1

The Ozone Layer

Chapter 15

2

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

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

– O2, as all molecules do, absorbs light selectively

10

A portion of the electromagnetic spectrum

O2 absorptionspectrum in UV

11

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

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, TiO2; white, inorganic

• Absorb UV before it reaches your skin = organic molecules

– Must not break down as they absorb light ----> useless

O

O

O

O

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

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

21

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

22

The Chapman Cycle

23

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

24

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

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 10-20 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 H2SO4 in these regions

• Dilution of Polar air masses definitely contributes

28

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

29

Chlorine Sources and Levels

30

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

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 = 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