Ozone depletion
From Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Ozone_depletion
Ozone cycle overview
The ozone cycle
Three forms (or allotropes) of oxygen are involved in the ozone-oxygen
cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic
oxygen), and ozone gas (O3 or triatomic oxygen).
Ozone is formed in the stratosphere when oxygen molecules photodissociate
after absorbing an ultraviolet photon whose wavelength is shorter than
240 nm.
This produces two oxygen atoms. The atomic oxygen then combines with
O2 to create O3.
Ozone molecules absorb UV light between 310 and 200 nm, following which
ozone splits into a molecule of O2 and an oxygen atom.
The oxygen atom then joins up with an oxygen molecule to regenerate
ozone.
This is a continuing process which terminates when an oxygen atom "recombines"
with an ozone molecule to make two O2 molecules.
O + O3 > 2 O2
The overall amount of ozone in the stratosphere is determined by a balance
between photochemical production and recombination.
Ozone can be destroyed by a number
of free radical catalysts, the most important of which are the hydroxyl
radical (OH·), the nitric oxide radical (NO·), atomic
chlorine (Cl·) and bromine (Br·).
All of these have both natural and manmade sources; at the present time,
most of the OH· and NO· in the stratosphere is of natural
origin, but human activity has dramatically increased the levels
of chlorine and bromine.
These elements are found in certain stable organic compounds, especially
chlorofluorocarbons (CFCs), which may find their way to the stratosphere
without being destroyed in the troposphere due to their low reactivity.
Once in the stratosphere, the Cl and Br atoms are liberated from the
parent compounds by the action of ultraviolet light.
The Cl and Br atoms can then destroy
ozone molecules through a variety of catalytic cycles.
In the simplest example of such a cycle, a chlorine atom reacts with
an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving
a normal oxygen molecule.
The chlorine monoxide (i.e., the ClO) can react with a second molecule
of ozone (i.e., O3) to yield another chlorine atom and two molecules
of oxygen. The chemical shorthand for these gas-phase reactions is:
Cl + O3 > ClO + O2
ClO + O3 > Cl + 2 O2
The overall effect is a decrease
in the amount of ozone.
More complicated mechanisms have been discovered that lead to ozone
destruction in the lower stratosphere as well.
A single chlorine atom would keep
on destroying ozone (thus a catalyst) for up to two years (the time
scale for transport back down to the troposphere) were it not for reactions
that remove them from this cycle by forming reservoir species such as
hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom
basis, bromine is even more efficient than chlorine at destroying ozone,
but there is much less bromine in the atmosphere at present.
As a result, both chlorine and bromine contribute significantly to the
overall ozone depletion.
Laboratory studies have shown that fluorine and iodine atoms participate
in analogous catalytic cycles.
However, in the Earth's stratosphere, fluorine atoms react rapidly with
water and methane to form strongly bound HF, while organic molecules
which contain iodine react so rapidly in the lower atmosphere that they
do not reach the stratosphere in significant quantities.
Furthermore, a single chlorine atom is able to react with 100,000 ozone
molecules.
This fact plus the amount of chlorine released into the atmosphere by
chlorofluorocarbons (CFCs) yearly demonstrates how dangerous CFCs are
to the environment.
Observations on ozone layer depletion
...
Reductions of up to 70% in
the ozone column observed in the austral (southern hemispheric) spring
over Antarctica and first reported in 1985 (Farman et al. 1985) are
continuing.
Through the 1990s, total column ozone in September and October have
continued to be 40–50% lower than pre-ozone-hole values.
In the Arctic the amount lost is more variable year-to-year than in
the Antarctic.
The greatest declines, up to 30%, are in the winter and spring, when
the stratosphere is colder.
Reactions that take place on
polar stratospheric clouds (PSCs) play an important role in enhancing
ozone depletion.
PSCs form more readily in the extreme cold of Antarctic stratosphere.
This is why ozone holes first formed, and are deeper, over Antarctica.
Early models failed to take PSCs into account and predicted a gradual
global depletion ( MHIP : from Antarctic to the rest of the world ),
which is why the sudden Antarctic ozone hole was such a surprise to
many scientists.
In middle latitudes it is preferable
to speak of ozone depletion rather than holes.
Declines are about 3% below pre-1980 values for 35–60°N
and about 6% for 35–60°S.
In the tropics, there are no significant trends ( MHIP : Myhouseinparadise
is at the maximum distance between the two holes ).
Ozone depletion also explains
much of the observed reduction in stratospheric and upper tropospheric
temperatures.
The source of the warmth of the stratosphere is the absorption of UV
radiation by ozone, hence reduced ozone leads to cooling.
Some stratospheric cooling is also predicted from increases in greenhouse
gases such as CO2; however the ozone-induced cooling appears
to be dominant.
Predictions of ozone levels remain
difficult.
The World Meteorological Organization Global Ozone Research and Monitoring
Project—Report No. 44 comes out strongly in favor for the Montreal
Protocol, but notes that a UNEP 1994 Assessment overestimated ozone
loss for the 1994–1997 period.
Chemicals in the atmosphere
CFCs and related compounds in the atmosphere
Chlorofluorocarbons (CFCs) and other halogenated ozone depleting substances
(ODS) are mainly responsible for man-made chemical ozone depletion.
The total amount of effective halogens (chlorine and bromine) in the
stratosphere can be calculated and are known as the equivalent effective
stratospheric chlorine (EESC).
CFCs were invented by Thomas
Midgley, Jr. in the 1920s.
They were used in air conditioning/cooling units, as aerosol spray propellants
prior to the 1980s, and in the cleaning processes of delicate electronic
equipment.
They also occur as by-products of some chemical processes.
No significant natural sources have ever been identified for these compounds
—
their presence in the atmosphere is due almost entirely to human manufacture.
As mentioned in the ozone cycle overview above, when such ozone-depleting
chemicals reach the stratosphere, they are dissociated by ultraviolet
light to release chlorine atoms.
The chlorine atoms act as a catalyst, and each can break down tens of
thousands of ozone molecules before being removed from the stratosphere.
Given the longevity of CFC molecules, recovery times are measured
in decades.
It is calculated that a CFC molecule takes an average of about five
to seven years to go from the ground level up to the upper atmosphere,
and it can stay there for about a century, destroying up to one hundred
thousand ozone molecules during that time.
Verification of observations
Scientists have been increasingly able to attribute the observed ozone
depletion to the increase of man-made (anthropogenic) halogen compounds
from CFCs by the use of complex chemistry transport models and their
validation against observational data (e.g. SLIMCAT, CLaMS - Chemical
Lagrangian Model of the Stratosphere).
These models work by combining satellite measurements of chemical concentrations
and meteorological fields with chemical reaction rate constants obtained
in lab experiments.
They are able to identify not only the key chemical reactions but also
the transport processes which bring CFC photolysis products into contact
with ozone.
The ozone hole and its causes
Ozone hole in North America during 1984 (abnormally warm reducing ozone
depletion) and 1997 (abnormally cold resulting in increased seasonal
depletion).
Source: NASA
The Antarctic ozone hole is an area of the Antarctic stratosphere
in which the recent ozone levels have dropped to as low as 33% of their
pre-1975 values.
The ozone hole occurs during the Antarctic spring, from September to
early December, as strong westerly winds start to circulate around the
continent and create an atmospheric container.
Within this polar vortex, over 50% of the lower stratospheric ozone
is destroyed during the Antarctic spring.
As explained above, the primary
cause of ozone depletion is the presence of chlorine-containing source
gases (primarily CFCs and related halocarbons).
In the presence of UV light, these gases dissociate, releasing chlorine
atoms, which then go on to catalyze ozone destruction.
The Cl-catalyzed ozone depletion can take place in the gas phase, but
it is dramatically enhanced in the presence of polar stratospheric
clouds (PSCs).
These polar stratospheric clouds(PSC)
form during winter, in the extreme cold.
Polar winters are dark, consisting of 3 months without solar radiation
(sunlight).
The lack of sunlight contributes to a decrease in temperature and the
polar vortex traps and chills air.
Temperatures hover around or below -80 °C.
These low temperatures form cloud particles.
There are three types of PSC clouds; nitric acid trihydrate clouds,
slowly cooling water-ice clouds, and rapid cooling water-ice(nacerous)
clouds; that provide surfaces for chemical reactions that lead to ozone
destruction.
...
The role of sunlight in ozone
depletion is the reason why the Antarctic ozone depletion is greatest
during spring.
During winter, even though PSCs are at their most abundant, there is
no light over the pole to drive the chemical reactions.
During the spring, however, the sun comes out, providing energy to drive
photochemical reactions, and melt the polar stratospheric clouds, releasing
the trapped compounds.
Warming temperatures near the end of spring break up the vortex around
mid-December.
As warm, ozone-rich air flows in from lower latitudes, the PSCs are
destroyed, the ozone depletion process shuts down, and the ozone hole
closes.
Most of the ozone that is destroyed
is in the lower stratosphere, in contrast to the much smaller ozone
depletion through homogeneous gas phase reactions, which occurs primarily
in the upper stratosphere.
Interest in ozone layer depletion
While the effect of the Antarctic ozone hole in decreasing the global
ozone is relatively small, estimated at about 4% per decade, the hole
has generated a great deal of interest because:
The decrease in the ozone layer
was predicted in the early 1980s to be roughly 7% over a 60 year period.
The sudden recognition in 1985 that there was a substantial "hole"
was widely reported in the press.
The especially rapid ozone depletion in Antarctica had previously
been dismissed as a measurement error.
Many were worried that ozone holes might start to appear over other
areas of the globe but to date the only other large-scale depletion
is a smaller ozone "dimple" observed during the Arctic spring
over the North Pole.
Ozone at middle latitudes has declined, but by a much smaller extent
(about 4–5% decrease).
If the conditions became more severe (cooler stratospheric temperatures,
more stratospheric clouds, more active chlorine), then global ozone
may decrease at a much greater pace.
Standard global warming theory predicts that the stratosphere will cool.
When the Antarctic ozone hole breaks up, the ozone-depleted air drifts
out into nearby areas.
Decreases in the ozone level of up to 10% have been reported in New
Zealand in the month following the break-up of the Antarctic ozone hole.
Consequences of ozone layer depletion
Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone
layer depletion is expected to increase surface UVB levels, which could
lead to damage, including increases in skin cancer.
This was the reason for the Montreal Protocol.
Although decreases in stratospheric ozone are well-tied to CFCs and
there are good theoretical reasons to believe that decreases in ozone
will lead to increases in surface UVB, there is no direct observational
evidence linking ozone depletion to higher incidence of skin cancer
in human beings.
This is partly because UVA, which has also been implicated in some forms
of skin cancer, is not absorbed by ozone, and it is nearly impossible
to control statistics for lifestyle changes in the populace.
Increased UV
Ozone, while a minority constituent in the Earth's atmosphere, is
responsible for most of the absorption of UVB radiation.
The amount of UVB radiation that penetrates through the ozone layer
decreases exponentially with the slant-path thickness/density of the
layer. Correspondingly, a decrease in atmospheric ozone is expected
to give rise to significantly increased levels of UVB near the surface.
Increases in surface UVB due to
the ozone hole can be partially inferred by radiative transfer model
calculations, but cannot be calculated from direct measurements because
of the lack of reliable historical (pre-ozone-hole) surface UV data,
although more recent surface UV observation measurement programmes exist
(e.g. at Lauder, New Zealand).
Because it is this same UV radiation
that creates ozone in the ozone layer from O2 (regular oxygen) in the
first place, a reduction in stratospheric ozone would actually tend
to increase photochemical production of ozone at lower levels (in the
troposphere), although the overall observed trends in total column ozone
still show a decrease, largely because ozone produced lower down has
a naturally shorter photochemical lifetime, so it is destroyed before
the concentrations could reach a level which would compensate for the
ozone reduction higher up.
Biological effects
The main public concern regarding the ozone hole has been the effects
of increased surface UV and microwave radiation on human health.
So far, ozone depletion in most locations has been typically a few percent
and, as noted above, no direct evidence of health damage is available
in most latitudes.
Were the high levels of depletion seen in the ozone hole ever to be
common across the globe, the effects could be substantially more dramatic.
As the ozone hole over Antarctica has in some instances grown so large
as to reach southern parts of Australia, New Zealand, Chile, Argentina,
and South Africa, environmentalists have been concerned that the increase
in surface UV could be significant.
Effects on humans
UVB (the higher energy UV radiation absorbed by ozone) is generally
accepted to be a contributory factor to skin cancer.
In addition, increased surface UV leads to increased tropospheric ozone,
which is a health risk to humans.
1. Basal and Squamous Cell Carcinomas
— The most common forms of skin cancer in humans, basal and squamous
cell carcinomas, have been strongly linked to UVB exposure.
The mechanism by which UVB induces these cancers is well understood—absorption
of UVB radiation causes the pyrimidine bases in the DNA molecule to
form dimers, resulting in transcription errors when the DNA replicates.
These cancers are relatively mild and rarely fatal, although the
treatment of squamous cell carcinoma sometimes requires extensive reconstructive
surgery.
By combining epidemiological data with results of animal studies, scientists
have estimated that a one percent decrease in stratospheric ozone would
increase the incidence of these cancers by 2%.
2. Malignant Melanoma — Another
form of skin cancer, malignant melanoma, is much less common but far
more dangerous, being lethal in about 15–20% of the cases diagnosed.
The relationship between malignant melanoma and ultraviolet exposure
is not yet well understood, but it appears that both UVB and UVA are
involved.
Experiments on fish suggest that 90 to 95% of malignant melanomas may
be due to UVA and visible radiation whereas experiments on opossums
suggest a larger role for UVB.
Because of this uncertainty, it is difficult to estimate the impact
of ozone depletion on melanoma incidence.
One study showed that a 10% increase in UVB radiation was associated
with a 19% increase in melanomas for men and 16% for women.
A study of people in Punta Arenas, at the southern tip of Chile, showed
a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer
over a period of seven years, along with decreased ozone and increased
UVB levels.
3. Cortical Cataracts — Studies
are suggestive of an association between ocular cortical cataracts and
UV-B exposure, using crude approximations of exposure and various cataract
assessment techniques.
...
4. Increased Tropospheric Ozone
— Increased surface UV leads to increased tropospheric ozone.
Ground-level ozone is generally recognized to be a health risk, as ozone
is toxic due to its strong oxidant properties. At this time, ozone at
ground level is produced mainly by the action of UV radiation on combustion
gases from vehicle exhausts.
Effects on non-human animals
A November 2010 report by scientists at the Institute of Zoology in
London found that whales off the coast of California have shown a sharp
rise in sun damage, and these scientists "fear that the thinning
ozone layer is to blame"
The study photographed and took
skin biopsies from over 150 whales in the Gulf of California and found
"widespread evidence of epidermal damage commonly associated with
acute and severe sunburn," having cells which form when the DNA
is damaged by UV radiation.
The findings suggest "rising UV levels as a result of ozone depletion
are to blame for the observed skin damage, in the same way that human
skin cancer rates have been on the increase in recent decades."
Effects on crops
An increase of UV radiation would be expected to affect crops. A number
of economically important species of plants, such as rice, depend on
cyanobacteria residing on their roots for the retention of nitrogen.
Cyanobacteria are sensitive to UV light and they would be affected by
its increase.
Public policy
NASA projections of stratospheric ozone concentrations if chlorofluorocarbons
had not been banned.
The full extent of the damage that CFCs have caused to the ozone
layer is not known and will not be known for decades; however, marked
decreases in column ozone have already been observed (as explained before).
After a 1976 report by the United
States National Academy of Sciences concluded that credible scientific
evidence supported the ozone depletion hypothesis a few countries, including
the United States, Canada, Sweden, Denmark, and Norway, moved to eliminate
the use of CFCs in aerosol spray cans.
...
Meanwhile, the halocarbon industry shifted its position and started
supporting a protocol to limit CFC production.
The reasons for this were in part explained by "Dr. Mostafa Tolba,
former head of the UN Environment Programme, who was quoted in the 30
June 1990 edition of The New Scientist, '...the chemical industry supported
the Montreal Protocol in 1987 because it set up a worldwide schedule
for phasing out CFCs, which [were] no longer protected by patents.
This provided companies with an equal opportunity to market new, more
profitable compounds.'"
...
It should be noted that for all substances controlled under the Protocol,
phaseout schedules were delayed for less developed ('Article 5(1)')
countries, and phaseout in these countries was supported by transfers
of expertise, technology, and money from non-Article 5(1) Parties to
the Protocol. Additionally, exemptions from the agreed schedules could
be applied for under the Essential Use Exemption (EUE) process for substances
other than methyl bromide and under the Critical Use Exemption (CUE)
process for methyl bromide.
See Gareau and DeCanio and Norman for more detail on the exemption processes.
To some extent, CFCs have been
replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although
concerns remain regarding HCFCs also.
In some applications, hydro-fluoro-carbons (HFCs) have been used to
replace CFCs.
HFCs, which contain no chlorine or bromine, do not contribute at all
to ozone depletion although they are potent greenhouse gases.
The best known of these compounds is probably HFC-134a (R-134a), which
in the United States has largely replaced CFC-12 (R-12) in automobile
air conditioners.
In laboratory analytics (a former "essential" use) the ozone
depleting substances can be replaced with various other solvents.
Ozone Diplomacy, by Richard Benedick
(Harvard University Press, 1991) gives a detailed account of the negotiation
process that led to the Montreal Protocol.
Pielke and Betsill provide an extensive review of early U.S. government
responses to the emerging science of ozone depletion by CFCs.
More recently, policy experts have
advocated for efforts to link ozone protection efforts to climate protection
efforts.
Many ODS are also greenhouse gasses, some significantly more powerful
agents of radiative forcing than carbon dioxide over the short and medium
term.
Policy decisions in one arena affect the costs and effectiveness of
environmental improvements in the other.
Prospects of ozone depletion
Ozone-depleting gas trends.
Since the adoption and strengthening of the Montreal Protocol has led
to reductions in the emissions of CFCs, atmospheric concentrations of
the most significant compounds have been declining.
These substances are being gradually removed from the atmosphere—since
peaking in 1994, the Effective Equivalent Chlorine (EECl) level in the
atmosphere had dropped about 10% by 2008.
It is estimated that by 2015, the Antarctic ozone hole will have reduced
by 1 million km² out of 25 (Newman et al., 2004); complete recovery
of the Antarctic ozone layer is not expected to occur until the year
2050 or later.
Work has suggested that a detectable (and statistically significant)
recovery will not occur until around 2024, with ozone levels recovering
to 1980 levels by around 2068.
The decrease in ozone-depleting chemicals has also been significantly
affected by a decrease in bromine-containing chemicals. The data suggest
that substantial natural sources exist for atmospheric methyl bromide
(CH3Br).
The phase-out of CFCs means that nitrous oxide (N2O), which is not
covered by the Montreal Protocol, has become the most highly emitted
ozone depleting substance and is expected to remain so throughout the
21st century.
When the 2004 ozone hole ended in
November 2004, daily minimum stratospheric temperatures in the Antarctic
lower stratosphere increased to levels that are too warm for the formation
of polar stratospheric clouds (PSCs) about 2 to 3 weeks earlier than
in most recent years.
The Arctic winter of 2005 was extremely
cold in the stratosphere; PSCs were abundant over many high-latitude
areas until dissipated by a big warming event, which started in the
upper stratosphere during February and spread throughout the Arctic
stratosphere in March.
The size of the Arctic area of anomalously low total ozone in 2004-2005
was larger than in any year since 1997.
The predominance of anomalously low total ozone values in the Arctic
region in the winter of 2004-2005 is attributed to the very low stratospheric
temperatures and meteorological conditions favorable for ozone destruction
along with the continued presence of ozone destroying chemicals in the
stratosphere.
A 2005 IPCC summary of ozone issues
concluded that observations and model calculations suggest that the
global average amount of ozone depletion has now approximately stabilized.
Although considerable variability in ozone is expected from year to
year, including in polar regions where depletion is largest, the ozone
layer is expected to begin to recover in coming decades due to declining
ozone-depleting substance concentrations, assuming full compliance with
the Montreal Protocol.
...
The area where total column
ozone is less than 220 DU (the accepted definition of the boundary of
the ozone hole) was relatively small until around 20 August 2006. Since
then the ozone hole area increased rapidly, peaking at 29 million km²
24 September. In October 2006, NASA reported that the year's ozone hole
set a new area record with a daily average of 26 million km² between
7 September and 13 October 2006; total ozone thicknesses fell as low
as 85 DU on 8 October. The two factors combined, 2006 sees the worst
level of depletion in recorded ozone history.
The depletion is attributed to the temperatures above the Antarctic
reaching the lowest recording since comprehensive records began in 1979.
On October 2008 the Ecuadorian Space
Agency published a report called HIPERION, a study of the last 28 years
data from 10 satellites and dozens of ground instruments around the
world among them their own, and found that the UV radiation reaching
equatorial latitudes was far greater than expected, climbing in some
very populated cities up to 24 UVI, the WHO UV Index standard considers
11 as an extreme index and a great risk to health.
The report concluded that the ozone depletion around mid latitudes
on the planet is already endangering large populations in this areas.
Later, the CONIDA, the Peruvian Space Agency, made its own study, which
found almost the same facts as the Ecuadorian study.
The Antarctic ozone hole is expected
to continue for decades.
Ozone concentrations in the lower stratosphere over Antarctica will
increase by 5%–10% by 2020 and return to pre-1980 levels by about
2060–2075, 10–25 years later than predicted in earlier assessments.
This is because of revised estimates of atmospheric concentrations of
Ozone Depleting Substances — and a larger predicted future usage
in developing countries.
Another factor which may aggravate ozone depletion is the draw-down
of nitrogen oxides from above the stratosphere due to changing wind
patterns.
History of the research
The basic physical and chemical processes that lead to the formation
of an ozone layer in the Earth's stratosphere were discovered by Sydney
Chapman in 1930.
These are discussed in the article Ozone-oxygen cycle — briefly,
short-wavelength UV radiation splits an oxygen (O2) molecule into two
oxygen (O) atoms, which then combine with other oxygen molecules to
form ozone.
Ozone is removed when an oxygen atom and an ozone molecule "recombine"
to form two oxygen molecules, i.e. O + O3 ? 2O2. In the 1950s, David
Bates and Marcel Nicolet presented evidence that various free radicals,
in particular hydroxyl (OH) and nitric oxide (NO), could catalyze this
recombination reaction, reducing the overall amount of ozone.
These free radicals were known to be present in the stratosphere, and
so were regarded as part of the natural balance –
it was estimated that in their absence, the ozone layer would be about
twice as thick as it currently is.
In 1970 Prof. Paul Crutzen pointed
out that emissions of nitrous oxide (N2O), a stable, long-lived gas
produced by soil bacteria, from the Earth's surface could affect the
amount of nitric oxide (NO) in the stratosphere.
Crutzen showed that nitrous oxide lives long enough to reach the stratosphere,
where it is converted into NO. Crutzen then noted that increasing use
of fertilizers might have led to an increase in nitrous oxide emissions
over the natural background, which would in turn result in an increase
in the amount of NO in the stratosphere. Thus human activity could have
an impact on the stratospheric ozone layer. In the following year, Crutzen
and (independently) Harold Johnston suggested that NO emissions from
supersonic aircraft, which fly in the lower stratosphere, could also
deplete the ozone layer.
The Rowland-Molina hypothesis
In 1974 Frank Sherwood Rowland, Chemistry Professor at the University
of California at Irvine, and his postdoctoral associate Mario J. Molina
suggested that long-lived organic halogen compounds, such as CFCs, might
behave in a similar fashion as Crutzen had proposed for nitrous oxide.
James Lovelock (most popularly known as the creator of the Gaia hypothesis)
had recently discovered, during a cruise in the South Atlantic in
1971, that almost all of the CFC compounds manufactured since their
invention in 1930 were still present in the atmosphere.
Molina and Rowland concluded that, like N2O, the CFCs would reach the
stratosphere where they would be dissociated by UV light, releasing
Cl atoms. (A year earlier, Richard Stolarski and Ralph Cicerone at the
University of Michigan had shown that Cl is even more efficient than
NO at catalyzing the destruction of ozone.
Similar conclusions were reached by Michael McElroy and Steven Wofsy
at Harvard University. Neither group, however, had realized that CFCs
were a potentially large source of stratospheric chlorine — instead,
they had been investigating the possible effects of HCl emissions from
the Space Shuttle, which are very much smaller.)
The Rowland-Molina hypothesis was
strongly disputed by representatives of the aerosol and halocarbon industries.
The Chair of the Board of DuPont was quoted as saying that ozone depletion
theory is "a science fiction tale...a load of rubbish...utter nonsense".
Robert Abplanalp, the President of Precision Valve Corporation (and
inventor of the first practical aerosol spray can valve), wrote to the
Chancellor of UC Irvine to complain about Rowland's public statements
(Roan, p 56.)
Nevertheless, within three years most of the basic assumptions made
by Rowland and Molina were confirmed by laboratory measurements and
by direct observation in the stratosphere. The concentrations of the
source gases (CFCs and related compounds) and the chlorine reservoir
species (HCl and ClONO2) were measured throughout the stratosphere,
and demonstrated that CFCs were indeed the major source of stratospheric
chlorine, and that nearly all of the CFCs emitted would eventually reach
the stratosphere. Even more convincing was the measurement, by James
G. Anderson and collaborators, of chlorine monoxide (ClO) in the stratosphere.
ClO is produced by the reaction of Cl with ozone — its observation
thus demonstrated that Cl radicals not only were present in the stratosphere
but also were actually involved in destroying ozone. McElroy and Wofsy
extended the work of Rowland and Molina by showing that bromine atoms
were even more effective catalysts for ozone loss than chlorine atoms
and argued that the brominated organic compounds known as halons, widely
used in fire extinguishers, were a potentially large source of stratospheric
bromine. In 1976 the United States National Academy of Sciences released
a report which concluded that the ozone depletion hypothesis was strongly
supported by the scientific evidence.
Scientists calculated that if CFC production continued to increase
at the going rate of 10% per year until 1990 and then remain steady,
CFCs would cause a global ozone loss of 5 to 7% by 1995, and a 30 to
50% loss by 2050.
In response the United States, Canada and Norway banned the use of CFCs
in aerosol spray cans in 1978. However, subsequent research, summarized
by the National Academy in reports issued between 1979 and 1984, appeared
to show that the earlier estimates of global ozone loss had been too
large.
Crutzen, Molina, and Rowland were
awarded the 1995 Nobel Prize in Chemistry for their work on stratospheric
ozone.
The ozone hole.
...
Arctic ozone hole
On March 15, 2011, a record ozone layer loss was observed, with about
half of the ozone present over the Arctic having been destroyed. The
change was attributed to increasingly cold winters in the Arctic stratosphere
at an altitude of approximately 20 km (12.4 miles), a change associated
with global warming in a relationship that is still under investigation.
By March 25, the ozone loss had become the largest compared to that
observed in all previous winters with the possibility that it would
become an ozone hole. This would require that the quantities of
ozone to fall below 200 Dobson units, from the 250 recorded over central
Siberia.
It is predicted that the thinning layer would affect parts of Scandinavia
and Eastern Europe on March 30–31.
Tibet ozone hole
As winters that are colder are more affected, at times there is an ozone
hole over Tibet.
In 2006, a 2.5 million square kilometer ozone hole was detected over
Tibet.
Also again in 2011 an ozone hole appeared over mountainous regions of
Tibet, Xinjiang, Qinghai and the Hindu Kush, along with an unprecedented
hole over the Arctic, though the Tibet one is far less intense than
the ones over the arctic or antarctic.
Ozone depletion and global warming
There are five areas of linkage between ozone depletion and global warming:
Radiative forcing from various greenhouse gases and other sources.The
same CO2 radiative forcing that produces global warming is expected
to cool the stratosphere.
This cooling, in turn, is expected to produce a relative increase in
ozone (O3) depletion in polar area and the frequency of ozone holes.
Conversely, ozone depletion represents a radiative forcing of the climate
system.
There are two opposing effects:
Reduced ozone causes the stratosphere to absorb less solar radiation,
thus cooling the stratosphere while warming the troposphere;
the resulting colder stratosphere emits less long-wave radiation downward,
thus cooling the troposphere.
Overall, the cooling dominates; the IPCC concludes that "observed
stratospheric O3 losses over the past two decades have caused a negative
forcing of the surface-troposphere system" of about -0.15 ±
0.10 watts per square meter (W/m²).
One of the strongest predictions of the greenhouse
effect is that the stratosphere will cool.
Although this cooling has been observed, it is not trivial to separate
the effects of changes in the concentration of greenhouse gases and
ozone depletion since both will lead to cooling.
However, this can be done by numerical stratospheric modeling.
Results from the National Oceanic and Atmospheric Administration's Geophysical
Fluid Dynamics Laboratory show that above 20 km (12.4 miles), the
greenhouse gases dominate the cooling.
As noted under 'Public Policy', ozone depleting chemicals are also
often greenhouse gases.
The increases in concentrations of these chemicals have produced 0.34
± 0.03 W/m² of radiative forcing, corresponding to about
14% of the total radiative forcing from increases in the concentrations
of well-mixed greenhouse gases.
The long term modeling of the process, its measurement, study, design
of theories and testing take decades to document, gain wide acceptance,
and ultimately become the dominant paradigm.
Several theories about the destruction of ozone were hypothesized in
the 1980s, published in the late 1990s, and are currently being proven.
Dr Drew Schindell, and Dr Paul Newman, NASA Goddard, proposed a theory
in the late 1990s, using a SGI Origin 2000 supercomputer, that modeled
ozone destruction, accounted for 78% of the ozone destroyed.
Further refinement of that model accounted for 89% of the ozone destroyed,
but pushed back the estimated recovery of the ozone hole from 75 years
to 150 years. (An important part of that model is the lack of stratospheric
flight due to depletion of fossil fuels.)
Misconceptions about ozone depletion
CFCs are "too heavy" to reach the stratosphere
It is commonly believed that CFC molecules are heavier than air (nitrogen
or oxygen), so that the CFC molecules cannot reach the stratosphere
in significant amount.
But atmospheric gases are not sorted by weight; the forces of wind can
fully mix the gases in the atmosphere.
Despite the fact that CFCs are heavier than air and with a long lifetime,
they are evenly distributed throughout the turbosphere and reach the
upper atmosphere.
Man-made chlorine is insignificant
compared to natural sources
Another misconception is that "it is generally accepted that natural
sources of tropospheric chlorine are four to five times larger than
man-made one". While strictly true, tropospheric chlorine is irrelevant;
it is stratospheric chlorine that affects ozone depletion.
Chlorine from ocean spray is soluble and thus is washed by rainfall
before it reaches the stratosphere.
CFCs, in contrast, are insoluble and long-lived, allowing them to
reach the stratosphere.
In the lower atmosphere, there is much more chlorine from CFCs and related
haloalkanes than there is in HCl from salt spray, and in the stratosphere
halocarbons are dominant .
Only methyl chloride which is one of these halocarbons has a mainly
natural source , and it is responsible for about 20 percent of the chlorine
in the stratosphere; the remaining 80% comes from man made sources.
Very violent volcanic eruptions
can inject HCl into the stratosphere, but researchers have shown that
the contribution is not significant compared to that from CFCs.
A similar erroneous assertion is that soluble halogen compounds from
the volcanic plume of Mount Erebus on Ross Island, Antarctica are a
major contributor to the Antarctic ozone hole.
...
The reason for occurrence of the ozone hole above Antarctica is not
because there are more CFCs concentrated but because the low temperatures
help form polar stratospheric clouds.
In fact, there are findings of significant and localized "ozone
holes" above other parts of the earth ( MHIP :Tibet and other mountaines
regions ).
...
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