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Ozone depletion

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Environment

   Global monthly average total ozone amount
   Enlarge
   Global monthly average total ozone amount

   The term ozone depletion is used to describe two distinct but related
   observations: a slow, steady decline, of about 3% per decade, in the
   total amount of ozone in the earth's stratosphere during the past
   twenty years and a much larger, but seasonal, decrease in stratospheric
   ozone over the earth's polar regions during the same period. (The
   latter phenomenon is commonly referred to as the "ozone hole".) The
   detailed mechanism by which the polar ozone holes form is different
   from that for the mid-latitude thinning, but the proximate cause of
   both trends is believed to be catalytic destruction of ozone by atomic
   chlorine and bromine. The primary source of these halogen atoms in the
   stratosphere is photodissociation of Chlorofluorocarbon ( CFC)
   compounds, commonly called freons, and bromofluorocarbon compounds
   known as Halons, which are transported into the stratosphere after
   being emitted at the surface. Both ozone depletion mechanisms
   strengthened as emissions of CFCs and Halons increased. CFCs, Halons
   and other contributary substances are commonly referred to as "ODS", or
   "Ozone Depleting Substances." Since the ozone layer prevents most
   harmful UVB wavelengths (270–315 nm) of ultraviolet light from passing
   through the Earth's atmosphere, observed and projected decreases in
   ozone have generated worldwide concern leading to adoption of the
   Montreal Protocol banning the production of CFCs and halons as well as
   related ozone depleting chemicals such as carbon tetrachloride and
   trichloroethane (also known as methyl chloroform). It is suspected that
   a variety of biological consequences, including, for example, increases
   skin cancer, damage to plants, and reduction of plankton populations in
   the ocean's photic zone, may result from the increased UV exposure due
   to ozone depletion.

Ozone cycle overview

   Three forms (or allotropes) of oxygen are involved in the ozone-oxygen
   cycle: Oxygen atoms (O or atomic oxygen), oxygen gas (O[2] or diatomic
   oxygen), and ozone gas (O[3] 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 O[2] to
   create O[3]. Ozone molecules absorb UV light between 310 and 200 nm,
   following which ozone splits into a molecule of O[2] 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 2 O[2]
   molecules. 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·) and atomic chlorine (Cl·) and bromine (Br·). All of these
   have both natural and anthropogenic (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 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. Once in the
   stratosphere, the Cl and Br atoms are liberated from the parent
   compounds by the action of ultraviolet light, and can 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. A free oxygen atom then takes away the oxygen from the ClO,
   and the final result is an oxygen molecule and a chlorine atom, which
   then reinitiates the cycle. The chemical shorthand for these reactions
   are:

   Cl + O[3] → ClO + O[2]

   ClO + O → Cl + O[2]

   In sum O[3] + O → O[2] + O[2]

   The overall effect is to increase the rate of recombination, leading to
   an overall decrease in the amount of ozone. For this particular
   mechanism to operate there must be a source of O atoms, which is
   primarily the photodissociation of O[3]; thus this mechanism is only
   important in the upper stratosphere where such atoms are abundant. 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 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
   (ClONO[2]. 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.

Observations

   The most pronounced decrease in ozone has been in the lower
   stratosphere. However, the ozone hole is most usually measured not in
   terms of ozone concentrations at these levels (which are typically of a
   few parts per million) but by reduction in the total column ozone,
   above a point on the Earth's surface, which is normally expressed in
   Dobson units. Marked decreases in column ozone in the Antarctic spring
   and early summer compared to the early 1970s and before have been
   observed using instruments such as the Total Ozone Mapping Spectrometer
   (TOMS) .
   Lowest value of ozone measured by TOMS each year in the ozone hole
   Enlarge
   Lowest value of ozone measured by TOMS each year in the ozone hole

   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 1990's, 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. Declines are greatest in
   winter/spring, when the stratosphere is colder. At these cold times
   ozone losses are greatest, with up to 30% loss.

   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, 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 35–60N and
   about 6% for 35–60S. In the tropics, there are no significant trends.

   Ozone depletion also explains much of the observed reduction in
   stratospheric and upper tropospheric temperatures . This is because the
   reason for the warmth of the stratosphere is 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
   CO[2]; however the ozone-induced cooling appears to be dominant.

   Predictions of ozone remain difficult. The World Meteorological
   Organization Global Ozone Research and Monitoring Project - Report No.
   44 comes out strongly in favour of the Montreal protocol, although it
   notes that projections of ozone loss for the 1994–1997 period made in
   the UNEP 1994 Assessment had been an overestimate.

Chemicals in the atmosphere

CFCs in the atmosphere

   Chlorofluorocarbons ( CFCs) were invented by Thomas Midgeley in the
   1920's. 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 can break
   down many 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 15 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 anthropogenic halogen compounds from CFCs
   by the use of complex chemistry transport models and their validation
   against observational data (e.g. SLIMCAT, CLaMS). These models work by
   combining satellite measurements of chemical concentrations and
   meteorological fields with chemical reaction rate constants obtained in
   lab experiments and 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

   Image of the largest Antarctic ozone hole ever recorded in September
   2006.
   Enlarge
   Image of the largest Antarctic ozone hole ever recorded in September
   2006.

   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 overall 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) FAQ, section 7.

   These polar stratospheric clouds form during winter, in the extreme
   cold. Polar winters are dark, consisting of 3 months without solar
   radiation (sunlight). Not only lack of sunlight contributes to a
   decrease in temperature but also the “polar vortex” traps and chills
   air. Temperatures hover around or below -80 °C. These low temperatures
   form cloud particles and are composed of either nitric acid (Type I
   PSC) or ice (Type II PSC). Both types provide surfaces for chemical
   reactions that lead to ozone destruction.

   The photochemical processes involved are complex but well understood.
   The key observation is that, ordinarily, most of the chlorine in the
   stratosphere resides in stable "reservoir" compounds, primarily
   hydrogen chloride (HCl) and chlorine nitrate (ClONO[2]). During the
   Antarctic winter and spring, however, reactions on the surface of the
   polar stratospheric cloud particles convert these "reservoir" compounds
   into reactive free radicals (Cl and ClO). The clouds can also remove
   NO[2] from the atmosphere by converting it to nitric acid, which
   prevents the newly formed ClO from being converted back into ClONO[2].

   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.

   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.

   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 heals.

Interest in ozone hole

   While the effect of the Antarctic 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 1980's
       to be roughly 7% over a sixty-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
       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 does break 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 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 due to
   the fact that UVA, which has also been implicated in some forms of skin
   cancer, is not absorbed by ozone.

Increased UV due to the ozone hole

   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 O[2] (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 of increased UV

   The main public concern regarding the ozone hole has been the effects
   of surface UV on human health. As the ozone hole over Antarctica has in
   some instances grown so large as to reach southern parts of Australia
   and New Zealand, environmentalists have been concerned that the
   increase in surface UV could be significant.

   UVB (the higher energy UV radiation absorbed by ozone) is generally
   accepted to be a contributory factor to skin cancer. 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%
   .

   Another form of skin cancer, malignant melanoma, is much less common
   but far more dangerous, being lethal in about 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 (Fears et al, Cancer Res. 2002,
   62(14):3992–6). 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 (Abarca, Jaime F. & Casiccia,
   Claudio C. (2002) Skin cancer and ultraviolet-B radiation under the
   Antarctic ozone hole: southern Chile, 1987-2000. Photodermatology,
   Photoimmunology & Photomedicine 18 (6), 294–302 ).

   So far, ozone depletion in most locations has been typically a few
   percent. 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. For example, recent research has analyzed a widespread
   extinction of plankton 2 million years ago that coincided with a nearby
   supernova. Researchers speculate that the extinction was caused by a
   significant weakening of the ozone layer at that time when the
   radiation from the supernova produced nitrogen oxides that catalyzed
   the destruction of ozone (plankton are particularly susceptible to
   effects of UV light, and are vitally important to marine food webs).

   An increase of UV radiation would also 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 very sensitive to UV light and they would be affected
   by its increase.

   Aside from the direct effect of ultraviolet radiation on organisms,
   increased surface UV leads to increased tropospheric ozone as noted
   above. Paradoxically, at 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.

Public policy in response to the ozone hole

   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
   above).

   After a 1976 report by the U.S. National Academy of Sciences concluded
   that credible scientific evidence supported the ozone depletion
   hypothesis, a few countries, including the United States, Canada,
   Sweden, and Norway, moved to eliminate the use of CFC's in aerosol
   spray cans. At the time this was widely regarded as a first step
   towards a more comprehensive regulation policy, but progress in this
   direction slowed in subsequent years, due to a combination of political
   factors (continued resistance from the halocarbon industry and a
   general change in attitude towards environmental regulation during the
   first two years of the Reagan administration) and scientific
   developments (subsequent National Academy assessments which indicated
   that the first estimates of the magnitude of ozone depletion had been
   overly large). The European Community rejected proposals to ban CFCs in
   aerosol sprays while even in the U.S., CFCs continued to be used as
   refrigerants and for cleaning electronic circuit boards. Worldwide CFC
   production fell sharply after the U.S. aerosol ban, but by 1986 had
   returned nearly to its 1976 level. In 1980, Dupont closed down its
   research program into halocarbon alternatives.

   The US Government's attitude began to change again in 1983, when
   William Ruckelshaus replaced Anne M. Burford as Administrator of the US
   Environmental Protection Agency. Under Ruckelshaus and his successor,
   Lee Thomas, the EPA pushed for an international approach to halocarbon
   regulations. In 1985 20 nations, including most of the major CFC
   producers, signed the Vienna Convention which established a framework
   for negotiating international regulations on ozone-depleting
   substances. That same year, the discovery of the antarctic ozone hole
   was announced, causing a revival in public attention to the issue. In
   1987, representatives from 43 nations signed the Montreal Protocol.
   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 June 30, 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.'"

   At Montreal, the participants agreed to freeze production of CFCs at
   1986 levels and to reduce production by 50% by 1999. After a series of
   scientific expeditions to the antarctic produced convincing evidence
   that the ozone hole was indeed caused by chlorine and bromine from
   manmade organohalogens, the Montreal Protocol was strengthened at a
   1990 meeting in London. The participants agreed to phase out CFC's and
   halons entirely (aside from a very small amount marked for certain
   "essential" uses, such as asthma inhalers) by 2000. At a 1992 meeting
   in Copenhagen, the phase out date was moved up to 1996.

   To some extent, CFC's 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. HFC's, 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.

   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 US govt responses to the emerging science of ozone depletion by
   CFCs.

Current events and future prospects of ozone depletion

   Ozone-depleting gas trends
   Enlarge
   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. By 2015, the Antarctic
   ozone hole would have reduced by only 1 million km² out of 25 (Newman
   et al., 2004); complete recovery of the Antarctic ozone layer will not
   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 (Newman et al., 2006).

   There is a slight caveat to this, however. Global warming from CO[2] is
   expected to cool the stratosphere. This, in turn, would lead to a
   relative increase in ozone depletion and the frequency of ozone holes.
   The effect may not be linear; ozone holes form because of polar
   stratospheric clouds; the formation of polar stratospheric clouds has a
   temperature threshold above which they will not form; cooling of the
   Arctic stratosphere might lead to Antarctic-ozone-hole-like conditions.
   But at the moment this is not clear.

   Even though the stratosphere as a whole is cooling, high-latitude areas
   may become increasingly predisposed to springtime stratospheric warming
   events as weather patterns change in response to higher greenhouse gas
   loading. This would cause PSCs to disappear earlier in the season, and
   may explain why Antarctic ozone hole seasons have tended to end
   somewhat earlier since 2000 as compared with the most prolonged ozone
   holes of the 1990s.

   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 (CH[3]Br).

   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 observed 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.

   Temperatures during the Arctic winter of 2006 stayed fairly close to
   the long-term average until late January, with minimum readings
   frequently cold enough to produce PSCs. During the last week of
   January, however, a major warming event sent temperatures well above
   normal — much too warm to support PSCs. By the time temperatures
   dropped back to near normal in March, the seasonal norm was well above
   the PSC threshold. . Preliminary satellite instrument-generated ozone
   maps show seasonal ozone buildup slightly below the long-term means for
   the Northern Hemisphere as a whole, although some high ozone events
   have occurred. . During March 2006, the Arctic stratosphere poleward of
   60 degrees North Latitude was free of anomalously low ozone areas
   except during the three-day period from March 17 to 19 when the total
   ozone cover fell below 300 DU over part of the North Atlantic region
   from Greenland to Scandinavia.

   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 September 24. 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 October 8. 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.

   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 (O[2])
   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
   + O[3] → 2O[2]. In the 1950's, 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
   (N[2]O), 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, a 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 N[2]O, 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 CFC's 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 (CFC's and related compounds) and the chlorine reservoir
   species (HCl and ClONO[2]) 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
   U.S. 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, Sweden and Norway banned the used 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.

The Ozone Hole

   The discovery of the Antarctic "ozone hole" by Farman, Gardiner and
   Shanklin (announced in a paper in Nature in May 1985) came as a shock
   to the scientific community, because the observed decline in polar
   ozone was far larger than anyone had anticipated. Satellite
   measurements showing massive depletion of ozone around the south pole
   were becoming available at the same time. However, these were initially
   rejected as unreasonable by data quality control algorithms (they were
   filtered out as errors since the values were unexpectedly low); the
   ozone hole was detected only in satellite data when the raw data was
   reprocessed following evidence of ozone thinning in in situ
   observations. Susan Solomon, an atmospheric chemist at the National
   Oceanic and Atmospheric Administration (NOAA), proposed that chemical
   reactions on polar stratospheric clouds (PSCs) in the cold Antarctic
   stratosphere caused a massive, though localized and seasonal, increase
   in the amount of chlorine present in active, ozone-destroying forms.
   This hypothesis was decisively confirmed, first by laboratory
   measurements and subsequently by direct measurements, from the ground
   and from high-altitude airplanes, of very high concentrations of
   chlorine monoxide (ClO) in the Antarctic stratosphere. Alternative
   hypotheses, which had attributed the ozone hole to variations in solar
   UV radiation or to changes in atmospheric circulation patterns, were
   also tested and shown to be untenable. Meanwhile, analysis of ozone
   measurements from the worldwide network of ground-based Dobson
   spectrophotometers led an international panel to conclude that the
   ozone layer was in fact being depleted, at all latitudes outside of the
   tropics. These trends were confirmed by satellite measurements. As a
   consequence, the major halocarbon producing nations agreed to phase out
   production of CFCs, halons, and related compounds, a process that was
   completed in 1996. Crutzen, Molina, and Rowland were awarded the 1995
   Nobel Prize in Chemistry for their work on stratospheric ozone.

   Since 1981 the United Nations Environment Programme has sponsored a
   series of reports on scientific assessment of ozone depletion. The most
   recent is from 2002.

Controversy regarding ozone science and policy

   There is no controversy or debate regarding ozone depletion. There is a
   general consensus among most atmospheric physicists and chemists that
   the scientific understanding has now reached a level where
   countermeasures to control CFC emissions are justified, although the
   decision is ultimately one for policy-makers.

   One atmospheric scientist, Fred Singer, questions the significance of
   the role that CFCs play in ozone depletion. Singer does not deny that
   CFC's play some role, but believes that other sources (including
   climatalogical and natural origins) play a larger role.

   Despite this general consensus, the science behind ozone depletion
   remains complex, and some who oppose the enforcement of countermeasures
   point to some of the difficulties experienced in these studies. For
   example, although increased UVB has been shown to constitute a melanoma
   risk (see above), it has been difficult for statistical studies to
   establish a direct link between ozone depletion and increased rates of
   melanoma. Although melanomas did increase significantly during the
   period 1970–1990, it is difficult to separate reliably the effect of
   ozone depletion from the effect of changes in lifestyle factors (e.g.
   increasing rates of foreign travel).

Myths about ozone depletion

   Various untruths and halftruths about ozone depletion are prevalent. A
   few of the most common are addressed briefly here; more detailed
   discussions can be found in the ozone FAQ.

Myth: CFCs are "too heavy" to reach the stratosphere

   A frequent point made is that since CFC molecules are much heavier than
   nitrogen or oxygen, they cannot reach the stratosphere in significant
   quantities. But atmospheric gases are not sorted by weight; the forces
   of wind (turbulence) are strong enough to fully intermix gases in the
   atmosphere. CFCs are heavier than air, but just like argon, krypton and
   other heavy gases with a long lifetime they are uniformly distributed
   throughout the turbosphere and reach the upper atmosphere. See here and
   the FAQ, part I, section 1.3.

Myth: An ozone hole was first observed in 1956

   G.M.B. Dobson (Exploring the Atmosphere, 2nd Edition, Oxford, 1968)
   mentioned that when springtime ozone levels over Halley Bay were first
   measured, he was surprised to find that they were ~320 DU, about 150 DU
   below spring levels, ~450 DU, in the Arctic. These, however, were the
   pre-ozone hole normal climatological values. What Dobson describes is
   essentially the baseline from which the ozone hole is measured: actual
   ozone hole values are in the 150–100 DU range.

   The discrepancy between the Arctic and Antarctic noted by Dobson was
   primarily a matter of timing: during the Arctic spring ozone levels
   rose smoothly, peaking in April, whereas in the Antarctic they stayed
   approximately constant during early spring, rising abruptly in November
   when the polar vortex broke down.

   The behaviour seen in the Antarctic ozone hole is completely different.
   Instead of staying constant, early springtime ozone levels suddenly
   drop from their already low winter values, by as much as 50%, and
   normal values are not reached again until December ( FAQ, Part III,
   section 6).

World Ozone Day

   In 1994, the United Nations General Assembly voted to designate
   September 16 as "World Ozone Day", to commemorate the signing of the
   Montreal Protocol on that date in 1987.

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