Geoscience Reference
In-Depth Information
Because stratospheric mixing ratios of CFCs require
along time to respond to emission changes from the
surface,
the stratospheric ozone layer is not expected
to recover fully until 2050 or beyond
(World Meteoro-
logical Organization, 1998). The annual occurrence of
the Antarctic ozone hole is expected to last for several
decades as well. Global warming, which is character-
ized by a warming of the troposphere but cooling of the
stratosphere, will likely delay the recovery of the ozone
layer further, as discussed in Chapter 12.
11.12. Problems
11.1. Explain why an ozone maximum occurs imme-
diately outside the polar vortex during the same season
that the ozone hole occurs over the Antarctic.
11.2. Briefly summarize the steps resulting in the
Antarctic ozone hole and its recovery. Explain why the
ozone dent is never so severe as the ozone hole.
11.3. List the chlorine catalytic cycles responsible for
ozone reduction (a) over the Antarctic and (b) in the
global stratosphere. Why is the Antarctic mechanism
unimportant on a global scale?
11.11. Summary
In this chapter, the stratospheric ozone layer is dis-
cussed, with an emphasis on long-term global ozone
reduction and seasonal Antarctic ozone depletion. Both
problems are caused by enhanced levels of chlorine
and bromine in the stratosphere. Damage to the ozone
layer is of concern because decreases in ozone per-
mit UV-B radiation to penetrate to the Earth's sur-
face, where it harms humans, animals, microorganisms,
and plants. The story of environmental damage to the
ozone layer may have a positive ending. CFCs, emit-
ted since the 1930s, slowly diffused into the middle
stratosphere with an overall transport time of 50 to
100 years. Most CFCs emitted during the 1930s did
not reach the middle stratosphere until the 1970s, and
those emitted during the 1990s will not reach the mid-
dle stratosphere until 2040 to 2090. In 1974, a relation-
ship between CFCs and ozone loss was hypothesized.
From the 1970s until today, measurements indicate a
global ozone reduction and a seasonal Antarctic deple-
tion. In 1978, regulations banning the release of CFCs
from spray cans were implemented in the United States.
Concerned that the ban on CFCs from spray cans was
not enough, the international community agreed to reg-
ulate all CFC emissions through the Montreal Proto-
col in 1987. In 2000, reported CFC emissions were
less than one-tenth those in 1976. In 2010, they were
nearly zero. Global ozone levels may be replenished
to their original levels by the year 2050 as CFCs cur-
rently in the atmosphere are slowly removed. Recovery,
though, will likely be delayed due to global warm-
ing, which warms the troposphere and cools the strato-
sphere. A cooler stratosphere increases the effective-
ness of chlorine destruction of ozone, as discussed in
Chapter 12.
11.4. What effect do you think volcanic aerosol parti-
cles that penetrate into the stratosphere have on strato-
spheric ozone during the daytime when chlorine reser-
voirs are present? What about during the nighttime?
11.5. If the entire global stratospheric ozone layer takes
only 1 year to form from scratch chemically in the pres-
ence of oxygen, why will today's ozone layer, which
has deteriorated a few percent since 1979, take up to 50
years to regenerate to its 1979 level?
11.6. Why does bromine destroy ozone more efficiently
than does chlorine?
11.7. Why do CFCs not cause damage to ozone in the
troposphere?
11.8. Some people argue that natural chlorine from vol-
canic eruptions and from the ocean is responsible for
global ozone reduction and Antarctic ozone depletion.
What argument contradicts this contention?
11.9. Explain why the ratio of UV-B to total solar radi-
ation reaching the top of the atmosphere is greater than
is that reaching the ground, as seen in Figure 11.5.
11.10. Explain how UV-B levels at the ground in pol-
luted air can be lower than those in unpolluted air, if
both the polluted air and clean air are situated under
an ozone layer that is partially depleted by the same
amount.
11.11. Why might you be exposed to more UV radia-
tion on a sunny June day at the Equator in Brazil than on
the same sunny day and at the same elevation in Mex-
ico, even though radiation at the top of the atmosphere
is greater on that day over Mexico? Ignore air pollution
effects.
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