The Ozone Layer
Chapter 15
1
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
2
Layers of the Atmosphere
3
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
4
Seasonal Ozone Fluctuations in the N. Hemisphere
5
Average October Ozone Levels over Antarctica
6
Extent of the Arctic Ozone Hole
7
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
8
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
9
A portion of the electromagnetic spectrum
O2 absorption
spectrum in UV
10
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
11
Ozone’s Absorption Spectrum
12
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
13
UV light damage to DNA
14
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
15
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
O
O
O
O
4-Methylbenzylidene camphor (4-MBC)
Octyl methoxycinnamate (OMC)
– Must not break down as they absorb light ----> useless
16
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
17
Normal Eye
Eye with a cataract
18
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
19
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
20
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
21
The Chapman Cycle
22
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
23
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
24
Antarctic Seasons and Ozone Levels
25
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
26
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
27
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
28
Chlorine Sources and Levels
29
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
30
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
31