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A Closer Look

Hole in the Ozone Layer?
zone is triatomic oxygen (O 3 ) that is down into the single-atom (O) form. This chemicals are broken down by UV radia-
Oconcentrated mainly in the upper reaction is significant because of (1) the tion, releasing chlorine (Cl), which reacts
portions of the stratosphere. Diatomic high-energy ultraviolet radiation that is with ozone. The reaction might be
molecules of oxygen (O 2 ) are concentrated removed from the sunlight and (2) the
CF Cl + UV → CF Cl−* + Cl−*
2
2
2
in the troposphere, and monatomic mol- monatomic oxygen that is formed, which
ecules of oxygen (O) are found in the will combine with diatomic oxygen to make Cl−* + O → *−ClO + O
2
3
outer edges of the atmosphere. Although triatomic ozone, which will absorb even *−ClO + O → O + Cl−*
2
the amount of ozone present in the strato- more ultraviolet radiation. This initial reac-
(−* is an unattached bond)
sphere is not great, its presence is vital to tion is
life on Earth’s surface. Ultraviolet radia- Note that ozone is decomposed in the
O + UV → O + O
2
tion (UV) causes mutations to the DNA second step, so it is removed from the UV-
of organisms. Without the stratospheric When the O molecule collides with an O 2 radiation-absorbing process. Furthermore,
ozone, much more ultraviolet radiation molecule and any third, neutral molecule the second and third steps repeat, so the
would reach the surface of Earth, causing (NM), the following reaction takes place: destruction of one molecule of Freon
mutations in all kinds of organisms. In O + O + NM → O + NM can result in the decomposition of many
3
2
humans, ultraviolet radiation is known to molecules of ozone.
When O 3 is exposed to ultraviolet, the ozone
cause skin cancer and damage the cornea A regional zone of decreased ozone
absorbs the UV radiation and breaks down to
of eyes. It is believed that the incidence availability in the stratosphere is referred
two forms of oxygen in the following reaction:
of skin cancer would rise dramatically to as a “hole in the ozone layer” since
without the protection offered by the ozone. O + UV → O + O much high-energy UV can now reach the
2
3
Here is how stratospheric ozone shields The monatomic molecule that is produced surface below. Concerns about the impact
Earth from ultraviolet radiation. The ozone combines with an ozone molecule to produce of chlorine- containing chemicals such as
concentration is not static because there two diatomic molecules Freon on the ozone layer led to an inter-
is an ongoing process of ozone formation national agreement known as the Montreal
O + O → 2 O
and destruction. For ozone to form, di- 3 2 Protocol. The agreement has resulted in a
atomic oxygen (O 2 ) must first be broken and the process starts all over again. phaseout of the use of Freon and similar
down into the monatomic form (O). Short- There is much concern about Freon compounds in most of the world. Subse-
wave ultraviolet (UV) radiation is absorbed (CF 2 Cl 2 ) and other similar chemicals that quently, the sizes of the holes in the ozone
by diatomic oxygen (O 2 ), which breaks it make their way into the stratosphere. These layer appear to be getting smaller.
On average, Earth as a whole reflects about 30 percent of the dioxide and water molecules. This match means that carbon
total radiation back into space, with two-thirds of the reflection dioxide and water molecules readily absorb infrared radiation
occurring from clouds. The amount reflected at any one time that is emitted from the surface of Earth. The absorbed infrared
depends on the extent of cloud cover, the amount of dust in the energy shows up as an increased kinetic energy of the mole-
atmosphere, and the extent of snow and vegetation on the sur- cules, which is indicated by an increase in temperature. Carbon
face. Substantial changes in any of these influencing variables dioxide and water vapor molecules in the atmosphere now emit
could increase or decrease the reflectivity, leading to increased infrared radiation of their own, this time in all directions. Some
heating or cooling of the atmosphere. of this reemitted radiation is again absorbed by other molecules
As Figure 22.6 shows, only about one-half of the incom- in the atmosphere, some is emitted to space, and significantly,
ing solar radiation reaches Earth’s surface. The reflection and some is absorbed by the surface to start the process all over
selective filtering by the atmosphere allow a global average of again. The net result is that less of the energy from the Sun es-
about 240 watts per square meter to reach the surface. Wide capes immediately to space after being absorbed and emitted
variations from the average occur with latitude as well as with as infrared. It is retained through the process of being re -
the season. directed to the surface, increasing the surface temperature more
The incoming sunlight that does reach Earth’s surface is than it would have otherwise been. The more carbon dioxide
mostly absorbed (Figure 22.6). Rocks, soil, water, and anything that is present in the atmosphere, the more energy that will
on the ground become warmer as a result. These materials emit be bounced around and redirected back toward the surface,
the absorbed solar energy as infrared radiation, wavelengths increasing the temperature near the surface. The process of
longer than the visible part of the electromagnetic spectrum. heating the atmosphere in the lower parts by the absorption
This longer-wavelength infrared radiation has a frequency that of solar radiation and reemission of infrared radiation is called
matches some of the natural frequencies of vibration of carbon the greenhouse eff ect. It is called the greenhouse effect because

546 CHAPTER 22 The Atmosphere of Earth 22-6