Stratospheric Ozone

AuthorArnold W. Reitze, Jr.
Page 435
Chapter 13:
Stratospheric Ozone
§1. Introduction
§1(a). Chemistry
Global atmospheric change is related to the
increase in CO2 a nd other greenhouse ga ses in the
troposphere (lower-atmosphere), increased ozone
levels in the troposphere, and the depletion of
stratospheric ozone. e stratosphere lies above the
troposphere and starts between 26,000 feet and
52,000 feet above sea level, depending on latitude,
and continues to approximately 160,000 feet. e
three problems are related, but it is the depletion
of stratospheric ozone that is the subject of this
e Earth’s atmosphere is approximately 21%
oxygen, 78% nitrogen, and 1% argon. In the lower
atmosphere water vapor makes up as much as 3%
of the air. Other gases are present in trace quanti-
ties measured in parts per million (ppm) or less.
e atmosphere is a uid attracted to the Earth’s
surface by gravity. A s a general rule, atmospheric
density decreases by a factor of 10 for every 50,000
feet in elevation. us, at 100,000 feet, or about 19
miles, the atmosphere is approximately 1% as dense
as at the surface.
e atmosphere is exposed to the rad iation of
the sun. About 99% of this radiation is in the form
of visible light. e rest is emitted from other por-
tions of the electromagnetic spectrum. e energy
is carried by photons. High-energy photons,
mainly in the ultraviolet wave (U V) length, have
sucient energy to break apart molecules in a pro-
1. is material on stratospheric ozone science primarily comes
from J F  R K, G A: T
O P C 23-56 (1990) and H M
 D H, L   O: A L’ G 
H T  A C 10 (1996).
cess called photolysis. Photons from solar radiation
strike diatomic oxygen and produce free oxygen.
O2 + high energy photon ¨ O + O
ese f ree oxygen atoms will combine to form
ozone if a neutral third molecule is available to
absorb the excess energy.
O + O2 + M ¨ O3 + M
M can be nitrogen (N2), oxygen (O2), or argon
(Ar). In the highest reaches of t he atmosphere
the ratio of O to O3 is high, as t here are fe w mol-
ecules present to form reactions, but a higher ratio
of ozone is found in the stratosphere. Before high
energy photons can reach the Earth their energy is
absorbed by oxygen molecules.
Once ozone is formed, it can be photolyzed back
to free oxygen.
O3 + photon ¨ O + O2
e free oxygen a lso can c onvert ozone to
O + O3 ¨ 2O2
e interplay among oxygen in its various forms
rst was described in 1930 by the British physi-
cist, Sydney Chapman. Since the 1930s, scien-
tic research has demonstrated the complexity of
atmospheric chemistry. In the 1950s, water vapor,
producing the hydrogen (H) and hydroxyl (OH)
radicals, was shown to be importa nt in determin-
ing the amount of ozone in the stratosphere. Free
oxygen that reacts with water does not produce the
ozone that would be created if the free oxygen com-
bined with O2. Hydroxyl radicals also can destroy
OH + O3 ¨ HO2 + O2
Page 436 Air Pollution Control and Climate Change Mitigation Law
HO2 + O3 ¨ OH + 2O2
In 1970, Paul Crutzen showed that nitrogen
oxides (NOx) could aect the chemistry of the
stratosphere. In 1973, the National Aeronau-
tics and Space Administration (NASA) began
researching the eects of rocket emissions on the
atmosphere. Little NOx, however, reaches this a lti-
tude. For a brief time, this concern led to oppo-
sition to supersonic aircraft development, but this
threat to the ozone layer eventually was shown to
be insignicant.
In 1974, hydrochloric acid (HCl) was shown to
be capable of photolyzation by ultraviolet radiation
that would free chlorine and destroy ozone.
Cl + O3 ¨ ClO + O2
ClO + O ¨ Cl + O2
is reaction is called a cata lytic cycle of ozone
destruction as each chlorine atom can react with
ozone 10,000 to 100,000 times to destroy ozone
molecules before the chlorine reacts w ith a mol-
ecule other than ozone. is chemistry became
more signicant when, in 1974, chlorouorocar-
bons (CFCs) were shown to be an important source
of free chlorine atoms in the stratosphere. Today,
the following reaction accounts for most of the
ozone loss:
CFC + UV lig ht = Free Chlorine Atom ¨ Free
Chlorine Atom + O3 = O2 + C1O (Chlorine
In this manner, CFCs destroy ozone and leave
normal oxygen plus the chlorine monoxide. e
chlorine monoxide may then react further:
C1O + NO2 = ClNO3 (Chlorine nitrate)
In the extreme cold over Antarctica the chlorine
nitrate reacts with hydrogen chloride in ice particles
and releases a pair of chlorine atoms. UV light then
splits the chlorine atoms, and they repeat the cycle.
In warmer environments this molecule eventually
is removed by precipitation. Chlorine also can react
with methane to produce HCl, which is soluble in
water and is eventually removed by precipitation.
In 1985, scientists from t he British Antarctic
Survey reported a “hole” in the ozone layer over
Antarctica. In 1986, a panel of 130 scientist s
from many nations reported that ozone levels in
the stratosphere were decreasing. e thickness of
the ozone layer is measured in Dobson units. In
1988, the panel concluded ozone had decreased
by 3% bet ween 1978 and 1987 in the southern
hemisphere and 2% in the northern hemisphere.
e Antarctic “hole” was believed to involve a
unique chemistry that depended on the presence
of temperatures below -80oC (-112oF) in pola r
stratospheric clouds. Studies also showed extensive
Arctic ozone destruction.2 Nearly all members of
the scientic community agreed an ozone “hole”
exists, but not a ll scholars agreed that there were
dire public health or environmental consequences.3
Ozone depletion continued more rapidly in the
1980s than in the 1970s. Concern was focusing on
methyl bromide, a crop fumigant, as another ozone-
depleting chemical that needed to be regulated.
Adding to the scientic confusion was the concern
that ozone depletion wa s cooling the planet, and
therefore, slowing ozone depletion could increase
global warming. Data showed summertime ozone
loss to be 3% in the 1980s. e ozone depletion
problem and the global warming problems might
be linked.4
On October 22, 1991, the United Nations Envi-
ronment Programme (UNEP) and World Meteo-
rological Organization (WMO) released a report
indicating that the ozone layer was being depleted
during the spring and summer; it was not just
occurring in the winter. Moreover, the ozone layer
was not only being depleted in polar regions but
was thinning over a wider area over both hemi-
spheres and for longer periods in the spring and
On February 3, 1992, NASA released prelimi-
nary data acquired from a series of high-altitude
instrument-laden plane ights over the nort h-
ern hemisphere. Additional data were obtained
from NASA’s Upper Atmosphere Rese arch Satel-
lite (UARS), launched in September 1991, which
showed higher levels of ch lorine monoxide (ClO;
the key a gent responsible for stratospheric ozone
depletion) over Canada and New England than
were previously observed. ClO levels over the
United States a nd Canada and as far south as the
Caribbean were many times greater tha n gas phase
models had predicted. ese levels were only par-
2. U N E P (UNEP), T
I  O L D 13 (1992)
3. B L, D D V: O D’
L  G W (Competitive Enterprise Inst.
1998).See alsoOcular and Dermatologic Health Eects of Ultraviolet
Radiation Exposure from the Ozone Hole in Southern Chile
4. Sharon Begley & Mary Hager, Bring Back the Ozone Layer!,
5. WMO  ., S A, supra note 2.

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