3 The atmosphere
3.1 Introduction
The atmosphere is in the news! Atmospheric chemistry has become a matter of
public concern in the last two decades. While the complexities of modern science
do not usually spark off great political and social debate, the changes in the atmos-
phere have evoked great interest. Heads of state have been forced to meetings in
Stockholm, Montreal, Kyoto and Johannesburg and given their attention to the
fate of our atmosphere. Television, which normally relegates scientific matters to
off-peak hours, has shown skilfully created colourful images from remotely sensed
measurements of the ozone (O 3 ) hole and huge emissions from forest fires of 1997
that have continued to raise concern into the current century. What has caused
this interest in the atmosphere?
The atmosphere is the smallest of the Earth’s geological reservoirs (Fig. 3.1).
It is this limited size that makes the atmosphere potentially so vulnerable to con-
tamination. Even the addition of a small amount of material can lead to signifi-
cant changes in the way the atmosphere behaves.
We should note that the mixing time of the atmosphere is very rapid. Debris
from a large accident, such as the one at the nuclear reactor at Chernobyl in 1986,
can quickly be detected all over the globe. Pollutant particles from Europe and
North America can be detected over China. This mixing, while distributing con-
taminants widely, dilutes them at the same time. By contrast, the spread of con-
taminants in the ocean is much slower and in the other reservoirs of the Earth
takes place only over geological timescales of millions of years.