An Introduction to Environmental Chemistry

violet radiation by oxygen (O 2 ) and O 3. The rise in temperature with height has

the effect of giving the upper part of the atmosphere great stability against ver-

tical mixing. This is because the heavy cold air at the bottom has no tendency to

rise. This region of the atmosphere has air in distinct layers or strata and is thus

called the stratosphere. The well-known O 3 layer forms at these altitudes. Despite

this stability, even the stratosphere is well mixed compared with the atmosphere

even higher up. Above about 120 km, turbulent mixing is so weak that

individual gas molecules can separate under gravitational settling. Thus the

The Atmosphere 33

200

180

160

140

120

100

80

60

40

20

Altitude (km)

10 –6 10 –4 10 –2 100 102 104

Heterosphere

Homosphere

Turbopause

Mesopause

Stratopause

Tropopause

Troposphere

Stratosphere

Mesosphere

Temperature

100 200 300 400 500 600 700

2000

1000

600

200

104

Concentration (cm–3)

Atomic

oxygen

Molecular

oxygen

Atomic

hydrogen

Helium

Heterosphere

Altitude (km)

Temperature (K)

106 108

Pre

ssu

re

Pressure (Pa)

Fig. 3.2The vertical structure of the atmosphere and associated temperature and pressure variation. Note the
logarithmic scale for pressure. The inset shows gas concentration as a function of height in the heterosphere
and illustrates the presence of lighter gases (hydrogen and helium) at greater heights.