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Ozone Layer Depletion
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Introduction Ozone is a bluish gas located in the stratosphere which
protects the earth by absorbing UV-B and prevents thisharmful radiation from reaching the earth.
Research has shown that the ozone is slowly beingdepleted.
We will discuss:
The causes of ozone depletion
The impacts ozone depletion has on the environment
The current status of the ozone
Solutions to the problem
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What is ozone?
Ozone is a stable moleculecomposed of three oxygen atoms.
While stable, it is highly reactive. The Greek word ozeinmeans to smell and O3 has a strong pungent odor.Electric discharges in air often produce significant
quantities of O3 and you may have smelled O3 near thesesources.
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Ozone in the atmosphere
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The ozone layer
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Ultraviolet protection by ozone
Ozone absorbs UV light in the solar irradiation that is harmful
to life
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Ultraviolet protection by ozone
The overlap of ground level radiation with the sunburnsensitivity curve would be much greater without the filtering
effects of the ozone layer.
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Ozone formation and destruction in the stratosphere
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Chapman Theory
a) O2+ hv ( 2Ob) O+O2+M -> O3+M
c) O3 + hv (
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Prediction by Chapman theory vs. Observation
Using Chapman theory
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There must be other O3 destruction pathways
Catalytic ozone destruction
X + O3 = XO + O2
XO + O = X + O2
O + O3 = 2 O2Net reaction
X is a regenerated in the process act as a catalyst.
The chain reaction continues until X is removed by some
side reaction.
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The important catalysts for stratospheric
O3 destruction Hydroxy radical (OH)
.OH + O3 = HO2. + O2
HO2.+ O =
.OH + O2
Net: O + O3 = 2 O2
Chlorine and bromine (Cl and Br)
Cl. + O3 = ClO. + O2
ClO. + O = Cl. + O2
Net: O + O3 = 2 O2
Nitric oxide (NO)NO + O3 = NO2 + O2
NO2 + O = NO + O2Net: O + O3 = 2 O2
HOx cycle
ClOx cycle
NOx cycle
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The two-sided effect of NOx
NOx provides a catalytic chain mechanism forO3 destruction.
NOx inhibit the HOx and ClOx cycles for O3destruction by removing radical species in the
two cycles. The relative magnitude of the two effects is
altitude dependent. >25 km, the net effect is to destruct O3.
(NOx accounts for >50% of total ozone destructionin the middle and upper troposphere.)
In the lower stratosphere, the net effect is toprotect O3 from destruction.
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The catalytic destruction reactions described so far,
together with the Chapman cycle, account for theobserved average levels of stratospheric ozone, they areunable to account for the ozone hole over Antarctica.
The ozone depletion in the Antarctica is limited bothregionally and seasonally. The depletion is too greatand too sudden. These observations can not beexplained by catalytic O3 destruction by ClOx alone.
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Causes of Depletion According to the Environmental Protection Agency,
the discovery of an ozone hole over Antarctica in 1985
focused attention on the idea that humans can have a
significant impact on the global environment. There arealso a number of natural causes of ozone depletion.
When the following substances reach the stratosphere,
they break down under intense ultraviolet light, and releasechlorine or bromine atoms, which degrade the ozone.
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Chlorofluorocarbons (CFCs)
CFCs is the abbreviated form of ChloroFluoroCarbons, a
collective name given to a series of compoundscontaining chlorine, fluorine and carbon atoms.Examples: CFCl3, CF2Cl2, and CF2ClCFCl2.
Related names HCFCs: Hydrochloroflorocarbons, halocarbonscontaining hydrogen atoms in addition to chlorine,
fluorine and carbon atoms. HFCs: hydroflorocarbons, halocarbons containing
atoms of hydrogen in addition to fluorine and carbon
atoms. Perhalocarbons: halocarbons in which every
available carbon bond contains a haloatoms. Halons: bromine-containing halocarbons, especially
used as fire extinguishing agents.
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Chlorine atom
Sources:
Photolysis of Cl-containing compounds in the stratosphere.CFCl3 + hv (185-210nm) CFCl2. + Cl.
CF2Cl2 + hv (185-210nm) CF2Cl. + Cl.
Subsequent reactions of CFCl2 and CF2Cl more Cl atoms
The principal Cl-containing species are:
CF2Cl2, CFCl3, CFCl2, CF2Cl, CCl4, CH3CCl3, CF2HCl, CH3Cl
Sources for Cl-containing compounds (need to be long-lived in the troposphere)
Man-made: e.g. CFCs
Natural: e.g. methyl chloride from biomass burning.
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Chlorine atom (Continued)
Termination reactions for Cl
Cl. + CH4 CH3. + HClStable in the stratosphere
Removed from air by precipitationwhen it migrates to the troposphere
ClO. + NO2 + M ClONO2 + MReservoir species
Relatively unreactive but can regeneratereactive species upon suitable conditions
ClONO2 + hvClO + NO2
ClONO2 + hvCl + NO3
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Nitric oxide
NO is produced abundantly in the troposphere, but all ofit is converted into NO2 HNO3 (removed throughprecipitation)
NO in the stratosphere produced from nitrous oxide
(N2O), which is much less reactive than NO.N2O + hv N2 + O (90%)
N2O + O 2 NO (~10%)
Removal processes:
NO2 + .OH HNO3ClO. + NO2 ClONO2
Inhibit the HOxand ClOx cycles
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Hydroxy radical
Accounts for nearly one-half of the total ozonedestruction in the lower stratosphere (16-20 km).
Sources
O3 + hv (
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How Humans Cause Depletion CFCs (chlorofluorocarbons)
Coolants for refrigerators
Aerosol propellants Cleaning solvents
Electric equipment
Blowing agents to produce plastic foam and insulation
Halon Fire Extinguishing agent (only until 1994)
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Carbon Tetra Chloride
Fire Extinguishers
Aerosol Spray Propellants
Dry Cleaning
Methyl Chloroform Industrial Solvents
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Natural Causes of Ozone Depletion Aerosols emitted from:
Volcanic Eruptions
The Ocean
Cow Farts
Burning Fossil Fuels
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Ozone Depletion Potential (ODP)
2
2
2
2
2
2
2
2
ODP
(averages)
CFC's Halons Carbon Tetra
Chloride
Methyl
Chloroform
Substances
Ozone Depletion Potentials (ODP)
The ratio of the impact on ozone of a chemical comparedto the impact of a similar mass of CFC-11.
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Environmental Impacts Increase in UV-B reaching the earths
surface, which causes harm to : Humans
Animals
Plants and Agriculture
The Ocean and Aquatic Ecosystems
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Impact on Humans and Animals Damaging health effects primarily with skin, eyes,
and immune system
Reduced air quality Human exposure to UV-B depends on
Individuals location
Duration and timing of outdoor activities Precautionary behavior
Skin color and age
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Plants and Agriculture Reduction of air quality reduces crop yields
Decrease in photosynthetic activity
Susceptibility to disease
Changes in plant structure and pigmentation
Retardation of growth
Field Study: Soybean Harvests
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Ocean and Aquatic Ecosystems Diminishes productivity of the oceans
Decreases species such as fish and shrimp Humans and other consumers are dependent
on these higher species
Populations outside the local ecosystem are
potentially at risk
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Status of Ozone Depletion Ban of production and consumption of
compounds that deplete the ozone layer.
Air Quality Improvements
Statistically
New Technology
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Solutions Many substitute products have been made
Increased public knowledge of ozone
depletion
New Technology
Policy and Regulations
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Policy 1987, The Montreal Protocol was signed
Ban of CFC production
More than 160 countries have signed the treaty 1990 Clean Air Act Amendments
Established U.S. regulatory program to protect
the stratospheric ozone layer Individual and Corporate Responsibility
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