An Introduction to Environmental Chemistry

3.2 Composition of the atmosphere

Bulk composition of the atmosphere is quite similar all over the Earth because

of the high degree of mixing within the atmosphere. This mixing is driven in a

horizontal sense by the rotation of the Earth. Vertical mixing is largely the

product of heating of the surface of the Earth by incoming solar radiation. The

oceans have a much slower mixing rate, but even this is sufficient to ensure a

relatively constant bulk composition in much the same way as the atmosphere.

However, some parts of the atmosphere are not so well mixed and here quite

profound changes in bulk composition are found.

The lower atmosphere, which is termed the troposphere (Fig. 3.2), is well

mixed by convection. Thunderstorms are the most apparent of the convective

driving forces. Temperature declines with height in the troposphere (Fig. 3.2);

solar energy heats the surface of the Earth and this in turn heats the directly over-

lying air, causing the convective mixing. This is because the warmer air that is in

contact with the surface of the Earth is lighter and tends to rise. However, at a

height of some 15–25 km, the atmosphere is heated by the absorption of ultra-

32 Chapter Three

Hydrosphere 2.4

Crust

Core

1900

Atmosphere 0.005

Crust 24

Mantle

4000

Fig. 3.1Relative sizes of the major reservoirs of the Earth. Units, 10^24 g.