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Ozone depletion
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Venus also has a very thin ozone layer at an altitude of 100 kilometers :by breakdown of CO
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Ozone layer
• UV light strike ordinary oxygen molecules containing two oxygen atoms (O2)
•Splitting them into individual oxygen atoms (atomic oxygen)
• Atomic oxygen then combines with unbroken O2 to create ozone, O3
•Ozone molecule is unstable (although, in the stratosphere, long-lived)
•UV light hits ozone it splits into a molecule of O2 and atomic oxygen
•A continuing process called the ozone-oxygen cycle
•Ozone concentrations are greatest between about 20 and 40 kilometres
•If all of the ozone were compressed to the pressure of the air at sea level, it would
be only 3 mm
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Ozone layer
•Ozone layer contains less than 10 ppm of ozone (O3)
•Earth's atmosphere contains about 0.3 ppm ozone
•Discovered in 1913 by French physicists Charles Fabry and Henri Buisson
• Properties explored by British meteorologist G. M. B. Dobson
• Spectrophotometer (the Dobsonmeter) to measure ozone from the ground
•Between 1928 and 1958, Dobson established a worldwide network of ozone
monitoring stations,
•The "Dobson unit", is named in his honor. (1 Dobson unit = 0.01 mm)
•One Dobson Unit is the number of molecules of ozone that would be required to
create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees
Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the
Earth) 5
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Ozone layer
•Majority of ozone is produced over the tropics and is transported towards the
poles by stratospheric wind patterns.
•In the northern hemisphere these patterns, known as the Brewer–Dobson
circulation, make the ozone layer thickest in the spring and thinnest in the fall
•Ozone layer is thinner near the equator and thicker near the pole
•Concentrations are greater in high northern than in high southern latitudes
•In US, highest in the spring (April and May) and lowest in October
Ozone layer
• Highest amounts of ozone found over the Arctic in spring (March & April), but the
Antarctic has the lowest amounts during the summer (September and October).
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Ozone layer
• Difference: Weaker polar vortex and stronger Brewer–Dobson circulation
Brewer–Dobson circulation in the ozone layer
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97-99% of UV rays are filtered by ozone layer
Extremely short or vacuum UV (10–100 nm) is screened out by nitrogen
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https://www.mrgscience.com/ess-topic-62-stratospheric-ozone.html
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https://dailytrust.com/preservation-of-the-ozone-layer-how-much-do-nigerians-know/
Ozone Depleting substances (ODS)
In 1976, atmospheric research revealed depletion of ozone layer
The ozone layer can be depleted by free radical catalysts, including:
1. Nitric oxide (NO)
2. Nitrous oxide (N2O)
3. Hydroxyl (OH)
4. Atomic chlorine (Cl) and
5. Atomic bromine (Br)
• The concentrations of chlorine and bromine increased markedly
due to release of large quantities of man-made organohalogen
compounds: chlorofluorocarbons (CFCs) & bromofluorocarbons.
• NOx make the largest contribution to stratospheric ozone loss, even
for elevated chlorine levels..??
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https://web.archive.org/web/20080629032506/http://www.eia.doe.gov/oiaf/1605/archive/gg97rpt/chap5.html
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Bromotrifluoromethane, commonly known as Halon 1301 used for gaseous fire suppression
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UV
Low temperature near poles result in separation of Chlorine and break down of Ozone
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Major Air pollutants
Chloroflurocarbons (CFCs)
• Effects:
• One chlorine atom can destroy 100,000 ozone molecule
•Net reaction: Ozone hole 28.4 million km² —September 2000
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Ozone destruction > Ozone formation = Ozone layer thinning
This thinning of ozone layer is named as Ozone hole
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Ozone layer status
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https://ourworldindata.org/ozone-layer
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Ozone Hole
•The ozone hole doesn’t exist year round: it’s seasonal.
•During the Antarctic winter, when polar stratospheric clouds become
widespread, chemical reactions convert less reactive forms of chlorine into large
amounts of highly reactive forms.
•These highly reactive gases have such a weak hold on their chlorine atoms that
they are only stable in the dark.
•When the Sun rises at the end of Southern Hemisphere winter, sunlight degrades
these reactive gases, releasing “free radicals” of chlorine.
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https://www.mrgscience.com/ess-topic-62-stratospheric-ozone.html
Ozone Hole
•Ozone destruction usually peaks in mid-October.
• Ozone loss tapers off in late spring as the polar vortex weakens.
• Temperatures rise, and fewer clouds form.
•Ozone-rich air from lower latitudes mixes back into the polar stratosphere, and
the ozone hole disappears until the next spring
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https://www.mrgscience.com/ess-topic-62-stratospheric-ozone.html
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Control of CFCs
• Banned under Montreal Protocol (16 September 1987): 200 countries ratified
• An international agreement on reduction of emission of ODS
• signatories agreed to freeze production of many CFCs and halons by 2000
• Most countries followed the rules but China and India
• In 1994, UN designated September 16 as the International Day for the Preservation of
the Ozone Layer: day of Montreal Agreement (1987).
• But still emissions and production increased then now decreasing
• India is world’s largest producer and smuggler
• China is leading in emissions
• Emissions:
• 2002 (14000 tons), 2012 (65000 tons), 2019 (52000 tons) decreasing
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https://www.nature.com/articles/s41586-021-03260-5
Reasons for the illegal trade:
1. ODS substitutes are often
costlier than CFCs
2. Updating equipment to enable
use of alternative chemicals is
expensive
3. The lifetime of CFC containing
equipment is long
4. Penalties in may countries are
small.
https://www.researchgate.net/figure/The-illegal-
trade-in-ozone-depleting-substances-in-the-Asia- 30
Pacific_fig9_304215688
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The need for coordination
and collaboration on
information and analysis:
The informal framework of
UN agencies, IGO's and
INTERPOL influenced by
environmental crimes. The
illustration is merely an
illustration of the range of
some of the UN entities,
protocols and conventions
engaged and significantly
affected by environmental
crime and with ability to
influence.
https://www.researchgate.net/figure/The-illegal-
trade-in-ozone-depleting-substances-in-the-Asia- 31
Pacific_fig9_304215688
Control of CFCs
Observation-based global CFC-11 emissions versus expected changes.
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https://www.nature.com/articles/s41586-021-03260-5
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Control of CFCs
Measured atmospheric mole fractions of CFC-11 and global mean rate of change
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https://www.nature.com/articles/s41586-021-03260-5
Control of CFCs
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https://ourworldindata.org/ozone-layer
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Control of CFCs
Effective Chlorine (all substances) in the atmosphere and the scenario with and without
the Montreal Protocol
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https://www.dcceew.gov.au/environment/protection/ozone/montreal-protocol/graphs
Control of CFCs
Effect of Montreal Protocol and its subsequent revisions
Montreal Protocol is often used as an exemplar of successful international cooperation
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https://www.mrgscience.com/ess-topic-62-stratospheric-ozone.html
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Atmospheric composition
Nitrogen 78.084%
Oxygen 20.95%
Argon 0.934%
Carbon Dioxide 0.036%
Neon 0.0018%
Helium 0.0005%
Methane 0.00017%
Hydrogen 0.00005%
Nitrous Oxide 0.00003%
Ozone 0.000004%
Water vapor is variable but typically makes up ~ 1-4% of
the atmosphere.
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Role of CO2 and NOx
• Increases in carbon dioxide (CO2) and nitrous oxide (N2O) will become
increasingly important in determining the future of the ozone layer.
• NOx increase can cause ozone depletion
• CO2 increases cool the stratosphere and affect ozone levels in several ways
• Cooling decreases the rate of many photochemical reactions, thus slowing ozone
loss rates.
•Cooling also increases the chemical destruction of nitrogen oxides, thereby
moderating the effect of increased N2O on ozone depletion.
•The stratospheric ozone level projected for the end of this century therefore
depends on future emissions of both CO2 and N2O as well
DOI 10.1088/1748-9326/10/3/034011 40
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