Page 488 - Environment: The Science Behind the Stories

thinned ozone concentration that was soon named the ozone
UV radiation O 3 hole (Figure 17.25). During each Southern Hemisphere spring
Ozone since then, ozone concentrations over this immense region
CFC
Cl have dipped to roughly half their historic levels.
Chlorine O 2 Extensive scientific detective work has revealed why sea-
Chlorofluorocarbons Oxygen sonal ozone depletion is so severe over Antarctica (and to a
Cl Stratosphere lesser extent, the Arctic). During the dark and frigid Antarc-
tic winter, temperatures in the stratosphere dip below –80°C
Chlorine Cl O (–112°F), enabling unusual high-altitude polar stratospheric
Chlorine monoxide
clouds to form. Many of these icy clouds contain condensed
nitric acid, which splits chlorine atoms off from compounds
O 2 O such as CFCs. The freed chlorine atoms accumulate in the
Refrigerators, aerosol Oxygen Oxygen atom clouds, trapped over Antarctica by wind currents that swirl in
spray cans, air conditioners a circular polar vortex that prevents air from mixing with the
(sources of chlorofluorocarbons)
rest of Earth’s atmosphere.
Figure 17.24 CFCs destroy ozone in a multistep process, In the Antarctic spring (starting in September), sun-
repeated many times. A chlorine atom released from a CFC shine returns, and UV radiation dissipates the clouds. This
molecule in the presence of UV radiation reacts with an ozone releases the chlorine atoms, which begin destroying ozone.
molecule, forming one molecule of oxygen gas and one chlorine The solar radiation also catalyzes chemical reactions, speed-
monoxide (ClO) molecule. The oxygen atom of the ClO molecule ing up ozone depletion as temperatures warm. The ozone
then binds with a stray oxygen atom to form oxygen gas, leaving hole lingers over Antarctica until December, when warmed
the chlorine atom to begin the destructive cycle anew. In this way, air shuts down the polar vortex, allowing ozone-depleted air
a single chlorine atom can destroy up to 100,000 ozone molecules.
to diffuse away and ozone-rich air from elsewhere to stream
in. The ozone hole vanishes until the following spring.
Synthetic chemicals deplete stratospheric By the time scientists had worked most of this out, plum-
ozone meting ozone levels were becoming a serious international
concern. Already worried that intensified UV exposure at
Researchers identifying ozone-depleting substances pinpointed Earth’s surface would lead to more skin cancer, scientists were
primarily halocarbons—human-made compounds derived from also predicting damage to crops and to ocean phytoplankton,
simple hydrocarbons (p. 46), such as ethane and methane, in the base of the marine food chain.
which hydrogen atoms are replaced by halogen atoms, such
as chlorine, bromine, or fluorine. Industry was mass-producing Figure 17.25 The “ozone hole” is a vast area of thinned
one type of halocarbon, chlorofluorocarbons (CFCs), at a rate ozone density in the stratosphere over the Antarctic region.
of a million tons per year in the early 1970s, and this rate was It has reappeared seasonally each September in recent decades.
growing by 20% a year. CFCs were useful as refrigerants, fire This colorized satellite imagery of Earth’s Southern Hemisphere
extinguishers, propellants for aerosol spray cans, cleaners for from September 24, 2006, shows the ozone hole (purple/blue) at
electronics, and for making polystyrene foam. Because CFCs its maximal recorded extent to date.
rarely reacted with other chemicals, scientists surmised that
they would be harmless to people and the environment.
Alas, the nonreactive qualities that made CFCs ideal
for industrial purposes were having disastrous consequences
for the ozone layer. Whereas reactive chemicals are broken CHAPTER 17 • AT m os PHER i C sC i E n CE , Ai R Qu A li T y, A nd Poll u T i on Con TR ol
down quickly in the troposphere, CFCs reach the strato-
sphere unchanged and can linger there for a century or more.
In the stratosphere, intense UV radiation from the sun even-
tually breaks CFCs into their constituent chlorine and car-
bon atoms. In a two-step chemical reaction (Figure 17.24),
each newly freed chlorine atom can split an ozone molecule
and then ready itself to split another one. During its long
residence time in the stratosphere, each free chlorine atom
can catalyze the destruction of as many as 100,000 ozone
molecules!

The Antarctic ozone hole appears
each spring

In 1985, researchers shocked the world by announcing that
stratospheric ozone levels over Antarctica in springtime had
declined by nearly half in just the previous decade, leaving a 487

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