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Ozone
Destruction
NASA
Earth Observatory (Illustration courtesy Barbara Summey, SSAI)
The
stratospheric ozone layer shields life on Earth from the Sun’s harmful
ultraviolet radiation. Chemicals that destroy ozone are formed by industrial and
natural processes. With the exception of volcanic injection and aircraft
exhaust, these chemicals are carried up into the stratosphere by strong
upward-moving air currents in the tropics. Methane (CH4),
chlorofluorocarbons (CFCs), nitrous oxide (N2O) and water are
injected into the stratosphere through towering tropical cumulus clouds. These
compounds are broken down by the ultraviolet radiation in the stratosphere.
Byproducts of the breakdown of these chemicals form “radicals”—such as
nitrogen dioxide (NO2) and chlorine monoxide (ClO)—that play an
active role in ozone destruction. Aerosols and clouds can accelerate ozone loss
through reactions on cloud surfaces. Thus, volcanic clouds and polar
stratospheric clouds can indirectly contribute to ozone loss.
The
animation illustrates how one chlorine atom in the stratosphere can destroy up
to 100,000 ozone molecules.
Credit
University Of Alaska
Ozone
is destroyed by reactions with chlorine, bromine, nitrogen, hydrogen, and oxygen
gases. Reactions with these gases typically occurs through catalytic processes.
A catalytic reaction cycle is a set of chemical reactions which result in the
destruction of many ozone molecules while the molecule that started the reaction
is reformed to continue the process. Because of catalytic reactions, an
individual chlorine atom can on average destroy nearly a thousand ozone
molecules before it is converted into a form harmless to ozone.
Environmental
Protection Agency graphic
Chlorofluorocarbon
(CFC): a compound consisting of chlorine(CI), fluorine, and carbon
How
ozone is destroyed by CFCs
When ultraviolet light waves (UV)
strike CFC* (CFCl3) molecules in the upper atmosphere, a carbon-chlorine
bond breaks, producing a chlorine (Cl) atom. The chlorine atom then
reacts with an ozone (O3) molecule breaking it apart and so destroying
the ozone. This forms an ordinary oxygen molecule(O2) and a chlorine
monoxide (ClO) molecule. Then a free oxygen** atom breaks up the chlorine
monoxide. The chlorine is free to repeat the process of destroying more ozone
molecules. A single CFC molecule can destroy 100,000 ozone molecules.
* CFC - chlorofluorocarbon: it
contains chlorine, fluorine and carbon atoms.
** UV radiation breaks oxygen molecules (O2) into single oxygen atoms.
Chemical equation
CFCl3
+ UV Light ==> CFCl2 + Cl
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2
The
free chlorine atom is then free to attack another ozone molecule
Cl
+ O3 ==> ClO + O2
ClO + O ==> Cl + O2
and
again ...
Cl
+ O3 ==> ClO + O2
ClO + O ==> Cl + O2
and
again... for thousands of times.
Source: http://www.bom.gov.au/lam/Students_Teachers/ozanim/ozoanim.shtml
Ozone
Depletion in the Antarctic Springtime
1)
HCl + ClONO2
→ HNO3 + Cl2
2)
Cl2 + sunlight → Cl
+ Cl
3)
2Cl + O3
→ 2ClO + 2O2
4)
2ClO + 2O → 2Cl
+ 2O2
______________________
NET
= 203 to 302
credit:NOAA
The
very thing that makes Ozone good for filtering UV radiation makes it easily
destroyed: it is very unstable.
Antarctic
Ozone Hole
As
winter arrives, a vortex of winds develops around the pole and isolates the
polar stratosphere. When temperatures drop below -78°C (-109°F), thin clouds
form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the
surfaces of ice crystals in the clouds release active forms of CFCs. Ozone
depletion begins, and the ozone “hole” appears.
Natural
events such as Volcanic Eruptions can strongly influence the amount of Ozone in
the atmosphere.
However,
man-made chemicals such as CFCs or chlorofluorocarbons are now known to have a
very dramatic influence on Ozone levels too. CFCs a were once widely used in
aerosol propellants, refrigerants, foams, and industrial processes.
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