18685116 ozone layer depletion

8/3/2019 18685116 Ozone Layer Depletion

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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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18685116 ozone layer depletion

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