How Earth got its ozone layer
The primordial Earth had an atmosphere quite different from today’s atmosphere.
Significant amounts of carbon dioxide were present as well as nitrogen, but there was little
oxygen. Earth receives radiated energy from the sun. The beginnings of life eventually led
to plants that could absorb energy from the sun and make the carbon dioxide into
molecular oxygen. At present and for many millions of years, Earth’s atmosphere has
been about 18% oxygen. All that oxygen was manufactured by living plants. It is mixed
evenly throughout the atmosphere.
Some of the solar radiation that reaches Earth is high energy ultraviolet radiation (this light
is of wavelengths less than the highest energy visible light, which has wavelength ~400
nm). (See Ch. 21 for a discussion of the meaning of wavelength.) In two specific
wavelength ranges (or energy ranges, the two are interchangeable because E = hc/λ, where
λ is the wavelength), there are reactions that produce single oxygen atoms, some of which
combine with normal molecular oxygen, O2, to make ozone, O3. Some single oxygen
atoms combine with ozone to make two normal oxygen atoms.
All of these transformations are encapsulated in what is known as the Chapman cycle
(after S. Chapman, who first described them in 1930).(4) The Chapman cycle is
O2 + solar energy of wavelength less than 242 nm → 2O,
O + O 2 → O3,
O + O 3 → 2O2,
O3 + solar energy of wavelength less than 336 nm → O* + O 2.
Here, O* is an excited state of oxygen. It can become de-excited through thermal collisions
and become a single oxygen atom. It can be seen that light with wavelengths from 336 nm
Energy, Ch. 1, extension 1 Earth’s ozone layer
on down will be absorbed. Only the lowest-energy ultraviolet radiation will reach the
surface.
How ozone is destroyed by nitrogen oxides
In the 1970s, it was found by Crutzen that the dissociation of ozone also proceeds
through reactions with oxides of nitrogen.(5) The Crutzen-identified nitrogen oxide cycle
involves formation of NO2 from NO:
NO + O3 → NO 2 + O 2,
NO 2 + O → NO + O2,
NO 2 + solar energy → NO + O.
Clearly, the net effect of this cycle is to eliminate ozone: 2O3 becomes 3O2. Note that
this cycle will continue until (somehow) the NO is removed. NO acts as a catalyst to
destroy ozone.
After a few kilometers penetration, most of the ultraviolet radiation has been absorbed by
the oxygen present and ozone has been formed or destroyed. The thickness of the upper
atmosphere containing the ozone produced is called the ozone layer. The key observation
is that the ultraviolet radiation that makes and destroys ozone is prevented from reaching
Earth’s surface. This is how the ozone layer protects the living things on the planet’s
surface from the effects of ultraviolet radiation.
How ozone is destroyed by chlorine oxides
Once chlorine (or other halogen) is released from its carrier (CFC or halon) by ultraviolet
radiation in the stratosphere, the major reactions are
Cl + O3 → ClO + O 2,
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Energy, Ch. 1, extension 1 Earth’s ozone layer
ClO + O → Cl + O2.
The net result of this reaction is O3 + O → 2O2. As with the NO cycle, chlorine will
continue its destruction until (somehow) it is removed from the stratosphere. It, too, is a
catalyst for ozone destruction.
Sources of methyl halides
A few years ago, it was thought that all chlorine and bromine came directly from
biological processes in the ocean. It had long been known that oceanic plankton (tiny
plants) played a starring role in stratospheric processes through release of dimethyl
sulfide.(6) Sulfur compounds work to cool the atmosphere. They also provide cloud
condensation nuclei. Similar origins were believed to produce natural bromine and
chlorine, but recently it was found that these sources can account for only about a tenth
of the total annual flux of around 3.5 billion kg.(7,8)
Several investigations have identified both biotic and nonliving (abiotic) sources of the
natural chlorine and bromine. Only one of these biotic sources is related to human activity
(rice cultivation). The chemical bond between bromine and chlorine is not quite as strong
as the chlorine-carbon bond, so bromine compounds are easier for sunlight to tear apart if
they reach the stratosphere (halons such as methyl bromide can be cleaned in the lower
atmosphere by the hydroxyl radical (a negatively-charged ion made up of oxygen and
hydrogen, written OH-), but in spite of this enough remains to reach the stratosphere).
Both ozone and the hydroxyl radical are part of the natural processes cleaning the
atmosphere. The hydroxyl radical removes an estimated 3.65 trillion kilograms of
pollutants (including those diminishing the ozone concentration in the stratosphere)
annually.(9)
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Energy, Ch. 1, extension 1 Earth’s ozone layer
The halons (book Table 1.2, page 9) contain bromine atoms that form bromine oxide,
BrO, and BrCl, which have characteristics similar to chlorine oxide. Bromine is more
effective per atom in destroying ozone.(4) The World Meterological Organization
estimates that the methyl bromide emissions total 97 to 298 kilotonnes (million
kilograms) per year, with the best estimate around 170 kilotonnes per year.(10) A bit less
than half this amount, around 75 kilotonnes, comes from human generation.
Humans release large amounts of such compounds as a result of the chemical industry, at
the present mostly from pesticides (for example, termite control), soil fumigants, (11)
CFCs, and chemicals from fire extinguishers (see book Table 1.2). The CFC atmospheric
concentrations are beginning to decline.(12) The others are expected to follow as the
additions to the Montreal Protocol come into effect.
Coastal salt marshes were found to produce chlorine and bromine from all vegetation
zones, about twenty times as much chlorine as bromine. (13) Another group, (7) operating
independently, found significant amounts of methyl chloride, CH3Cl, coming from warm
coastal land (tropical islands) and common tropical plants (nearly a teragram—a billion
kilograms—per year). Yokouchi et al. also note that measured atmospheric concentrations
are larger in the tropics than farther north or south. Atmospheric mixing is believed to be
responsible for transport to the temperate and polar regions. Rice paddies, in addition to
emitting methane in copious amounts, also produce these compounds. About 1% of
methyl bromide and 5% of methyl iodide come from rice cultivation.(14) The unplanted
flooded fields emit as much as the cultivated fields.
The inorganic source comes from reactions in organic-laden soils.(8) Reactions in such
soils and sediments can produce halide ions through oxidation of organic matter by an
electron acceptor (for example, a state of iron). The result is compounds such as CH3Cl,
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Energy, Ch. 1, extension 1 Earth’s ozone layer
C2H5Cl, C3H7Cl, C4H9Br, CH3Br, C2H5Br, C3H7Br and C4H9Br, and CH3I, C2H5I,
C3H7I and C4H9I for soils containing chlorine, bromine, and iodine, respectively. These
abiotic processes could therefore be a large source of methyl halides. The halons (Table
1.1) contain bromine atoms that form bromine oxide, BrO, and BrCl, which have
characteristics similar to chlorine oxide. Methyl bromide, a pesticide, is heavily used in
agriculture and is responsible for much of the stratospheric bromine.
Generally, iodine-containing compounds react more readily in the lower atmosphere and
so do not cause ozone depletion because they do not survive long enough in the
atmosphere to reach the stratosphere.(11) However, convective clouds could inject
localized high iodine concentrations into the stratosphere occasionally, where it would
destroy ozone just as its lower-mass cousins.
How ozone is destroyed at the poles
Ozone destruction is discussed clearly in references 4 and 15–18. Ozone is destroyed
through catalytic cycles involving reactive nitrogen (NOy), Cl and Br (“halons” are
compounds containing bromine),(19) and hydrogen species (HO x).(20) In the upper
atmosphere, chlorine is found in “reservoirs,” in combination with nitrogen compounds.
The CFCs, which were stable in the lower atmosphere, break apart under the action of
sunlight and release chlorine to the “reservoir molecules,”(21) where chlorine (or bromine)
is inactivated.
In winter and spring, ozone destruction is mostly due to Cl and Br because at low
temperatures, the reservoir molecules don’t reform after they break apart. In summer, the
halon reservoirs return through reactions involving nitric acid. However, when the sunlight
is available in the summer, NO3 is photolyzed within seconds. It can’t form into N2O5,
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Energy, Ch. 1, extension 1 Earth’s ozone layer
the inactive form of nitrogen oxide, so ozone is also destroyed at high rates in summer by
active nitrogen oxides.
Fig. E01.1.1 Total Ozone Mapping Spectrometer (TOMS) satellite data for ozone concentrations in the
Antarctic for selected years.
(NASA)
Polar stratospheric clouds play a crucial role in the cycle of ozone destruction. In polar
stratospheric clouds, nitric acid can be held away from contact with chlorine, allowing it to
do its dastardly deeds.(15) The ozone-depleting reactions can take place on ice crystals,
thought to be primarily nitric acid trihydrate (NAT; HNO3•3H2O), frozen nitric acid with
water. The clouds help destroy ozone by mixing chlorine nitrate and hydrochloric acid to
form nitric acid and molecular chlorine.(21) When winter comes to polar regions, there is no
sunlight and temperatures drop. When the temperature is low enough, clouds of nitric acid
and water condense on sulfuric acid drops. The ice crystals can form only in extreme cold,
around -78 °C, in the extremely dry air. The ice is not pure water ice (which forms at -83
°C) but is NAT. The NAT removes nitrogen from the region of the cloud;(21) the nitrogen
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Energy, Ch. 1, extension 1 Earth’s ozone layer
compounds are covered with water, freeze, and eventually fall as snow, especially those
on the largest ice crystals.(15) This depletion of ozone is apparent in the Total Ozone
Mapping Spectrometer (TOMS) data of Fig. E01.1.1 and in the more complete yearly
evolution shown in Fig. E01.1.2.
Fig. E01.1.2 TOMS October averages beginning in 1979.
(NASA satellite data)
By spring, most nitrogen has been removed by the clouds; the nitrogen concentration is so
low that the chlorine does not get to recombine with nitrogen, but forms chlorine oxide,
ClO. The ClO combines with another ClO molecule to form the chlorine oxide dimer
(ClO)2. Without nitrogen around, the (ClO)2 breaks up through the action of sunlight into
free chlorine atoms, keeping the cycle of ozone destruction going.(21)
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Energy, Ch. 1, extension 1 Earth’s ozone layer
The largest Antarctic ozone hole on record as of this writing occurred in the Antarctic
Spring of 2000 (Fig. E01.1.3).(22) A comparison with the depletion shown in Fig. E01.1.1
shows how much bigger the 2000 depletion was.
Fig. E01.1.3 TOMS image of the largest ozone hole on record, Sept. 6, 2000.
(NASA satellite data)
With the success of the Montreal protocol in controlling chlorofluorocarbon emissions, it
is to be hoped that the ozone holes in the Arctic and Antarctic will begin to shrink. (23,24)
Given the complexity of the interaction of chlorine concentrations and weather, the
shrinkage may exhibit fits and starts, but the stratospheric concentration of chlorine is
expected to begin declining in the early 2000s. There is a lag time between emissions
reduction and polar chlorine concentration because it takes some years for the lower
atmospheric chlorofluorocarbons to reach the stratosphere and be broken up through
action of ultraviolet solar radiation. The best estimate for the lifetime of CFC-11, for
example, is about 50 years.(24)
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