Ozone Depletion
Watch the NASA Video (5 minutes)
Position In Syllabus
Global Enviroental Crisis, Biogeochemical Cycles, Climate Change,
Ozone Hole, Overpopulation, Energy Crisis, Water Crisis.
Periodical 1
Ecology, Biodiversity Loss, Deforestation and Land Degradation,
Food Crisis, Pollution (Water, Air, Land, Thermal, Noise), Sewage
Domestic and Industrial and Effluent treatment
Periodical 2
Solid waste (municipal, medical, e-waste, nuclear), Enviroental
Management, Conventions, Regulations and Impact Assessment,
Sustainable Delopment.
Final Exam
Ozone
About 90% of Ozone resides in the stratosphere
It protects us from UV radiations
Increase in the Troposphere ozone can contribute
to raising global temperature
Ozone is continuously created in the stratosphere
by absorption of UV radiation, while it is
continuously being removed by various chemical
reactions that convert it back to molecular oxygen.
Classification of UV Rays
Energy/
range in
Name Abbr. photon
nm
(eV)
UV A, long wave, or black light UVA 400 315 3.103.94
Near NUV 400 300 3.104.13
UV B or medium wave UVB 315 280 3.944.43
Middle MUV 300 200 4.136.20
UV C, short wave, or
UVC 280 100 4.4312.4
germicidal
Far FUV 200 122 6.2010.2
Vacuum VUV 200 100 6.2012.4
Low LUV 100 88 12.414.1
Super SUV 150 10 8.28124
UV Skin Penetration
UVC and much of UVB rays are absorbed by
the Earths ozone layer.
UVA and some UVB rays are transmitted
through the atmosphere,
UVB rays (short wavelength) reach only the
outer layer of your skin (the epidermis)
UVA rays (longer wavelength) and can
penetrate the middle layer of your skin (the
dermis).
Both Cause sunburns.
SunburnSkin damage due to UV
UVB radiation (280 - 320 ) has been linked to
skin cancer, cataracts, damage to materials like
plastics, and harm to certain crops and marine
organisms. Although some UVB reaches the
surface without ozone depletion, its harmful
effects will increase as a result of this problem.
Natural Formation and Removal of Ozone
Ozone Formation:
O2 + h 2 O*................ (< 240 nm)
O* + O 2 O3
Ozone Removal:
O3 + h O* + O2.............. ( 200 nm<<310)
O3 + O * 2 O2
Catalytic Ozone Removal (by Cl, Br, OH*, NO*):
The catalysts can be anthropogenic or natural
Cl + O3 ClO + O2
ClO + O3 Cl + 2 O2.....catalyst regenerated.
Anthropogenic Ozone Depletion
Emission of chlorofluorocarbons and other ozone depleting
substances.
These molecules are inert and non water soluble,
They are not destroyed through atmospheric chemical
reactions in the troposphere or removed with precipitation.
They rise to the stratosphere and are broken down to yield Cl,
Br. e.g.
CFCl3 +h CFCl2 + Cl
This results in a drastically higher concentration of
ozone-destroying catalysts (Cl, Br) in the stratosphere.
Accelerates the catalytic destruction of ozone.
A single chlorine molecule can break down tens of thousands
of ozone molecules before it gets deactivated.
Ozone Depletion
Chemical Application O3 Dep.
Pot.
CFC Stable in troposphere. Rise to the stratosphere, break down 0.6-1
in presence of UV-B and attack O3.
CCl4 Wide industrial uses: CFC manufacture, solvent and 1.2.
catalyst. Potent carcinogen.
Halons Contain Br, F, C. Fire extinguishing agent. Production
Stopped in the U.S. ended on 12/31/93. Br is many times
more effective at destroying ozone than Cl
HCFC CFC-replacements. Lesser depletion potential. 0.01- 0.1.
HFC CFC replacements. Because they do not contain chlorine or 0.
bromine, they do not deplete the ozone layer.
MeBr CH3Br Methyl Bromide. Pesticide for soil and many 0.6.
agricultural products. Production stopped in the U.S. on
12/31/2000.
CH3CCl3 Methyl Chloroform. Industrial solvent. 0.11.
HBFC Hydrobromofluorocarbon Class I substances.
Chlorofluorocarbons
CFCs can last in stratosphere > 100 yr.
Production ban since December 31, 1995.
Only recycled and stockpiled CFCs can now be
used on a limited basis.
CFCs are also a greenhouse gas
Class I substances have an ozone-depletion
potential of 0.2 or higher. These include CFCs,
halons, carbon tetrachloride, methyl chloroform,
HBFC and methyl bromide.
Hydrochlorofluorocarbons
HCFCs and hydroflurocarbons (HFC) are
safer.
Less stable in the atmosphere
Less likely to reach the stratosphere to affect
the ozone layer.
HFCs even lack Cl and hence are safer.
But HCFCs an HFCs are highly potent
greenhouse gasses.
Measurement of Ozone: Dobson Unit
Measurement of stratospheric ozone columnar density:
Dobson unit (DU)
Gordon Dobson, University of Oxford researcher.
1920s, he built the Dobson ozone spectrophotometer.
1 DU = a layer of ozone that would be 10 m thick
under standard temperature and pressure. 2.691016
ozone molecules/ sq. cm
300 DU of ozone brought down to the surface of the
Earth (atmospheric pressure) at 0C would occupy a
layer only 3 mm thick.
Ozone hole: Layer < 220 DU
Global warming and Ozone
Depletion impacts of CFCs
Fully halogenated CFCs have long
atmospheric lifetimes,
They contain relatively large amounts of
chlorine
They absorb strongly within the 7 to 13
micron atmospheric window
Therefore, they have considerable potential
for both global warming and ozone depletion.
They are 15000 times potent than carbon
India & CFC's
According to UNEP, in 2008, India produced almost all the
CFC in the world, and the amount it officially exported was
far lower than the amount reported by other countries to be
imported from India.
However, CFC's are still the most smuggled commodity,
next to
India drugs.
and South Korea are the two major manufacturers
that still produce CFC's, but are sold on the black
market.
In 2008, UNEP estimated these two countries accounted
for over 70 per cent of global CFC production, which has
come down from a million tonnes a year to 50,000.
All developed nations have already banned
CFC production, and India officially banned
CFC's in late 2008, after signing the Montreal
Protocol.
Ozone Hole Largest observed ozone hole,
September 21-30 Sept, 2006
(10.6 mi. sq miles). This image,
from Sept. 24, the Antarctic
ozone hole was equal to the
record single-day largest area
of 11.4 million square miles,
reached on Sept. 9, 2000. The
blue and purple colors are
where there is the least ozone,
and the greens, yellows, and
reds are where there is more
ozone. Credit: NASA
Antarctic Ozone Hole
In the 3-month long dark polar winter, the Polar
Vortex develops.
It is more pronounced in the southern hemisphere
due to fewer land masses.
It traps and chills the polar stratosphere to <80 C.
Polar Stratospheric Clouds (PSCs) form at these
temperatures.
Chlorine accumulates on these ice crystals in the
form of reservoir compounds like ClONO2, HOCl
Cl + O3 ClO + O2
ClO + NO2 ClONO2
Antarctic Ozone Hole
In the spring, solar radiation triggers the release of
massive quantities of Cl from the reservoir
compounds.
The released Cl causes a rapid catalytic
degradation of ozone in a short span.
This leads to the formation of the ozone hole.
By mid-December (in the Antarctic), the polar
vortex breaks up, PSCs disappear, and ozone from
lower latitudes comes in and the ozone hole shuts
down.
Two types of depletion:
Homogeneous and heterogeneous: Ozone
hole
http://www.epa.gov/ozone/science/hole/whya
nt.html
Why does the polar vortex form only above
Antarctica and not the Arctic?
http://science.nasa.gov/headlines/y2000/ast
02oct_1.htm
Never doubt that a small group of
thoughtful, committed citizens
can change the world indeed
its the only thing that er has
- Margaret Meed
Extra Slides
How ozone is destroyed by
CFCs
Ozone layer as protective shield
Atomic oxygen is produced by the
photolytic decomposition of molecular
oxygen
O2 + h O + O
Atomic oxygen, in turn, reacts rapidly
with molecular oxygen to form ozone
Ozone layer as protective shield
O + O2 + M O3 + M
where M represents a third atom (Nitrogen
or oxygen)
Ozone removal by photo dissociation
O3 + h O2 + O
H2O + ClONO2 HOCl + HNO3
The sun light frees Cl from HOCl by photolysis
HOCl + h Cl + OH
Cl + O3 ClO + O2
OH + O3 HO2 + O2
Impacts of other greenhouse gases
Methane helps to remove ozone destroying
chlorine
Cl + CH4 HCl + CH3
CH3 + O2 CH3O2
CH3O2 + NO CH3O + NO2
NO2 + h NO + O
M + O + O2 O3 + M
2. Developing International agreements that
control production and emission of damaging
trace gases
(a) Reduction of CFC
(b) Reduction of CO2 emission
3. Supporting efforts to halt deforestation and
encourage reforestation
(a) Financial assistance for developing
countries from developed countries
(b) Environmental tax
(c) Urban reforestation
