Geology Reference
In-Depth Information
PART B. GLOBAL RESPONSE TO DECREASE
IN STRATOSPHERIC OZONE
Stratospheric Ozone and the Ozone Hole
Similar reactions occur in the Arctic, however to a
lesser degree because it is warmer. The ozone decrease
extends to the mid-latitudes and beyond. See Shanklin
(2005) and other sources for additional information on
Antarctic ozone.
The Antarctic thinning in October was discov-
ered in 1985 at the British Antarctic station, Halley,
where ozone had been measured in atmospheric stud-
ies since 1955. Swift international action produced the
Montreal Protocol in 1987; later agreements hastened
further the end of CFC production. The quick response
occurred because less harmful substitutes were devel-
oped and there were only a limited number of manu-
facturers. The ozone problem showed that the world
could unite to solve a global environmental-change
problem. (The C0
2
global warming problem is much
more difficult because there are many sources of
C0
2
/
almost everyone depends on fossil fuel energy,
and no readily available substitutes for fossil fuels
exist.) The concentration of ozone-destroying chemi-
cals in the stratosphere is near its peak. According to
Shanklin (2004), the ozone hole over Antarctica is not
expected to completely heal until mid-century—barring
massive volcanic eruptions or a bolide impact such as
the Tunguska event.
Questions on ozone are based primarily on data
from Halley, Antarctica (Figure 18.7).
Ozone
(O3)
is produced in the stratosphere when O2
molecules are split by solar radiation and an oxygen
atom (O) then bonds with 0
2
to form
O3.
This trace
gas makes up only 0.000007% of the atmosphere (oxy-
gen makes up 20.95%), so why were humans con-
cerned about it in the 1970s when scientists (who later
won Nobel prizes for their work on ozone) predicted
that it would decrease due to interaction with CFCs?
One reason is the fact that ozone blocks UV radiation,
too much of which is harmful for humans (skin cancer
and cataracts), animals, and some plants. Such a
change would present human health problems and
impact our life-support system. A second concern was
economical. A large industry depended on CFCs—
refrigerants for air conditioners and freezers, propel-
lants in aerosol cans, etc. Industry's first response was
that the CFC
-O3
Connection was bad science. How
could heavy inert CFCs (chlorofluorocarbons) reach
the stratosphere and then how would their chlorine
interact with ozone molecules? Figure 18.6 contains
the model for the reaction. CFCs may remain in the
atmosphere for many years. Chlorine and bromine
atoms remain in the atmosphere for one or two years
and a chlorine atom may destroy 100,000 ozone mole-
cules as it cycles through the O3-CIO reaction!
Although most ozone is produced in the trop-
ics, it makes its way to the polar regions, too. During
the winter over isolated Antarctica,
O3
attaches to
ice particles in the cold atmosphere. With sunrise in
September/October, reactions with chlorine decrease
the concentration of ozone (an ozone "hole" or thin-
ning develops when the concentration of ozone is less
than 220 Dobson Units or ppb). It is here in the Antarc-
tic during spring that the lowest ozone readings occur.
QUESTIONS 18, PART B
1. In Figure 18.6, identify the following in the space below or
by marking on the diagram: CFC, ozone, and diatomic oxygen
molecules and chlorine and oxygen atoms.
2. In step 5 to step 6 of Figure 18.6, what causes the chlorine
atom to separate from the chlorine-oxygen molecule?
3.
a.
Why does one chlorine atom destroy many
O3
molecules?
b. Where and when in the year are the lowest values for
stratospheric ozone?
c.
Why is the ozone "hole" over Antarctica?
4. The concentration of ozone is measured in "Dobson
units." From Figure 18.7 one Dobson unit is equal to
©
FIGURE 18.6
Chemical reactions for destruction of ozone in
the stratosphere.
(Modified from
NASA, 1993)
