Geoscience Reference
In-Depth Information
2
CHAPTER TWO
Atmospheric composition,
mass and structure
LEARNING OBJECTIVES
When you have read this chapter you will:
n
be familiar with the composition of the atmosphere - its gases and other constituents
n
understand how and why the distribution of trace gases and aerosols varies with height, latitude
and time
n
know how atmospheric pressure, density and water vapor pressure vary with altitude
n
be familiar with the vertical layers of the atmosphere, their terminology and significance.
This chapter describes the composition of the
atmosphere - its major gases and impurities, their
vertical distribution, and variations through time.
The various greenhouse gases and their signifi-
cance are discussed. It also examines the vertical
distribution of atmospheric mass and the struc-
ture of the atmosphere, particularly the vertical
variation of temperature.
Table 2.1
Average composition of the dry
atmosphere below 25km
Component
Symbol Volume
Molecular
% (dry air)
weight
Nitrogen
N
2
78.08
28.02
Oxygen
O
2
20.95
32.00
*
‡
Argon
Ar
0.93
39.88
Carbon dioxide
CO
2
0.037
44.00
A COMPOSITION OF THE
ATMOSPHERE
‡
Neon
Ne
0.0018
20.18
*‡Helium
He
0.0005
4.00
†
Ozone
O
3
0.00006
48.00
1 Primary gases
Air is a mechanical mixture of gases, not a
chemical compound. Dry air, by volume, is more
than 99 percent composed of nitrogen and oxygen
(
Table 2.1
). Rocket observations show that these
gases are mixed in remarkably constant propor-
tions up to about 100km altitude. Yet, despite
Hydrogen
H
0.00005
2.02
‡
Krypton Kr 0.00011
‡
Xenon Xe 0.00009
§
Methane CH
4
0.00017
Notes: *Decay products of potassium and uranium.
†
Recombination of oxygen.
‡
Inert gases.
§
At surface.
