Download The Ozone layer

Stratospheric Ozone (O3)
•Formation and Depletion
•History of problem and solution
•Tie to Climate Change
The Ozone layer
•Diffuse layer
containing ozone
(O3) molecules
•Reacts with UV
light from the sun
•Blocks some UV
that causes skin
cancer where layer
is thick
Stratospheric Ozone Formation
In the mid-stratosphere over the tropics ozone
(O3) is created:
1.
2.
3.
Oxygen is split by UV light
Ozone formation
Ozone absorbs UV light
and is decomposed,
protecting us
The Ozone Hole
• “Stratospheric ozone depletion”
• Sources
– Anthropogenic (“human-caused”: compounds
containing Chlorine, such as Chlorofluorocarbons
(CFCs))
– Natural depletion at a rate equal to rate ozone is
created
• Former uses of chlorine-containing compounds:
solvents to clean computer circuit boards,
coolants in air conditioners & refrigerators, fire
extinguishers, aerosol cans, styrofoam
Natural Ozone Depletion
• Ozone photolysis – degradation by sunlight
• Normally in equilibrium with ozone formation
Anthropogenic Ozone Depletion
• Chlorine-containing compounds released into the
atmosphere react with ozone
• Chlorofluorocarbons (CFCs)
ex: freon
Cl + O3
ClO + O2
ClO + O
Cl + O2
net:
O + O3
2O2
ozone destruction
More on the ozone hole
• CFCs (like freon) are stable near Earth’s
surface at lower atmospheric and thus can
make it up to stratosphere
• In the upper atmosphere, CFCs decompose to
Cl (chlorine radicals)
• One Cl atom can break down 100,000 Ozone
(03 ) molecules
• Poles and high altitudes most affected
- O3 cannot form at the poles
Impacts of Ozone Hole
Harm to human health:
• More skin cancers, sunburns and premature aging of the skin.
• More cataracts, blindness and other eye diseases.
Adverse impacts on agriculture, forestry and natural ecosystems:
• Reduced growth, photosynthesis and flowering
Damage to marine life:
• Plankton are threatened by increased UVB radiation.
-Plankton are the first vital step in aquatic food chains.
Animals:
• May cause eye and skin cancers.
• Can harm marine animals in their developmental stage (e.g. young
fish, shrimp larvae and crab larvae)
History of Finding Problem and Fixing
• 1974-Sherwood Rowland and Mario Molina predict CFCs
will break down stratospheric ozone. (Later won Nobel
Prize)
• 1976-UN calls for an international conference
• 1981 UN creates global convention to protect ozone
• 1985-Hole in ozone discovered over Antarctic (7.3 million
square miles)
• 1987- 24 countries sign Montreal Protocol to reduce and
eliminate Chlorine containing compounds
• 1990s-peak of ozone depleting compounds in atmosphere
Benefits of Montreal Protocol
• By 2165, actions to protect and restore the ozone
layer will prevent an estimated 6.3 million U.S.
deaths from skin cancer.
• By 2165, protecting stratospheric ozone layer will
produce an estimated $4.2 trillion in societal
health benefit in the U.S.
• The ozone layer has not grown thinner since 1998
over most of the world and it appears to be
recovering. Antartic ozone is projected to return to
pre-1980 levels by 2060 to 2075.
-EPA (2007)
Is Ozone Hole related to Climate
Change?
• Process of replacing CFCs created more energy
efficient appliances (refrigerators)-decreasing
greenhouse gases (GHGs)
• Climate change may slow Ozone recovery
• Many ozone-depleting substances are also potent
GHGs
– thus the decrease in amount of CFCs has been positive
for climate change