur in March—April in N. Hem.
• The explanation of this phenomenon is not as clear– Do pollution droplets function like polar PCS clouds?
– Volcanoes have given off H2SO4 in these regions
• Dilution of Polar air masses definitely contributes
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Ozone Depleting Chemicals• Sink = natural process to regulate the concentration
of a compound: CO2 has vegetation, oceans as a sink
• Sources of Chlorine and Bromine– CH3Cl (methyl chloride) produced by decaying plants
– CCl4 (carbon tetrachloride) used in dry cleaning
– Manmade CFC’s have no sink in nature• Not water soluble
• Doesn’t react with most other gases
• Not reactive with visible or UV-A light
– Drift to stratosphere by natural buoyancy (60 yr lifetime)• CF2Cl2 + UV-C ----> CF2Cl + Cl
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Chlorine Sources and Levels
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CFC Replacements• The presence of C—H bonds allows decomposition
OH + H—CCl3 ----> H2O + decomposed products
– CH3Cl is partially removed before reaching stratosphere
• HCFC’s = hydrofluorochlorocarbons– CHF2Cl = HCFC-22
– Air conditioners and Refrigerators currently use these
– Only about 5% of the risk to ozone as CFC’s
– Temporary bridges to even better compounds
• HFC’s = hydrofluorocarbons – FCH2CF3 = HFC-134a already being used
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Ozone Hole: the success story• International Agreements have limited damage
– Rowland/Molina predicted problem in 1970’s (Nobel)– CFC’s banned in Europe by late 1970’s– 1987 Montreal Protocol: phase out of ozone depleters
• 1995 all legal CFC production ceased
• 2010 goal for developing countries
• Halon = CF3Br, CF2BrCl illegal by 1994
• HCFC use to end by 2030—2040 (no increase after 2015)
• Future: chlorine pollution will be removed as HCl– Cl peak 1999– No ozone hole after 2050