Geoscience Reference
In-Depth Information
Atmospheric composition, mass
and structure
Learning objectives
When you have read this chapter you will:
Be familiar with the composition of the atmosphere - its gases and other constituents,
Understand how and why the distribution of trace gases and aerosols varies with height, latitude and time,
Know how atmospheric pressure, density and water vapour pressure vary with altitude,
Be familiar with the vertical layers of the atmosphere, their terminology and significance.
This chapter describes the composition of the atmos-
phere - its major gases and impurities, their vertical
distribution, and variations through time. The various
greenhouse gases and their significance are discussed.
It also examines the vertical distribution of atmospheric
mass and the structure of the atmosphere, particularly
the vertical variation of temperature.
Table 2.1
Average composition of the dry atmosphere
below 25 km.
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
1 Primary gases
†Ozone
O
3
0.00006
48.00
Hydrogen
H
0.00005
2.02
Air is a mechanical mixture of gases, not a chemical
compound. Dry air, by volume, is more than 99 per cent
composed of nitrogen and oxygen (Table 2.1). Rocket
observations show that these gases are mixed in remark-
ably constant proportions up to about 100 km altitude.
Yet, despite their predominance, these gases are of little
climatic importance.
‡Krypton
Kr
0.0011
‡Xenon
Xe
0.00009
§Methane
CH
4
0.00017
Notes: * Decay products of potassium and uranium.
† Recombination of oxygen.
‡ Inert gases.
§ At surface.
