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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.
Stratospheric
air temperatures in both polar regions reach minimum values in the lower
stratosphere in the winter season. Average minimum values over Antarctica are as
low as –90°C in July and August in a typical year. Over the Arctic, average
minimum values are near –80°C in January and February. Polar stratospheric
clouds (PSCs) are formed when winter minimum temperatures fall below the
formation temperature (about –78°C). This occurs on average for 1 to 2 months
over the Arctic and 5 to 6 months over Antarctica (see heavy red and blue
lines). Reactions on PSCs cause the highly reactive chlorine gas ClO to be
formed, which increases the destruction of ozone. The range of winter minimum
temperatures found in the Arctic is much greater than in the Antarctic. In some
years, PSC formation temperatures are not reached in the Arctic, and significant
ozone depletion does not occur. In the Antarctic, PSCs are present for many
months, and severe ozone depletion now occurs in each winter season.
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
Ozone
Destruction Cycles

The destruction of
ozone in Cycle 1 involves two separate chemical reactions. The net or overall
reaction is that of atomic oxygen with ozone, forming two oxygen molecules.
The cycle can be considered to begin with either ClO or Cl. When starting with
ClO, the first reaction is ClO with O to form Cl. Cl then reacts with (and
thereby destroys) ozone and reforms ClO. The cycle then begins again with
another reaction of ClO with O. Because Cl or ClO is reformed each time an
ozone molecule is destroyed, chlorine is considered a catalyst for ozone
destruction. Atomic oxygen (O) is formed when ultraviolet sunlight reacts with
ozone and oxygen molecules. Cycle 1 is most important in the stratosphere at
tropical and middle latitudes, where ultraviolet sunlight is most intense.

Significant
destruction of ozone occurs in polar regions because ClO abundances reach
large values. In this case, the cycles initiated by the reaction of ClO with
another ClO (Cycle 2) or the reaction of ClO with BrO (Cycle 3) efficiently
destroy ozone. The net reaction in both cases is two ozone molecules forming
three oxygen molecules. The reaction of ClO with BrO has two pathways to form
the Cl and Br product gases. Ozone destruction Cycles 2 and 3 are catalytic,
as illustrated for Cycle 1, because chlorine and bromine gases react and are
reformed in each cycle. Sunlight is required to complete each cycle and to
help form and maintain ClO abundances.
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.


Global
Total Ozone Change

Satellite
observations show a decrease in global total ozone values over more than two
decades. The graph above compares global ozone values (annual averages) with the
average from the period 1964 to 1980. Seasonal and solar effects have been
removed from the data. On average, global ozone decreased each year between 1980
and the early 1990s. The decrease worsened during the few years when volcanic
aerosol from the Mt. Pinatubo eruption in 1991 remained in the stratosphere. Now
global ozone is about 4% below the 1964- to-1980 average.

The
graph above compares ozone changes between 1980 and 2004 for different
latitudes. The largest decreases have occurred at the highest latitudes in both
hemispheres because of the large winter/spring depletion in polar regions. The
losses in the Southern Hemisphere are greater than those in the Northern
Hemisphere because of the Antarctic ozone hole. Long-term changes in the tropics
are much smaller because reactive halogen gases are less abundant in the
tropical lower stratosphere.
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