Roger Harris/Photo Researchers, Inc.192 CHAPTER 8 Air and Air Pollution
The Atmosphere
LEARNING OBJECTIVES
- Define atmosphere and list the major gases
comprising the atmosphere. - Briefly describe the four major concentric
layers of the atmosphere. - Explain the causes of wind, including the
Coriolis effect.
O
xygen and nitrogen are the predominant
gases in the atmosphere, accounting for
about 99 percent of dry air (Figure 8.1).
Other gases make up the remaining
1 percent. In addition, water vapor (the most variable gas
in the atmosphere) and trace amounts of air pollutants
are present in the air. The atmosphere b ecomes less dense
as it extends outward into space.
Ulf Merbold, a German space
shuttle astronaut, felt differently
about the atmosphere after view-
ing it in space (Figure 8.2): “For
the first time in my life, I saw the horizon as a curved line.
It was accentuated by a thin seam of dark blue light—
our atmosphere. Obviously, this wasn’t the ‘ocean’ of air
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The “ocean of air” is a thin blue layer that separates the planet
from the blackness of space.Carbon dioxide 0.04%
Argon 0.93%Oxygen
21%Nitrogen
78%Other gases 0.03%Composition of the atmosphere
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Nitrogen and oxygen form most of the atmosphere. Air also
contains water vapor and various pollutants (methane, ozone,
dust particles, microorganisms, and chlorofluorocarbons [CFCs]).
I had been told it was so many times in my life. I was
terrified by its fragile appearance.” The atmosphere is
composed of four major concentric layers—the tropo-
sphere, stratosphere, mesosphere, and thermosphere
(Figure 8.3). These layers vary in altitude and tempera-
ture, depending on the latitude and season.
The atmosphere performs several valuable ecosystem
services. First, it protects Earth’s surface from most of the
sun’s ultraviolet (UV) radiation and x-rays, and from lethal
exposures to cosmic rays from space. Life as we know it would
cease to exist without this shielding. Second, atmospheric
greenhouse gases absorb some of the heat reradiated from
Earth’s surface, which keeps the lower atmosphere within
the range of temperatures that support life.
Organisms depend on the atmosphere for existence,
but they also maintain and, in certain instances, modify its
composition. Atmospheric oxygen is thought to have in-
creased to its present level as a result of billions of years
of photosynthesis. Over the course of a year, oxygen-
producing photosynthesis and oxygen-using cellular respi-
ration roughly balance, although carbon dioxide levels have
increased each year over the past century (see Chapter 9).atmosphere The
gaseous envelope
surrounding Earth.