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1. The normal ozone cycle: Naturally occurring ozone is formed in the
stratosphere. Two reactions are involved in ozone formation. In the
first (1), a molecule of oxygen, O2, absorbs a photon or quantum of
electromagnetic radiation. The specific wavelength of EMR that
reacts with oxygen is in the ultraviolet, UV, portion of the EMR
spectrum - specifically UV-C, that portion of the UV spectrum with
wavelengths between 0.1 and 0.24 µm.
The energy contained in the photon is sufficient to split the
oxygen molecule into two highly reactive oxygen atoms, O*, called
singlet oxygen:
O2 + W -> 2O* (1) In the second reaction (2) singlet oxygen spontaneously combines with another oxygen molecule to create a molecule of ozone, O3: O* + O2 -> O3 (2) Since reaction 1 generates 2 singlet oxygen atoms, reaction 2 generates 2 molecules of ozone for every photon of UV radiation absorbed in reaction 1. These ozone molecules can now absorb a second photon, also from the UV portion of the EMR spectrum, although this time UV-B with a wavelength between 0.2 and 0.32 µm. The energy contained in this photon splits the ozone molecule into its constituent parts, a molecule of oxygen and a singlet oxygen atom: O3 + W -> O* + O2 (3) Equation 3 consumes a molecule of ozone while absorbing a photon of UV, but because the reaction generates a singlet oxygen, there is no net loss of ozone, since the singlet oxygen simply proceeds to generate more ozone via equation 2. 2. The catalytic chlorine cycle: Chlorofluorocarbons, CFC's, are a class of compounds composed of various numbers of carbon atoms, C; chlorine atoms, Cl; and fluorine atoms, Fl. CFC's belong to a larger class of compounds known generically as organochlorines. Carbon tetrachloride, CCl4, a constituent of some dry-cleaning processes is an organochlorine. CCl4 can contribute to loss of ozone even though it is not technically a CFC (because it does not contain any fluorine). Organochlorines are virtually unreactive or inert in the troposphere. Unfortunately, their inertness means that when they eventually reach the stratosphere (which may take as long as 5 years), they are virtually unchanged. Two of the most common CFC's , CFC-11 (CCl3F) and CFC-12 (CCl2F2) are estimated to have half-lives of 75 and 140 years respectively. (A half-life represents the time required to remove 63% of the compound from the atmosphere.) Once in the stratosphere, CFC's are exposed to much higher energy EMR than can penetrate into the troposphere. Some wavelengths of this high energy EMR are absorbed by the CFC molecules, as well as by other stratospheric compounds. The energy contained in these photons is sufficient to break up the organochlorine molecules, releasing the chlorine atoms in a highly reactive form called a chlorine radical, Cl-: CFC + W -> Cl- (4) Like singlet oxygen, the chlorine radical is highly energized. It combines spontaneously with a molecule of ozone and has enough energy to split the ozone molecule, reaction 5. One atom of oxygen from the ozone molecule combines with the chlorine radical to form chlorine monoxide, ClO, leaving the other two atoms as molecular oxygen, O2: Cl- + O3 -> ClO + O2 (5) The highly reactive singlet oxygen then reacts with chlorine monoxide, regenerating the chlorine radical and another molecule of oxygen: O* + ClO -> Cl- + O2 (6) Together equations 5 and 6 are called the chlorine catalytic cycle or CCC. The CCC virtually inactivates the formation and regeneration of ozone, as well as the UV absorbing ability that accompanies the reactions. This happens because Equation 5 consumes a molecule of ozone similarly to equation 3, but without the concomitant adsorption of a photon of UV radiation. Equation 6 consumes a singlet oxygen that otherwise would have generated a molecule of ozone via equation 2. Cl- is not consumed in reactions 5 and 6 but simply keeps regenerating itself. Therefore, Cl- is called a catalyst. Each chlorine radical has the potential to destroy thousands of molecules of ozone and to consume thousands of singlet oxygen atoms as it cycles through the CCC over and over again. Concern over the potential implications of the CCC were sufficiently great that Canada, the U.S. and other industrialized nations banned the use of CFC's as an aerosol propellant in 1978. However, efforts to control other, far more significant, uses of CFC's were not successful at that time. 3. The Reservoir Reactions: Given the amounts of CFC released and the agressive nature of the CCC, a fundamental question arose: Why did we have as much ozone left in the stratosphere as we did? While fortunate, the situation should have been much worse. Work under the auspices of the National Academies of Sciences continued and a second set of chlorine reactions were discovered. In this second set of reactions, chlorine ended up "inactivated" in products that were at least quasi-stable. While chlorine was "tied-up" in these stable compounds, it was not available to attack ozone. These stable compounds came to be known as "reservoirs". One of the most important of these chlorine reservoirs is chlorine nitrate, ClONO2, which forms as a result of a reaction between chlorine monoxide and nitrogen dioxide: ClO + NO2 -> ClONO2 (7) Another important reservoir is hydrochloric acid, HCl, which forms as a result of a reaction between the chlorine radical and methane, CH4: Cl- + CH4 -> HCl (8) The difference between reactions 7 and 8 and any of the other reactions we have seen so far, is that the end products of reaction 7, ClONO2 and the end product of reaction 8, HCl, do not immediately go on to react with other compounds. They are relatively "inert" although not permanently so. Furthermore, HCl can precipitate and represents one way that Cl can be removed from the atmosphere. Eventually these quasi-stable reservoir compounds will be attacked by a photon of EMR or may enter into a reaction with another chemical releasing their chlorine atoms. Nevertheless, they can sequester ClO and Cl- for considerable periods of time thus inactivating the CCC. 4. The Antarctic Ozone Hole: Once scientists had worked out the details of the reservoir reactions, they came to the conclusion that CFC's should have only a minimal effect on global ozone. Therefore the British Antarctic Survey's 1985 announcement that springtime stratospheric ozone at Halley Bay Antarctica had decreased by 40% between 1977 and 1984 came as a rather rude awakening. What was inactivating the reservoir reactions and allowing ozone to be lost?
The whole point about the reservoir reactions is that they
inactivate chlorine as quasi-inert chlorine nitrate, ClONO
ClONO2 + HCl ->
Cl2 + HNO3
(9)
At these temperatures, HNO3 condenses and along with water begins to
form clouds made of HNO3- 3H2O, nitric acid trihydrate or NAT. Because
these clouds form in the polar stratosphere, they are called polar
stratospheric clouds or PSC's. PSC's do not form as a result of uplift,
the way clouds form in the troposphere. Instead they form slowly,
though the long austral winter. The strong polar vortex plays an
important role in isolating Antarctic air masses allowing vast regions
of the Antarctic stratosphere to reach sufficiently low temperatures
for condensation. The situation is not simple. We know there are at
least three different kinds of PSC's that form under different climatic
criteria. Furthermore, it appears that different processes are going
on in the upper and lower stratosphere involving complicated reactions
between Cl2, O*, O3, ClO and the so-called chlorine dimer, ClOOCl, two
molecules of chlorine monoxide stuck together. Scientists also suspect
that other compounds such as sulphur oxides or SOX's are important in
the formation of PSC's and the inactivation of the chlorine reservoir
system.
In spite of the number of unknowns, it appears
that large quantities of ClOOCl build up through the austral winter
and are confined to the Antarctic by the polar vortex. Estimates are
that chlorine levels, likely as the non-reactive chlorine dimer, reach
concentrations 500 times those outside the polar vortex. In the austral
spring, light returns to the region. When the chlorine dimer, ClOOCl,
absorbs EMR, it is converted into chlorine dioxide and the chlorine
radical though a series of steps:
ClOOCl + W -> Cl- + ClO2
(10)
ClO2 -> Cl- +
O2 (11)
At the temperatures present in the Antarctic, these reactions proceed
extremely quickly creating huge concentrations of Cl-, initiating the
CCC and triggering massive destruction of ozone. Eventually as the
austral spring proceeds, the polar vortex breaks down, the PSC's
vaporize and the ozone "hole" dissipates as it mixes with ozone rich
air from lower latitudes. However this mixing dilutes mid-latitude
ozone concentrations and the high concentrations of Cl- and ClO
carried along from the ozone hole destroy mid-latitude ozone.
Public opinion as to the risks associated with ozone depletion shifted
very rapidly with the "discovery" of the Antarctic ozone hole in 1985
(although a careful review of existing data suggests the hole first
appeared in 1976). By 1987 the Montreal Protocol, a political accord,
had been negotiated. The Protocol required CFC production to be cut by
50% by 1998. Public concern continued to grow with revelations of high
levels of mid-latitude stratospheric ClO and the findings that ozone
depletion was not restricted to Antarctica but was also occurring at
mid-latitudes and in the Arctic. In 1990 the London amendment to the
Montreal Protocol required production of CFC's to cease by 2000. The
1992 Copenhagen amendment accelerated the CFC phase-out to 1996.
The time required for CFC's to make their way to the stratosphere (+/- 5
years), let alone be washed out the atmosphere (i.e. their long
residence times) means that stratospheric chlorine levels are just now
reaching their peak concentrations
and indeed this year (2001) for the first time since measurement began,
the Antarctic Ozone hole did not increase in size. If this trend continues,
Antarctic ozone losses should end by about 2050 and natural processes should
start to rebuild the stratospheric ozone layer.
5. The Next Step: A number of questions remain.
1. Is the ozone hole now stable?
While the size of the hole did not increase this year,
one year is not enough time to discern a trend.
Stabilization is what would be expected given model
projections of atmospheric
chlorine concentrations,
but you can check out
NOAA's Stratospheric Ozone Page
to see what's happening now!
2. Is it possible that other processes might start to consume ozone, an Arctic ozone hole for example? We don't know. Periodically, large
depletions in ozone (more than 10-20%) have been reported from the high
northern latitudes. While the processes that lead to this decrease
are similar to those at the Antarctic, the pattern has not
been similar. The Arctic polar vortex is weaker than the Antarctic one.
Stratospheric temperatures at the Arctic are not as low as those
at the Antarctic, hence PSC formation is reduced. However, global warming
. may strengthen the Arctic polar vortex.
We already know that stratospheric temperatures are falling, so it is possible that an
Arctic
ozone hole could develop in the future. It would likely be
smaller and weaker than the Antarctic hole, but could have serious consequences for Canadians.
3. Can the ozone hole be "plugged"? There is no point in trying to
replace ozone. The amounts are far too large to be transported to
the stratosphere. Furthermore, the Antarctic mechanisms are so
efficient that they would easily destroy any added ozone. It is
far better to remove the organochlorines that catalyze ozone
depletion in the first place. (
Links
to a number of sources of additional information.)
4. Is loss of ozone something to be concerned about? As far as
direct impacts on human health is concerned,
the worry about loss of ozone is that more
UV-B radiation will reach the earth's surface. 90% of skin cancers
are attributed to UV exposure. One of the most important
studies in this area was reported in the journal Science by T.B. Kerr
and C.T. McElroy on work done in Toronto which suggested that UV-B levels
increased by 35% over the period 1989-1993 and fluctuated as a function of
stratospheric ozone levels over eastern North America.
Nevertheless, scientists believe that 20 years or more of exposure
are required before cancer is initiated. Ozone levels have
only been reduced for about 20 years.
So while there is reason to be concerned, there is yet no
irrefutable evidence that losses of
ozone will translate into increases in skin cancer
over and above the increases we are already seeing, as a result of
inappropriate exposure to sunlight in the 50's, 60's and 70's.
Furthermore, there are UV-B exposure effects postulated for
plant communities as well as other organisms.
How do we
assess risk in the absence of information?
Another source of concern is that increases in UV-B will lead to
increases in cataracts, an occluding of the lens of the eye. The
lens is a very efficient UV filter and there is a correlation
between exposure to UV and the incidence of cataracts among people
who live at high latitudes where UV levels are higher than they
are at sea level. However the effects of prolonged, repeated
exposure of the human eye to UV are not known.
Other concerns have been raised about the relationship between
UV-B exposure and the immune system.
5. Aren't the Montreal Protocols and subsequent aggrements supposed to
solve the ozone problem? Many ozone depleting substances, ODS, are
being controlled, but the long residence times of ODSs means it will
take time to move significant concentrations of ODS from the
atmosphere and to see improvements.
If agreements subsequent to
Montreal are adhered to, significant improvements in global ozone should
be seen by
about
2050.
6. Where can I get more information? There are several places on
the InterNet where you can obtain extremely current information.
You might want to start with something like Environment Canada's
Website
on the stratospheric ozone layer and see where it takes you.
Or you could try looking up ozone on one of the search engines available
on the ENV200Y Home Page and see what happens!
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