Atmosphere
CompositionOzone LayerHuman Impact
Our Atmosphere
Is fixed – i.e. closed system for matter, but not energy!
Has many functions– Protection from cosmic rays– Keeping us warm– Oxygen
Its all relative
Is a very thin layer of gas
Is small compared to Earth itself
Composition & Layers
Troposphere
The lowest layer– Home of life– Home of weather– Home of pollution – mainly from human impact
Sources of pollution
Humans – industry, transport and domestic– Is particulate (carbon, dust and metals) and
gaseous (carbon dioxide, smog, unburnt hydrocarbons, etc.
Can be natural – Chief source is volcanic activity
Stratosphere
Home of the ozone layer Where the Concorde once flew Where global warming originates
What is ozone
Allotrope of oxygen Formed naturally Oxygen molecule split
by UV radiation Unstable Oxygen atom
forms Atom joins with
molecule to form ozone
O2 + UV O + O
Spliting of the oxygen molecule by UV light. The oxygen atoms formed are not stable and quickly "attack" other oxygen molecules to form ozone molecules.
O + O2 O3
The oxygen free radical
This is another name for the unstable oxygen atom
contains an unpaired electron in its outermost shell of electrons. – This is an extremely unstable configuration, – radicals quickly react with other molecules or
radicals to achieve the stable configuration of 4 pairs of electrons in their outermost shell
How can Ozone be stable?
Simply put, a special bond forms It is called a co-ordinate covalent bond One of the oxygen molecule atoms has to
share two electrons with the atom!
How the bond forms
O O O
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x x
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The oxygen molecule has a double bond. These 4 electrons are shared so that each atom has the "magic 8"
These two electrons are shared by the molecule with the atom. This also means that both atoms have access to 8 electrons.
Decomposing ozone
Ozone is readily split apart to form oxygen molecules This is a natural process occurring in the atmosphere
O3 + O 2 O2
CFC’s the real villains!
CFC = chloro fluoro carbons CFCs were developed in the early 1930s and are
used in a variety of industrial, commercial, and household applications
Used as coolants for commercial and home refrigeration units, aerosol propellants, electronic cleaning solvents, and blowing agents
Halons are also to blame
Are compounds of bromine, fluorine and carbon
The halons are used as fire extinguishing agents, both in built-in systems and in handheld portable fire extinguishers
CFC and ozone
In 1973 chlorine was found to be a catalytic agent in ozone destruction
It essentially cleaves the ozone to produce oxygen molecules – see decomposing ozone
The chemistry – we think
O3 + Cl O2 + ClO
ClO + O O2 + Cl
As we see, the chlorine atom is used and produced by the reaction. This is similar to recycling chemicals in a reaction - the net result is the reaction is self-perpetuating.
Simply put
1. Human activity puts chlorides and bromides into the atmosphere.
2. Cold conditions are present in the Polar Regions that allow the accumulation of the CFC’s.
3. Sunlight breaks down the CFC’s. 4. Under certain conditions atomic chlorine forms and
this has a major part in the breakdown of ozone.5. The breakdown products destroy ozone.6. The ozone is broken into oxygen molecules.7. No more Ozone protection in the stratosphere.
So what’s being done
The CFC problem may be hard to solve because there are already great quantities of CFCs in the environment.
CFCs would remain in the stratosphere for another 100 years even if none were ever produced again
In the Montreal Protocol, 30 nations worldwide agreed to reduce usage of CFCs and encouraged other countries to do so as well
Montreal Protocol
The governments of the world got together to try to stop the problems that were clearly evident – the major push was the gradual phasing out of CFC’s.
The Montreal Protocol was a convention signed in 1987 by many countries to greatly reduce the production and use of CFCs which had been shown to be responsible for damage to the ozone layer.
Since 1987, further amendments to the protocol have imposed even greater restrictions of the production and use of potentially damaging compounds.
What’s it mean to me?
Small steps are being taken to reduce CFCs including the use of alternate propellants in:
Refrigeration units Air conditioning units Propellants for spray cans Using pump action instead of gas propellants
But surely there are alternatives!
You bet! In place of CFCs and Halons we now use:
Two CFC replacements have been developed for medical uses; they are referred to as hydrofluoroalkanes (HFAs) or as hydrofluorocarbons (HFCs), and contain only carbon, hydrogen and fluorine.
They are non-flammable and chlorine-free, so they have no impact on the stratospheric ozone layer.
Being more specific
Cyclopentane and cyclohexane have been put forth as replacements for coolants
Nitrogen gas can be used as a blowing agent
C C
CC
CH
H H
H H
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C
C C
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CC
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cyclopentane
cyclohexane