Download Ozone Layer: Existence and Anthropogenic Depletion

Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
LECTURE 7 - OZONE LAYER: EXISTENCE AND
ANTHROPOGENIC DEPLETION
Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture.
Ozone Layer, slide 1 here
THE OZONE LAYER
The stratosphere has a layer of relatively concentrated ozone at a height of about 16-18
kilometers in the polar regions to about 25 kilometers near the equator. The absorption of ultraviolet
radiation by this layer heats the stratosphere, which creates a steep temperature inversion
(temperature increases with altitude) between altitudes of 15 and 50 kilometers. This inversion of
temperature affects global atmospheric circulation that influences weather and climate.
Ozone Layer, slide 2 here
Another major effect of the ozone layer is the regulation of ultraviolet light levels at the
earth's surface. Ultraviolet light affects all organisms that live on the surface of the land or near
the surface of the sea. Marine phytoplankton, marine plants that live near the surface, may be
one the most important species affected. Marine phytoplankton provide food for nearly all
marine fish, directly or indirectly. Any decrease in the marine phytoplankton will be mirrored by
a decrease in marine fish populations. Such a decrease will have severe consequences for marine
mammals, for human populations that depend on the sea for food, and for other animals, such as
bears, which get part of their food supply from fish returning to freshwater rivers to spawn.
Ozone Layer, slide 3 here
Ultraviolet light occupies the part of the electromagnetic spectrum from 100 to 400
nanometers (nm). According to the American National Standards Institute (2005), the UV part of
the spectrum is divided into UV-A (315 to 400 nm), UV-B (280-315 nm), and UV-C (100-280
nm).
Ozone Layer, slide 4 here
Different wavelengths are absorbed to different degrees. As the figure shows, the most dangerous
type of UV, UV-C, is completely absorbed in the stratosphere. Most, but not all, of the UV-B is
absorbed. UV-A is attenuated, but much reaches the ground.
Ozone Layer, slide 5 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Below 290 nm, absorption of solar radiation is extremely strong. The figure shows the
amount of solar energy in watts falling perpendicularly on a surface one square centimeter, and
the units are watts per cm2 per nm. The solar energy flux is plotted versus wavelength for four
different regions in the atmosphere - the surface, at 20 and 30 kilometers above the surface, and
at the “top” of the atmosphere, above 100 kilometers altitude. Below 290 nm in wavelength, the
intensity of solar radiation is attenuated by a factor of 108 or more. The blue line, labeled “DNA
Action Spectra”, is, “a measure of the relative effectiveness of radiation in generating a certain
biological response over a range of wavelengths. This response might be erythema (sunburn),
changes in plant growth, or changes in molecular DNA.” (Newman, date unknown) As the figure
shows, where DNA damage is likely, the ozone layer in the stratosphere strongly absorbs UV
light. The red line in the figure shows the calculated UV spectrum if the ozone levels were
reduced 10%, which would lead to an increase of about 22% in DNA damage.
Ozone Layer, slide 6 here
Increased ultraviolet light levels modify the rate of photosynthesis. Marine
phytoplankton are susceptible to such changes. (Goudie, 1990) Crop damage on land is also
possible (Stolarski, 1988). Loss of marine phytoplankton could also affect global warming.
Phytoplankton absorb carbon dioxide and convert it to oxygen, in the same manner as terrestrial
plants. More than half of all the carbon dioxide emitted each year is consumed by plants. Of that
amount, more than half is used by phytoplankton. Thus, the phytoplankton are responsible for
removal of at least 25% of the carbon dioxide emitted each year. Any reduction in phytoplankton
will result in increased carbon dioxide levels in the atmosphere (Vogel, 1993).
In 1990 a study of marine phytoplankton near Antarctica by Ray Smith and colleagues
showed that phytoplankton losses were about 6-12% in areas exposed to increased UV. Since
the ozone hole lasts about three to four months, the annual losses are more like 2-4% (Vogel,
1993). There were substantial differences between species. Diatoms, with siliceous shells that
help to protect them, suffered much less than Phaeocystis, a colonial animal growing encased in
jellylike material. This can lead to differences in the entire ecosystem supported by the
phytoplankton.
Ozone Layer, slide 7 here
Another potential effect is a change in the emission of dimethylsulfide (DMS). Some
phytoplankton manufacture this substance as a kind of natural antifreeze. When they die, the
DMS is emitted into the atmosphere. In the atmosphere, DMS acts as a nucleus for cloud
condensation. Clouds help to shield UV, so any reduction in DMS emission could lead to fewer
clouds and a possible increase in UV (Vogel, 1993). However, this is just speculation. No one
knows if the increased UV favors species that make DMS or not. If increased UV favored
species that emit DMS, it could act as a negative feedback, increasing cloud cover and decreasing
UV. Many such feedbacks exist, but work is needed to gather data.
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Ozone Layer, slide 8 here
Foraminiferans, or forams, are single-celled animals who armor their bodies with
calcium carbonate shells. In Hawaii, about 25% of the sand on the beach consists of foram
shells. Forams sometimes harbor within their bodies photosynthetic organisms capable of
providing energy derived from the sun. This is very similar to the process that occurs in corals.
Ozone Layer, slide 9 here
Since 1987, corals around the world have been ”bleaching.” That is, they suddenly lose their
pigment. Now the same phenomenon has been observed in forams. In both cases, the cause is
the same. Bleaching is caused by the loss of the photosynthetic organisms. One Florida
foraminifera, Amphistegina gibbosa, is now at populations as low as 10% of previous levels
(Mirsky, 1994). Remaining animals show developmental defects and malformed shells. High
water temperatures have been linked to coral bleaching. Pamela Hollock, of the University of
South Florida, investigated whether high-water temperatures were correlated with foram damage.
They were not. She found that the damage was strongly correlated with UV light levels at the
earth’s surface. She confirmed in the laboratory that UV light levels could cause the kind of
bleaching observed in the Florida Keys. The effect may have been triggered by the eruption of
Mount Pinatubo, which reduced atmospheric ozone levels and injected dust into the atmosphere.
The reduced ozone increased UV levels at the surface, while the dust reduced visible light the
organisms could sense. Thinking they were not getting enough light may have made them seek
sun, and “fry” themselves (Mirsky, 1994).
Ozone Layer, slide 10 here
Increased levels of UV-B (UV-B; 280-315 nm) radiation has deleterious effects on living
organisms, such as DNA damage (Rozema et al., 1997; Rousseaux, et al., 1999) When exposed
to elevated ultraviolet-B radiation, plants display a wide variety of physiological and
morphological responses characterized as acclimation and adaptation (Jansen et al., 1998)
Ozone Layer, slide 11 here
Exposure to different levels of elevated UV-B radiation induced morphological changes and
markedly affected the somatic recombination frequency in all the plant lines tested (Ries et al.,
2000)
To humans, increased ultraviolet levels will translate into increasing rates of skin cancer,
including the most deadly form, melanoma. Other effects include an increase in the number of
cataracts, and immune deficiencies.
Ozone Layer, slide 12 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Changes in the ozone layer became of concern in the 1970's. Research on the ozone layer
was spurred by perceived anthropogenic threats to the layer. From 1977 to 1984 the ozone levels
over Halley Bay, Antarctica decreased by 40% in the region from 12 to 24 kilometers. The
problem grew progressively worse into the early 1990's, and the area with the worst ozone
depletion is in the Southern Hemisphere. There had been frequent talk of a "hole" in the ozone
layer over parts of the Southern Hemisphere, and this hole has grew increasingly large for many
years. There have been some signs that a hole may develop over the northern hemisphere.
These "holes" are areas of severe ozone depletion, not areas where the ozone layer has
disappeared entirely. Over Antarctica the total ozone depletion exceeds 50%. The cause of the
ozone hole(s) has been the subject of intense research and debate since the 1970's. It has been
suggested that several factors might cause ozone depletion. Some of these are anthropogenic,
while others are natural. There is some indication that natural effects are working with
anthropogenic effects to significantly lower the ozone levels in the stratosphere.
Ozone Layer, slide 13 here
The ER-2, a civilian model of the U-2 spy plane, has been used to study the stratosphere.
(NOAA/CDML Internet Data). The ER-2 flights are part of the Airborne Arctic Stratospheric
Expedition. The ER-2 differs from the U.S. Air Force U-2 in the lack of defensive systems,
absence of classified electronics, completely different electrical wiring to support NASA sensors,
and a different paint scheme. It also is 30% larger, has 20 ft greater wingspan, and supports a
considerably larger payload than the older airframe. To date, NASA U-2 and ER-2 aircraft have
flown more than 4,000 data missions and test flights in support of scientific research. (NOAA
Climate Monitoring and Diagnostics Laboratory, ER-2, date unknown)
Ozone Layer, slide 14 here
Other evidence comes from NASA's Upper Atmosphere Research Satellite (UARS). It
now appears that significant depletion of ozone could occur in the mid-latitudes and, in a
complete surprise to atmospheric scientists, in the tropics.
Ozone Layer, slide 15 here
In 1992, it was suggested that a hole does might develop over the Arctic, due to findings that
suggested conditions favorable for substantial ozone reductions existed over northern Europe,
roughly from London to Moscow (Kerr, 1992). At that time, it was suggested ozone losses might
reach a depletion of 30-40%. Vogel (1993) stated that harmful ultraviolet striking the northern
hemisphere rose 5% in the preceding decade. Over Toronto, Canada, there is evidence of
increasing UV-B radiation. It is the most energetic, and most dangerous, form of ultraviolet
radiation. Kerr and McElroy (1993) report that ozone levels have decreased 4.1% per year
during the winter (December - March) and 1.8% per year during the summer (May to August)
between 1989 and 1993. Ninety-three percent of the observed change occurred in the lower
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
stratosphere. UV-B measurements correlate with this change. At 300 nm (UV-B), the radiation
levels increase 35% per year in the winter and 6.7% in the summer. At 324 nm, the trend was 0.4% per year in the winter and -0.1% per year in the summer. Ozone is strongly implicated
because ozone has almost no absorption at 324 nm, whereas it strongly absorbs at 300 nm.
Ozone Layer, slides 16-17 here
In a 2000 study, it was found that, in one of the Arctic stratosphere's coldest winters on
record, measured ozone losses were as high as 55 percent at about 60,000 feet altitude in the
ozone layer. The findings may be an indication that future cold winters in the Arctic could
prolong the depletion of ozone by manmade chlorine compounds, despite the fact that chlorine is
now diminishing in the atmosphere in response to international agreements. (NOAA Climate
Monitoring and Diagnostics Laboratory, Large Ozone Depletion Reoccurs this Year in the Arctic,
2000) In answer to the question “Is there depletion of the Arctic ozone layer?” in a 2006 report it
was stated:
“Yes, significant depletion of the Arctic ozone layer now occurs in some years in the late
winter/early spring period (January-April). However, the maximum depletion is less
severe than that observed in the Antarctic and is more variable from year to year. A large
and recurrent “ozone hole,” as found in the Antarctic stratosphere, does not occur in the
Arctic.” (NOAA et al., 2006 Update)
Ozone Layer, slide 18 here
The results of Kerr and McElroy appear to demonstrate directly that the loss of ozone is
responsible for the increase in UV-B radiation (Appenzeller, 1993). Changes during the winter
months are large fractional increases in small values. Biologically, they may not be of great
significance. Increases during the spring and summer, although of smaller degree, may be more
significant. This increase is occurring when many species are reproducing and the biological
effects may therefore be larger.
The problem will not get better soon. Levels of stratospheric chlorine were about 3.4 ppb
in 1990, with a predicted rise to 4.1 ppb by the year 2000, even if all new additions of ozone
depleting chemicals to the atmosphere are stopped by the year 2000. Until late 1991 it was
thought that further damage to the ozone layer during the 1990's would be comparable to losses
experienced during the 1980's. More recent work suggests that the atmosphere's ability to
minimize ozone losses is less than was thought. It remains to be seen whether future losses will
take the form of scattered spot depletions of ozone within the nonpolar regions, or whether a
complete shredding of the stratospheric ozone will occur.
In recent years, opinion on the future of the ozone layer has see-sawed back and forth. In
a May 2002 article in Nature, Virginia Gewin examined current thinking about ozone and
suggested the ozone hole might be repaired by 2040.
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
In an excellent commentary in Nature, Dr. Susan Solomon, of the NOAA Aeronomy
Laboratory, has made clear that the ozone hole is a problem that is far from cured, and will take
considerable time to repair. She warns both scientists and journalists against jumping to conclusions
based on yearly data, since so many variable affect the atmosphere.
Ozone Layer, slide 19 here
The figure shows satellite maps of total ozone over Antarctica on September 24, when the ozone hole
is near its annual peak, in 1980, 1981, 2000, 2001, 2002 and 2003. The color scale shows the amount
of ozone in Dobson units, indicating the depth of the hole. The images are based on multiple satellite
records and analyses. (Solomon, 2004)
Ozone Layer, slide 20 here
The slide shows the area of the ozone hole in recent years compared to longer term averages, and an
image of the current year September 25 ozone hole.
Solomon’s warning is well taken. In April and May of 2005, Nature News articles reported
that the Arctic ozone hole suffered the biggest losses ever reported, and that the protective ozone layer
over northern and central Europe was thinner in 2005 than it had been since measurements began 50
years previously. Scientists were arguing whether global warming was to blame. The argument is that
increasing levels of greenhouse gases have a cooling effect on the stratosphere, because heat is locked
near the surface. As we shall see, it takes very cold temperatures in the stratosphere to produce an
ozone hole. Once again, Susan Solomon urged caution because the temperature drop during the 20042005 winter was far too large to be explained by greenhouse cooling. She added that natural
fluctuations in meteorological conditions probably far outweigh greenhouse effects. (Schiermeier,
2005a and 2005b)
Current theories on ozone depletion will be examined. We will first examine anthropogenic
causes, then look at natural contributions and possible synergistic effects of the natural and
anthropogenic causes that together might pose a greater danger than either alone.
ANTHROPOGENIC OZONE DEPLETION
Ozone Layer, slide 21 here
Serious changes in the ozone levels in the stratosphere began to be investigated in the 1970's.
At first, it was thought that fleets of supersonic transport planes (SST's) would introduce water vapor
and nitrous oxides into the stratosphere, degrading the atmosphere. Fleets of these aircraft have not
materialized.
Ozone Layer, slide 22 here
In 1974, another threat was recognized. This threat was the use of chlorofluoro-carbons (CFC's)
(Molina and Rowland, 1974). As previously discussed, from 1977 to 1984 the ozone levels over
Halley Bay, Antarctica decreased by 40% in the region from 12 to 24 kilometers.
Ozone Layer, slide 23 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Molina and Rowland, together with Paul Creuzen, shared the 1995 Nobel Prize for Chemistry for
the timely work and warnings about the ozone hole problem.
Ozone Layer, slide 24 here
In the Antarctic the ozone hole typically begins developing in August,
reaches a maximum in early October, and disappears by early December. The
cause of this ozone depletion has been attributed to several anthropogenic causes.
These include:
Ozone Layer, slide 25 here
1. Combustion products from high-flying military and civilian aircraft,
particularly supersonic aircraft
2. Nitrous oxides released from nitrogenous fertilizers
3. Chlorofluorocarbons (CFC's), first introduced in the late 1920's, are used
as refrigerants, in the manufacture of foam fast-food containers, as
cleansers for electronic parts, and as propellants in aerosol cans
4. Other compounds, such as Halon and methyl bromide, which contain
bromide, and which are capable or releasing substances capable of
destroying ozone.
Of the four, the CFC's might be the most serious problem, and have received the
most attention from scientific researchers and from the press. However, as the
CFC problem has been addressed, the bromide compounds assume larger relative
importance. Therefore, we shall also examine these compounds and their effect on
the ozone layer.
Ozone Layer, slide 26 here
There are many common CFC's. Some are called Freon. Freon is not a
single compound. There are various Freons, usually designated by number (see
Table 7-1). Freons are often used as refrigerant gases. Today, HFC or HCFC
compounds are being substituted for Freon. Halon gases are used in fire
extinguishers. This use has now been largely banned. All CFC compounds have
chlorine and/or fluorine atoms substituted for hydrogen. The Halon compounds
also have bromine, which causes its own problems
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Table 7-1 Common Chlorofluorocarbons
Normal Designation,
Other name
Formula
Chemical name
CFC-11, Freon 11
CFCl3
Trichlorofluoromethane
CFC-12, Freon 12
CF2Cl2
Dichloro-difluoromethane
CFC-13, Freon 13
CF3Cl
Chloro-trifluoromethane
CFC-113
C2F3Cl3
Trichloro-trifluoroethane
CFC-114
C2F4Cl2
Dichloro-tetrafluoroethane
CFC-115
C2F5Cl
Chloro-pentafluoroethane
HCFC-22
CHF2Cl
Chloro-difluoromethane
HCFC-123
CHCl2CF3
Dichloro-trifluoroethane
HCFC-124
CHFClCF3
Chloro-trifluoroethane
HFC-125
CHF2CF3
Pentafluoroethane
HCFC-132b
C2H2F2Cl2
Dichloro-difluoroethane
HFC-134a
CH2FCF3
Tetrafluoroethane
HCFC-141b
CH3CFCl2
Dichlorofluoroethane
HCFC-142b
CH3CF2Cl
Chloro-difluoroethane
HFC-143a
CH3CF3
Trifluoroethane
HFC-152a
CH3CHF2
Difluoroethane
HALON 1211
CF2BrCl
Bromochloro-difluoromethane
HALON 1301
CF3Br
Bromo-trifluoroethane
HALON 2402
C2F4Br2
Dibromo-tetrafluoroethane
After Houghton et al., 1990, Appendix 8
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Ozone Layer, slide 27 here
The bonds between carbon and chlorine, carbon and fluorine, or carbon and
bromine, are stronger than the bonds between carbon and hydrogen (Table 7-2).
Table 7-2
Relative bond strengths
Bond
Bond Strength,
kcal/mole
C-H
80.9
C-Br
95.6
C-F
107
Data from Weast, 1966
Thus, CFC compounds are very stable. They do not break up in the troposphere,
of the earth's atmosphere. They circulate without change in the troposphere and
some eventually rise into the stratosphere.
Sherwood Rowland recounted his work with then graduate student
Mario Molina in elucidating the removal mechanisms of ozone in the
atmosphere. He said in his Nobel Prize lecture, (Rowland, 2007)
Ozone Layer, slide 28 here
“When Mario Molina joined my research group as a postdoctoral
research associate later in 1973, he elected the chlorofluorocarbon
problem among several offered to him, and we began the scientific
search for the ultimate fate of such molecules. At the time, neither of us
had any significant experience in treating chemical problems of the
atmosphere, and each of us was now operating well away from our
previous areas of expertise.
Ozone Layer, slide 29 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
The search for any removal process which might affect CCl3F began
with the reactions which normally affect molecules released to the
atmosphere at the surface of the Earth. Several well-established
tropospheric sinks - chemical or physical removal processes in the
lower atmosphere - exist for most molecules released at ground level:
Ozone Layer, slide 30 here
1. Colored species such as the green molecular chlorine, Cl2,
absorb visible solar radiation, and break apart, or photodissociate,
into individual atoms as the consequence;
2. Highly polar molecules, such as hydrogen chloride, HCl,
dissolve in raindrops to form hydrochloric acid, and are removed
when the drops actually fall; and
Ozone Layer, slide 31 here
3. Almost all compounds containing carbon-hydrogen bonds,
for example CH3Cl, are oxidized in our oxygen-rich atmosphere,
often by hydroxyl radical as in reaction (1).
CH3Cl + HO  H2O + CH2Cl
(1)
Ozone Layer, slide 32 here
However, CCl3F and the other chlorofluorocarbons such as CCl2F2 and
CCl2FCClF2* are transparent to visible solar radiation and those
wavelengths or ultraviolet (UV) radiation which penetrate to the lower
atmosphere, are basically insoluble in water, and do not react with HO,
O2, O3, or other oxidizing agents in the lower atmosphere. When all of
these usual decomposition routes are closed, what happens to such
survivor molecules?”
Ozone Layer, slide 33 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Their stability was at first thought to make them an ideal industrial
compounds because they are unreactive and therefore nontoxic. Carbonchlorine bonds can be broken by ultraviolet radiation, releasing chlorine free
radicals (equation 7-1).
Ozone Layer, slide 34 here
7-1
The chlorine free radical then reacts with ozone:
7-2
Chlorine monoxide can further react,
7-3
or,
7-4
Chlorine atoms thus released by equations 7-3 or 7-4 can again react
according to equation 7-2. The net result is a chain reaction that destroys
thousands of ozone atoms for each chlorine atom initially released. Chlorine
is acting as a catalyst, since it is not consumed in the total reactions. It is the
magnification of the chain reaction process that makes the CFC's so
dangerous. Eventually the chlorine atom may migrate back to the
troposphere. Here the chlorine will react to form hydrochloric acid, which
will be removed by rain (and thus become a minor contributor to acid rain).
When the CFC compounds reach 25 kilometers or more altitude, the
ultraviolet light is strong enough to initiate bond breakage. The ozone levels
above 25 kilometers are small and thus the ultraviolet levels are higher. The
release of chlorine triggers the rapid depletion of ozone because of the chain
reaction.
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Even if all CFC emission were to stop immediately, the problem would
not disappear. Freon 11 persists for about 75 years and Freon 12 for about
100 years in the atmosphere (Stolarski, 1988). Since most of the CFC's are in
the troposphere, and are only slowly migrating into the stratosphere, the
stratospheric CFC levels will increase for years.
Ozone Layer, slide 35 here Freon 11 (Elkins 2016)
Ozone Layer, slide 36 here Freon 12 (Elkins 2016)
Monthly means of the dry mixing ratios in parts per trillion of Freon 11 and
Freon 12 are shown.
Ozone Layer, slide 37 here HCFC
Plots of HCFC data for three compounds. Note the scales. HCFC-22 is about
10 times as much as HCFC-141b or HCFC-142b.
Ozone Layer, slide 38 here Halons and methyl bromide
Plots for two halon compounds are shown. Halon compounds contain
bromine, and are often used as fire suppressants because the vapor is heavy.
Methyl bromide was heavily used as a soil fumigant for killing and
controlling a variety of pests. Because the vapor is heavy, it has low volatility
and is retained by soils for long periods.
Ozone Layer, slide 39 here
There have been attempts to control CFC use. In 1978, the United
States banned the use of CFC's in aerosol products such as hair sprays and
deodorants. Britain, in 1984, announced their data showing 40% ozone
depletion. In September, 1987 23 nations, including the United States,
signed an agreement, known as the Montreal Protocol, to reduce consumption
of chlorofluoro-carbons (United Nations Environment Programme, 2000). It
required the signatory nations to reduce consumption of CFC's to 1986 levels
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by the mid-1990's and to halve the usage by 1999. Methyl bromide produced
and imported in the U.S. was reduced incrementally until it was phased out in
January 1, 2005, pursuant to our obligations under the Montreal Protocol on
Substances that Deplete the Ozone Layer (Protocol) and the Clean Air Act
(CAA). (EPA Ozone Layer Protection, 2014)
Ozone Layer, slide 40 here
Chlorine loading is a measure of the real contribution of a substance to ozone
depletion. A projection of future halocarbon contributions to chlorine loading
is shown in the figure. All ozone depleting substances are shown on a
common scale that takes account of differences in potency. Bromine is
expressed as its equivalent in chlorine. (European Fluorocarbons Technical
Committee, 2006).
Fortunately there are reactions that destroy chlorine in the stratosphere.
An important degradation reaction involves either chlorine atoms or chlorine
monoxide molecules. These react with another molecule to form a "stable"
product that temporarily removes the chlorine into a chlorine reservoir.
Chlorine trapped in the reservoir cannot attack ozone.
Ozone Layer, slide 41 here
Two important reactions are the reaction of chlorine monoxide and nitrogen
dioxide to form chlorine nitrate,
7-5
and the reaction of chlorine and methane to form hydrochloric acid:
7-6
The CH3 radical is reactive and will undergo further reactions. Unfortunately
the molecules in the reservoir will absorb a photon and break apart, releasing
chlorine. Thus these reactions slow the destruction of ozone, but probably
prolong the destruction over many years. It is ironic that the various nitrogen
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oxides are themselves thought to destroy ozone, but here serve to break the
chlorine chain reaction and thus conserve ozone.
Ozone Layer, slide 42 here
Nitrogen gases attack ozone approximately as follows. Nitrous oxide
(N2O) is produced as a by-product of nitrification and denitrification in soils
and oceans. In the troposphere, there are no known degradation reactions for
nitrous oxide, so it migrates upward into the stratosphere. Most of the
nitrous oxide will be destroyed by photodissociation into oxygen atoms and
N2. Approximately 10% escapes this fate and is destroyed by reaction with
activated atomic oxygen. This reaction produces either nitric oxide (NO) or
N2 and O2. It is the nitric oxide that is important because of the ability of NO
to catalytically destroy ozone (Kinzig and Socolow, 1994). Nitric oxide can
also be directly injected into the stratosphere by high-flying supersonic
planes. This was originally thought to be the biggest danger to the ozone
layer.
Ozone Layer, slide 43 here
7-7
7-8
This yields a net reaction of:
7-9
Nitrous oxide additions to the atmosphere can be divided into natural
and anthropogenic components. Natural additions, from temperature and
tropical soils and the world’s oceans are estimated at 10 million tons per year.
Anthropogenic emissions are thought to be of similar magnitude. The most
important source is agricultural use of nitrogen fertilizers. Other sources
include industry (production of fertilizer and nylon), fossil fuel use in power
production, biomass burning, cattle and cattle feed production. (Reay, 2007)
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POLAR STRATOSPHERIC CLOUDS AND
HETEROGENEOUS CHEMICAL REACTIONS
Ozone Layer, slide 44 here
The presence of the inhibitor reactions led the computer models to
predict that CFC's should have had little effect on the ozone layer by the late
1980's. Yet the ozone hole over Antarctica was unmistakably present, so
something was wrong.
Ozone Layer, slide 45 here
Antarctica is different from the temperate parts of the globe in many ways.
One important way is the presence of polar stratospheric clouds, or PSC's.
These clouds form during the Antarctica winter, when the absence of the sun
for long periods lets the stratospheric temperatures dip below -78C (<195
Kelvins).
Ozone Layer, slide 46 here
The figure in the PowerPoint presentation shows the temperatures over the
Neumayer station, at 70 south latitude, during 1997.
It has been known for most of the last century that stratospheric clouds
occasionally formed over both the north and south pole.
Ozone Layer, slide 47 here - Polar Stratospheric Cloud Photos
They often glow with seashell-like iridescence. This has led to the name
nacreous, or mother-of-pearl, clouds. Besides the nacreous clouds (called
Type II), composed of water ice, two other types exist.
Ozone Layer, slide 48 here
The video shows nacreous clouds. Description by Laura Candler, the
photographer: “On January 11, 2010, a beautiful group of nacreous clouds
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appeared over Kiruna, Sweden. Also known as mother-of-pearl clouds,
nacreous clouds sometimes form in the polar stratosphere in wintertime and
glow with a silky iridescence as they undulate across the sky. I created this
time lapse using images shot at 10 second intervals over the course of about 2
hours, from 10:25 a.m. until 12:14 p.m.. The images have not been
manipulated in any way to enhance color, exposure, “
Ozone Layer, slide 49 here
One consists of nitric acid ice (Type I). The third type is similar in chemistry
to the nacreous clouds, but the cloud is larger and shows no iridescence
(Type III). In 1986, Susan Solomon and colleagues and Michael McElroy
and coworkers provided the first hint as to the connection between PSC's and
ozone depletion. They suggested that chemical reactions on the surface of ice
crystals in the clouds release chlorine. This chlorine in turn reacts with
thousands of ozone molecules before again being captured into a reservoir.
Reactions on the surface of particles are called heterogeneous chemical
reactions. This suggestion spurred research on the PSC's.
Ozone Layer, slide 50 here
The Type I cloud, containing nitric acid, is usually hard to observe
from the ground. The Stratospheric Aerosol Measurement (SAM) II,
launched on the NIMBUS satellite in 1978, detected particles in the air by
examining sunlight grazing the earth and passing through the stratosphere.
SAM II indicated the presence of clouds even when the temperature was 195
Kelvins, too warm for water ice to form. It was proposed in 1986 that these
clouds consist of nitric acid trihydrate (NAT =HNO3.3 H2O) (Toon et al.,
1986). At these temperatures, which are above the freezing point of water ice
under ambient conditions, the nitrogen compounds may freeze onto aerosol
particles (probably sulfuric acid aerosol), becoming part of the ice in the
clouds. This would remove the inhibitor molecules, allowing more chlorine
atoms to form. Such clouds are produced by slow cooling, due to radiation
cooling in the long polar nights. Consequently, they are more common in the
Antarctic than the Arctic, which experiences more mixing with the warmer
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
mid-latitude air. The nitric acid clouds are more tenuous than nacreous
clouds, but are often much larger, reaching sizes up to several thousand
kilometers.
Ozone Layer, slide 51 here
The ice particles also serve as catalysts for the destruction of chlorine
reservoir molecules. Laboratory studies showed that a reaction between
hydrochloric acid and chlorine nitrate (HCl and ClONO2) produces
photolytically unstable species such as Cl2, ClNO2, or HOCl on the surface of
ice crystals (Toon and Turco, 1991). Sunlight transforms these species into
the ozone-reactive species Cl or ClO (Cicerone et al. 1991). Reactions often
take place much faster on a solid surface than in a purely gaseous phase.
Without ice, the reactions are negligibly slow. Overall, the lack of sunlight
would be expected to slow most chemical reactions to near zero levels.
However, these processes will release chlorine or chlorine monoxide quickly
when the sun returns. This would lead to a rapid ozone depletion in the
stratosphere above Antarctica at the start of the southern spring, in agreement
with what has been observed.
Ozone Layer, slide 52 here
The nacreous clouds look like lenticular clouds which form in
mountainous regions. However, lenticular clouds form in the troposphere.
In the stratosphere, the relative humidity is about 1%, and the average water
vapor content is a few parts per million. It is difficult to imagine ice clouds
forming under these conditions. The lenticular clouds are known to form as
air rushes up a mountain. Such an air mass undergoes rapid cooling,
increasing the relative humidity. This can create a series of clouds in a
standing wave pattern downwind from the mountain. Under certain
conditions, the standing wave pattern can propagate upward into the
stratosphere. Nacreous clouds are thought to form by condensation of ice on
any aerosols present on the crests of the standing waves. Air is constantly
passing through the nacreous clouds, but the clouds are stationary. At the
leading edge, ice begins to nucleate to form a cloud. The ice particles grow
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
to about 2 microns in size before exhausting their supply of water. As the
crystals move with the air they eventually reach the descending portion of the
airmass, on the lee side. As they descend, they sublimate. The cloud
disappears. Thus the clouds appear stationary, although the air and ice
crystals are constantly moving. Nacreous clouds are important because they
tell us that temperatures are below 190 Kelvin, the temperature at which ice
crystallizes under conditions in the stratosphere. (Toon and Turco, 1991)
The iridescence observed in nacreous clouds is due to their size and the
density of the ice particles in the clouds. As light passes through the cloud it
passes through many crystals, and is refracted in each. Since different
wavelengths of light are refracted at different angles and absorbed
differently, iridescence is produced. The effect is similar to iridescence in
minerals.
Ozone Layer, slide 53 here
Work by Webster et al. (1993) has shown that the reaction,
7-10
occurs in Type I or II PSC’s at temperatures below 196 ± 4K on particle
surfaces in the clouds. This reaction begins with an excess of ClONO2, not
HCl, as was earlier believed. It is this reaction that appears responsible for
partitioning of chlorine between various phases. This work was based on
actual in situ measurements of species in the atmosphere. Toohey et al.
(1993) have shown increased ClO levels in the Arctic polar vortex during the
1991-92 winter when temperatures descended below 195 K. They also
observed rapid decrease in ClO concentrations in February 1992 as
temperatures warmed and ClONO2 formed. Profitt et al. (1993) showed that
there was an altitude dependent change in ozone concentration in the Arctic
polar vortex. There was a substantial decrease toward the bottom of the
vortex, which they attributed to ozone-rich air entering the top of the vortex.
The ozone-rich air reacts with high concentrations of reactive chlorine within
the cloud. The ozone-depleted air released from the bottom of the vortex is
of sufficient quantity to significantly lower the mid-latitude ozone levels.
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Ozone Layer, slide 54 here
Type III polar stratospheric clouds are similar to the nacreous clouds,
except that the particle size is much larger, about ten microns. Temperature
must drop slowly below 190 Kelvin. Water ice begins to form on seed
particles, either nitric acid particles or any remaining sulfuric acid aerosol.
Cooling is slow and the particles can grow to large size. The particle density
is much less than in a nacreous cloud. Since the particle size and density are
different from that of nacreous clouds, these clouds are not iridescent.
Slowly-cooled water ice clouds are important because they remove nitrogen
from the atmosphere. They form on nitric acid substrate and remove nitric
acid vapors from the air. This decreases the nitrogen available for
complexing the chlorine into a reservoir. There is one additional problem that
must be overcome during the southern spring. The sun is low on the horizon,
which reduces the radiation driven breakdown of ozone. This, in turn,
reduces the number of oxygen atoms available for the chlorine catalytic cycle
of ozone destruction (equation 7-4). It has been suggested (Stolarski, 1988)
that the presence of bromine in the stratosphere could help overcome the lack
of oxygen atoms. Bromine comes from several sources. One is naturally
occurring methyl bromide. Anthropogenic sources include fumigants and
certain fire extinguishers.
Ozone Layer, slide 55 here
Bromine can interact with ozone as follows:
7-11
The bromine monoxide can interact with chlorine monoxide to form a
bromine atom plus a chlorine atom.
7-12
The net result is the conversion of ozone to oxygen. Thus, a brominechlorine cycle could operate even lacking free oxygen atoms. Br can also
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
destroy ozone via chain reactions similar to the chlorine catalytic reaction,
but not requiring free oxygen atoms. Methyl bromide is now regulated, and
has been eliminated by law (see Appendix A).
Ozone Layer, slide 56 here
There is some data to support the ideas outlined above. Data from a
major study in 1986 at McMurdo Sound, Antarctica, and other work indicate
that the springtime levels of chlorine monoxide are elevated over Antarctica
relative to mid-latitude stratospheric sites, and that nitrous oxide levels are
severely depressed. Also, both chlorine nitrate and hydrochloric acid levels
are low simultaneously. Work in the Arctic during the winter of 1991-92
showed that the levels of ClO increase in December, when the stratosphere
first gets cold enough for PSC’s to form. ClO concentration reaches a
maximum in January, when temperatures are cold enough that most of the air
inside the polar vortex has been processed inside PSC’s. In February, ClO
concentration begins to decrease because air temperatures are warming.
Without the huge polar icecap, the Arctic temperatures warm much earlier
than the Antarctica temperatures. Thus most of the ClO is gone before
sufficient sunlight is available for catalytic ozone removal to occur. In
Antarctica, the ozone removal reaches a maximum in early to mid-October,
which is equivalent to April in the northern hemisphere (Rodriguez, 1993).
HETEROGENEOUS REACTIONS OUTSIDE POLAR
VORTICES
Ozone Layer, slide 57 here
Heterogeneous reactions have also been observed outside polar
stratospheric clouds. Hydrochloric acid molecules have been expected to be
the major component of inorganic chlorine in the atmosphere. Normally,
hydrochloric acid is expected to be very stable and inactive. Reactions
between HCl and ClNO3 have been shown to occur rapidly on PSC -like
laboratory surfaces. HCl values do indeed decrease in the PSC’s. However,
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
measurements of HCl outside polar vertices also showed that HCl was
sometimes lower than predicted. This means that HCl may not be the
dominant species, and raises the question of what unexpected reactions are
occurring. The results of the second Airborne Arctic Stratospheric
Expedition (AASE II) have provided some possible answers.
Ozone Layer, slide 58 here
After the eruption of Mount Pinatubo in 1991, there was a substantial
increase in global aerosol loading. The spreading volcanic cloud reached a
maximum enhancement in particle surface area of thirty times that observed
during AASE I in 1989. Airborne instruments can sample and analyze
changes in aerosol size and surface area along with monitoring changes in
chemistry (Rodriguez, 1993). It was found that a “footprint” of
heterogeneous reactions on ClO existed despite the absence of any polar
stratospheric clouds in some areas (Wilson et al., 1993). Similar reductions
were found in nitrogen oxides. If temperatures are cold enough, the reactions
on sulfate aerosol can be as efficient as those within PSC’s. This will be
discussed more fully in lecture 8.
Ozone Layer, slide 59 here
A Dobson unit (DU) is the standard method of measuring ozone levels.
The Dobson unit is 0.01mm, or 10 microns. For ozone, the Dobson units
represent the thickness of the layer of ozone above a given point if all of the
ozone could be brought together at a standard pressure and temperature.
ATMOSPHERIC DYNAMICS AND OZONE
Ozone Layer, slide 60 here
Geochemical degradation may not be the only cause of the ozone hole.
The earth's atmosphere is far from static. There is circulation at all levels of
the atmosphere. This circulation helps to keep the temperature of the earth
more uniform than solar heating levels would otherwise suggest. In the
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
stratosphere, ozone is produced most strongly at high altitudes and low
latitudes. Thus, we might expect that the greatest concentrations of ozone
would be found above the equator at the top of the stratosphere. Actually,
the ozone levels peak in the mid-stratosphere and near the poles. This
distribution is caused by stratospheric circulation from high altitudes above
the equator to lower altitudes near the poles. The ozone is carried along with
the air mass. In the northern hemisphere peak ozone levels reach 450
Dobson units in the late winter or early spring, over the North Pole. In the
southern hemisphere circulation is limited to about 60 south latitude. There
is little exchange between the polar vortex and mid-latitude regions. Peak
ozone levels reach 380 Dobson units at 60 south latitude. At the equator,
ozone levels are typically about 260 Dobson units (Stolarski, 1988, p.35)
Ozone Layer, slide 61 here
In the polar vortex above Antarctica the ozone levels once were about
300 Dobson units during the winter and early spring. The amount would
increase to about 400 Dobson units in the late spring when the polar vortex
broke down, allowing exchange with mid-latitude air masses. Now the ozone
levels are steady throughout the winter but fall rapidly in the spring to less
than 200 Dobson units. The early winter ozone minimum is a natural
phenomenon.
Ozone Layer, slide 62 here
It is the depletion below 300 Dobson units that represents the ozone hole. In
1991 the record low value of ozone minima first fell below 100 DU over
Antarctica, and that has had in ten additional years since then. The record
low was September 30, 1994, with the lowest value ever recorded at 73 DU.
It was anticipated that the 1994 values would be much higher than the 1993
values, since most of the sulfur gases from the 1991 Mount Pinatubo eruption
had washed out of the atmosphere. This was proven not to be the case (Kerr,
1994, and NASA Goddard Space Flight Center, 20130.
Ozone Layer, slide 63 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
Thus atmospheric circulation may contribute to changing ozone levels,
but do not appear able to explain the ozone hole observed over Antarctica.
CFC's remain the most likely culprit. A plot of the ozone depletion over the
South Pole station at the maximum ozone hole formation in 1994 and 2015
shows little difference.
Appendix A
The following is taken from the Internet (at
http://aceis.agr.ca/policy/env/methbrom.html), and was written by Linda
Dunn, of the Environment Bureau, Agriculture & Agri-Food Canada.
OPTIONS FOR METHYL BROMIDE
When Methyl Bromide, used worldwide as a fumigant for pest control
in soils, structures, spaces and commodities, was identified as an
ozone-depleting substance, it came under the control of the Montreal
Protocol. Parties agreed in November 1992 to freeze production and
consumption of Methyl Bromide at 1991 levels by 1995, except for
quarantine and pre-shipment applications.
Parties are required to meet this commitment, however they can also
implement more stringent domestic actions. Canada committed to a further
25% reduction by 1998, an action recently matched by the European Union.
The freeze and reduction in Canada have been incorporated into changes to
the Canadian Environmental Protection Act (CEPA) which came into effect
January 1, 1995. The United States will eliminate production and
consumption by the year 2001, under their Clean Air Act.
As agriculture is the primary user of Methyl Bromide in Canada, it falls
on our sector to find ways to reduce use. The Environment Bureau of
Agriculture & Agri-Food Canada (AAFC) has been co-chairing the process
with Environment Canada.
A workshop was held in November 1993 with interested stakeholders.
As a follow-up, an informal industry / government working group was
established to look at research priorities and possibilities for joint projects,
effective ways to implement alternatives and regulations, ways the industry
can reduce emissions, and assisting in the development of Canadian positions
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2016
on future controls.
The industry has responded to this challenge by designing its own
program of tradable permits. Through the creation of a market for these
permits, the industry has taken ownership of this issue and the approach to
addressing it.
Last year's meetings to the Montreal Protocol focused on clearly
defining quarantine and pre-shipment exemptions. Canada's goal, which was
attained at the 1994 meeting of the Parties, was to push for definitions which
closely matched those under CEPA.
When Parties to the Protocol placed Methyl Bromide under interim
controls in 1992, they requested a number of studies to be undertaken,
including one on alternatives. The Plant Industry Directorate of AAFC was
Canada's representative on this committee. All of the studies are now
completed, and in time for the November
1995 meeting of the Parties. Two working group sessions will be held prior
to this meeting, and will make recommendations on further controls
(including phase-out dates).
A national consultation was held in March, 1995, to discuss issues
related to further controls and advise on a Canadian position leading up to the
1995 meetings. AAFC will continue working with our stakeholders and
Environment Canada to develop and refine Canada's position on these issues.
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