1 The Ozone Layer Chapter 15

2 Good Ozone The ozone layer = 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 –N 2, O 2 are still the majority of the gases in ozone layer –O 3 would make a 3.5 mm thick layer at Earth’s surface Ozone is produced at the tropics, flows to poles

3 Layers of the Atmosphere

4 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

5 Seasonal Ozone Fluctuations in the N. Hemisphere

6 Average October Ozone Levels over Antarctica

7 Extent of the Arctic Ozone Hole

8 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

9 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 –O 2, as all molecules do, absorbs light selectively

10 A portion of the electromagnetic spectrum O 2 absorption spectrum in UV

11 Ozone absorbs UV light O 2 absorbs all UV between 120—220 nm –Above the stratosphere UV between 50—120 nm is absorbed by N 2, O 2 O 3 vital because it absorbs 220—320 nm UV –UV-C = 220—280 nm O 3 + O 2 remove all –UV-B = 280—320 nm O 3 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

12 Ozone’s Absorption Spectrum

13 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

14 UV light damage to DNA

15 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

16 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, TiO 2 ; white, inorganic Absorb UV before it reaches your skin = organic molecules –Must not break down as they absorb light ----> useless 4-Methylbenzylidene camphor (4-MBC) Octyl methoxycinnamate (OMC)

17 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

18 Normal Eye Eye with a cataract

19 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

20 Creation of Ozone Ozone formation occurs in the upper stratosphere –UV-C not yet filtered out at this altitude O 2 + UV-C ----> 2 O O + O > O 3 + heat Temperature of stratosphere is warmer than those below –Temperature Inversion (usually cools as you move upward) –Little mixing of the O 3 layer that forms

21 Recycling of Ozone Ozone is destroyed as it filters UV O 3 + UV-B/UV-A ----> O 2 + O O 2 + O ----> O 3 (mostly) O 3 + O ----> 2 O 2 (some) –NO and other gaseous pollutants speed up O 3 destruction –O 3 “lives” about 30 minutes at 30 km altitude –Formation/Destruction is called the Chapman Cycle –Ozone Layer: < 10 ppm O 3 << O 2 and N 2

22 The Chapman Cycle

23 Chlorine and Ozone Chlorine initiates ozone destruction Cl + O > ClO + O 2 (chorine monoxide) 2 ClO ----> ClOOCl (dichloroperoxide) ClOOCl ----> ----> ----> 2 Cl + O 2 Total: 2 O > 3 O 2 –Cl is a catalyst (speeds up reaction); not used up –Each Cl atom can destroy 50 O 3 molecules per day –Bromine (Br) reacts in the same way Small amount compared to Cl pollution

24 Chlorine Activation Only Cl, ClO are active ozone destroying Cl forms Cl is mostly found in inactive form –ClONO 2 = chlorine nitrate ClO + NO > ClONO 2 –HCl = hydrogen chloride Cl + CH > HCl + CH 3 Antarctica’s weather favors active forms –Dark/cold: PCS = polar stratospheric clouds (ice crystals) –Air pressure drops = vortex = whirling cold air is isolated –HCl + ClONO 2 + PCS ----> Cl 2 + HNO 3 (dark) –Cl 2 + light ----> 2 Cl (beginning of spring) –Ozone decreases 2% per day until PCS clouds melt

25 Antarctic Seasons and Ozone Levels

26 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 years

27 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 H 2 SO 4 in these regions Dilution of Polar air masses definitely contributes

28 Ozone Depleting Chemicals Sink = natural process to regulate the concentration of a compound: CO 2 has vegetation, oceans as a sink Sources of Chlorine and Bromine –CH 3 Cl (methyl chloride) produced by decaying plants –CCl 4 (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) CF 2 Cl 2 + UV-C ----> CF 2 Cl + Cl

29 Chlorine Sources and Levels

30 CFC Replacements The presence of C—H bonds allows decomposition OH + H—CCl > H 2 O + decomposed products –CH 3 Cl is partially removed before reaching stratosphere HCFC’s = hydrofluorochlorocarbons –CHF 2 Cl = 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 – FCH 2 CF 3 = HFC-134a already being used

31 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 = CF 3 Br, CF 2 BrCl 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