Page 104 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

68 part I The energy–atmosphere System
Kilometres 480
400 320
200 160
80
50
20
10
Pressure Profile
Lower density, lower pressure
Higher density, higher pressure
EARTH
Thermosphere
Auroras
Measurement equivalents
% of sea level mb pressure
Thermopause
(a) Density is higher nearer Earth’s surface, and decreases with altitude.
▲Figure 3.2 Density decreasing with altitude. Have you experi- enced pressure changes that you could feel on your eardrums? How high above sea level were you at the time?
Mesosphere Stratosphere
Kittinger balloon
Troposphere
.004 .001
0.78 0.1
12 1
55
175 10
507 50
1013.2 100 1013.2
(b) Pressure profile plots the decrease in pressure with increasing altitude. Pressure is in millibars and as a
percentage of sea level pressure. Note that the troposphere holds about 90% of the atmospheric mass (far-right % column).
Earth’s surface—the blend of gases is nearly uniform throughout. The only exceptions are the concentration of 0 ozone (O3) in the “ozone layer,” from 19 to 50 km above
sea level, and the variations in water vapour, pollutants,
and some trace chemicals in the lowest portion of the atmosphere.
0 200 500 Millibars (mb)
The present blend of gases evolved approximately 500 million years ago. Table 3.1 lists by volume the gases that constitute dry, clean air in the homosphere, divided into constant (showing almost no change throughout Earth’s history) and variable (present in small but vari- able amounts). Air sampling occurs at the Mauna Loa Observatory, Hawai‘i, operational since 1957. A marine inversion layer keeps volcanic emissions from nearby K¯ılauea volcano and dust to a minimum.
The air of the homosphere is a vast reservoir of rela- tively inert nitrogen, originating principally from vol- canic sources. A key element of life, nitrogen integrates into our bodies not from the air we breathe but through compounds in food. In the soil, nitrogen-fixing bacteria incorporate nitrogen from air into compounds that can be used by plants; later, the nitrogen returns to the at- mosphere through the work of denitrifying bacteria that remove nitrogen from organic materials. A complete dis- cussion of the nitrogen cycle is in Chapter 19.
Oxygen, a by-product of photosynthesis, also is es- sential for life processes. The percentage of atmospheric oxygen varies slightly over space with changes in pho- tosynthetic rates of vegetation with latitude, season, and the lag time as atmospheric circulation slowly mixes the air. Although it makes up about one-fifth of the at- mosphere, oxygen forms compounds that compose about half of Earth’s crust. Oxygen readily reacts with many
Constant Gases
nitrogen (n2) argon (ar) Helium (He) Xenon (Xe)
Water vapour (H2O) Methane (CH4) nitrous oxide (n2O)
78.084
0.934
0.000525 5.2
Trace ~0.1
0–4% (max at tropics, min at poles) 0.00018 1.8 Trace ~0.3
TABLE 3.1 Composition of the Modern Homosphere
Gas (Symbol)
Percentage by Volume
Parts per Million (ppm)
780,840 9,340
Oxygen (O2)
20.946
209,460
neon (ne)
0.001818
18
krypton (kr)
0.00010
1.0
Variable Gases (change over time and space)
Carbon dioxide (CO2)*
0.0402
402
Hydrogen (H)
Trace
~0.6
Ozone (O3)
Variable
*May 2014 average CO2 measured at Mauna loa, Hawai‘i
(see ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_mlo.txt).