Stratospheric Ozone

• Author: Arnold W. Reitze, Jr.
• Pages: 435-462

Page 435

Chapter 13:

Stratospheric Ozone

§1. Introduction

§1(a). Chemistry

Global atmospheric change is related to the

increase in CO2 and other greenhouse gases 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

chapter.1

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 radiation 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 free 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 the atmosphere

the ratio of O to O3 is high, as there are few 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 also can convert ozone to

oxygen.

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 important 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

ozone.

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 alti-

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 catalytic 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 with 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

Monoxide)

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 the British Antarctic

Survey reported a “hole” in the ozone layer over

Antarctica. In 1986, a panel of 130 scientists

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% between 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 polar

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 all 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 was 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

summer.5

On February 3, 1992, NASA released prelimi-

nary data acquired from a series of high-altitude

instrument-laden plane ights over the north-

ern hemisphere. Additional data were obtained

from NASA’s Upper Atmosphere Research Satel-

lite (UARS), launched in September 1991, which

showed higher levels of chlorine monoxide (ClO;

the key agent 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 than 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.