Environmental Engineering Reference
In-Depth Information
As a result, many analysts suggest we should be-
gin to prepare for the possible harmful effects of long-
term atmospheric warming and climate change. Fig-
ure 16-16 shows some ways to implement this
adapta-
tion strategy.
Rowland and Molina's research led them to four
major conclusions.
First,
CFCs remain in the tropo-
sphere because they are insoluble in water and chemi-
cally unreactive.
Second,
over 11-20 years these heavier-than-air
compounds rise into the stratosphere mostly through
convection, random drift, and the turbulent mixing of
air in the troposphere.
Third,
once they reach the stratosphere, the CFC
molecules break down under the influence of high-
energy UV radiation. This releases highly reactive chlo-
rine atoms (Cl), as well as atoms of fluorine (F),
bromine (Br), and iodine (I), which accelerate the break-
down of ozone (O
3
) into O
2
and O in a cyclic chain of
chemical reactions. As a consequence, ozone in some
parts of the stratosphere is destroyed faster than it
forms.
Fourth,
each CFC molecule can last in the strato-
sphere for 65-385 years, depending on its type. During
that time, each chlorine atom released during the
breakdown of CFC can convert hundreds of O
3
mole-
cules to O
2
.
According to Rowland and Molina's calculations
and later models and atmospheric measurements of
CFCs in the stratosphere, these dream molecules had
turned into global ozone destroyers.
The CFC industry (led by DuPont), a powerful,
well-funded adversary with a lot of profits and jobs at
stake, attacked Rowland and Molina's calculations
and conclusions. The two researchers held their
ground, expanded their research, and explained its
meaning to other scientists, elected officials, and the
media. After 14 years of delaying tactics, DuPont offi-
cials acknowledged in 1988 that CFCs were depleting
the ozone layer and agreed to stop producing them
once they found substitutes.
In 1995, Rowland and Molina received the Nobel
Prize in chemistry for their work. In awarding the
prize, the Royal Swedish Academy of Sciences said
that they contributed to “our salvation from a global
environmental problem that could have catastrophic
consequences.”
Measurements and models indicate that 75-85%
of the observed ozone losses in the stratosphere since
1976 are the result of CFCs and other ozone-depleting
chemicals (ODCs) released into the atmosphere by hu-
man activities beginning in the 1950s.
16-6 OZONE DEPLETION
IN THE STRATOSPHERE
Science: Annual Drops in Ozone Levels
over the Earth's Poles
Less ozone in the stratosphere allows more harmful
UV radiation to reach the earth's surface.
A layer of ozone in the lower stratosphere keeps about
95% of the sun's harmful ultraviolet (UV) radiation
from reaching the earth's surface. Measuring instru-
ments on balloons, aircraft, and satellites show consid-
erable seasonal depletion (thinning) of ozone concen-
trations in the stratosphere above Antarctica and the
Arctic. Similar measurements reveal a lower overall
thinning everywhere except over the tropics.
Based on these measurements and mathematical
and chemical models, the overwhelming consensus of
researchers in this field is that ozone depletion (thin-
ning) in the stratosphere poses a serious threat to hu-
mans, other animals, and some of the sunlight-driven
primary producers (mostly plants) that support the
earth's food webs.
Science: What Causes Ozone Depletion?
Widespread use of a number of long-lived chemicals
has reduced ozone levels in the stratosphere.
Thomas Midgley, Jr., a General Motors chemist, dis-
covered the first chlorofluorocarbon (CFC) in 1930.
Chemists soon developed similar compounds to create
a family of highly useful CFCs, known by their trade
name as Freons.
These chemically unreactive, odorless, nonflam-
mable, nontoxic, and noncorrosive compounds seemed
to be dream chemicals. Inexpensive to manufacture,
they became popular as coolants in air conditioners and
refrigerators, propellants in aerosol spray cans, cleaners
for electronic parts such as computer chips, fumigants
for granaries and ship cargo holds, and bubbles in plas-
tic foam used for insulation and packaging. Between
1960 and the early 1990s, CFC production rose sharply.
But it turned out that CFCs were too good to
be true. In 1974, calculations by chemists Sherwood
Rowland and Mario Molina at the University of
California-Irvine indicated that CFCs were lowering
the average concentration of ozone in the stratosphere.
They shocked both the scientific community and the
$28-billion-per-year CFC industry by calling for an im-
mediate ban of CFCs in spray cans (for which substi-
tutes were available).
Learn more about how chlorine-containing compounds
destroy stratospheric ozone at Environmental ScienceNow.
Science: Effects of Ozone Depletion
During four months of each year, as much as half of
the ozone in the stratosphere over Antarctica and a
smaller amount over the Arctic is depleted.
