Page 470 - Environment: The Science Behind the Stories

Ozone concentration (milli-Pascals) stratosphere’s air becomes warmer with altitude, attaining a
maximum temperature of –3°C (27°F) at its top. The reason
0 5 10 15 20 25 30 35
120 is that ozone and oxygen absorb and scatter the sun’s ultravi-
olet (UV) radiation (p. 49), so that much of the UV radiation
110 that penetrates the upper stratosphere fails to reach the lower
Thermosphere stratosphere. Most of the atmosphere’s ozone concentrates
(up to 500 km)
100 in a portion of the stratosphere roughly 17–30 km (10–19 mi)
above sea level, a region we have come to call Earth’s ozone
90 layer. The ozone layer greatly reduces the amount of UV
radiation that reaches Earth’s surface. Because UV light
Temperature profile
80 can damage living tissue and induce mutations in DNA, the
Mesosphere ozone layer’s protective effects are vital for life on Earth.
70 Above the stratosphere lies the mesosphere, which
extends 50–80 km (31–56 mi) above sea level. Air pressure
Altitude (km) 50 tude. The thermosphere, our atmosphere’s top layer, extends
60
is extremely low here, and temperatures decrease with alti-
upward to an altitude of 500 km (300 mi).

40 Ozone
concentration
Stratosphere Temperature, pressure, and humidity
30
vary within the atmosphere
20 Air moves dynamically within the lower atmosphere as a
Tropopause
result of differences in the physical properties of air masses.
10 Among these properties are pressure and density, relative
Troposphere
humidity, and temperature.
0 Gravity pulls gas molecules toward Earth’s surface, caus-
-80 -60 -40 -20 0 ing air to be most dense near the surface and less dense as
Temperature (ºC)
altitude increases. Atmospheric pressure, which measures
Figure 17.2 The atmosphere’s layers differ in their properties. the force per unit area produced by a column of air, also
Temperature (red line) drops with altitude in the troposphere, rises decreases with altitude, because at higher altitudes there are
with altitude in the stratosphere, drops in the mesosphere, and rises fewer molecules being pulled down by gravity (Figure 17.3).
in the thermosphere. The tropopause separates the troposphere
from the stratosphere. Ozone (blue-shaded area) reaches a peak in
the lower stratosphere, giving rise to the term ozone layer. Adapted 50
from Jacobson, M.Z., 2002. Atmospheric pollution: History, science, and regula-
tion. Cambridge: Cambridge University Press; and Parson, E.A., 2003. Protecting
the ozone layer: Science and strategy. Oxford: Oxford University Press.
40
Weather balloon
diameter, like the fuzzy skin of a peach. It consists of four
layers that differ in temperature, density, and composition 30
(Figure 17.2). 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
The bottommost layer, the troposphere, blankets Earth’s Altitude (km) Above 90% of Density of air molecules
surface and provides us the air we breathe. Movement of air molecules
air within the troposphere also drives the planet’s weather. 20
Although it is thin (averaging 11 km or 7 mi high) relative Airplane
Mt. Everest
to the atmosphere’s other layers, the troposphere contains
three-quarters of the atmosphere’s mass, because gravity 10 Above 50% of
air molecules
pulls mass downwards, making air denser near Earth’s sur- 5.5
face. Tropospheric air gets colder with altitude, dropping to
roughly –52°C (–62°F) at the top of the troposphere. At this 0
point, temperatures stabilize, marking a boundary called the 0 100 300 500 700 900
tropopause. The tropopause acts like a cap, limiting mixing (low) (high)
between the troposphere and the atmospheric layer above it, Atmospheric pressure (millibars)
the stratosphere. Figure 17.3 As one climbs higher through the atmosphere,
The stratosphere extends 11–50 km (7–31 mi) above sea gas molecules become less densely packed, and atmos-
level. Similar in composition to the troposphere, the strato- pheric pressure decreases. One needs to be only 5.5 km
sphere is 1000 times drier and less dense. Its gases expe- (3.4 mi) high to be above half the planet’s air molecules. Adapted
rience little vertical mixing, so once substances (including from Ahrens, C.D., 2007. Meteorology today, 8th ed. Fig 1.9. © 2007. Belmont,
pollutants) enter it, they tend to remain for a long time. The CA: Brooks/Cole. By permission of Cengage Learning. 469

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