An Introduction to Environmental Chemistry

a long residence time, then it will have ample time to become well mixed in the

atmosphere and thus would be expected to show great constancy in concentra-

tion all around the globe. This is the case and the results of measurements are

illustrated in Fig. 3.3.

3.4 Natural sources

Since the atmosphere can be treated, on a large scale, as if it were in steady state,

we have a model that views the atmosphere as having sources, a reservoir (i.e. the

atmosphere itself) and removal processes, all in delicate balance. The sources

need to be quite stable over the long term. If they are not, then the balance will

shift. In terms of our earlier analogy, the level in the leaking bucket will change.

The best-known, and most worrying, example of such a shift is the increasing

magnitude of the CO 2 source because of the consumption of vast amounts of

38 Chapter Three

Residence time (days)

Residence time (years)

Coefficient of variation

100

100

100

101

101

10 –2

10 –4

10 –6

102

102

103

103 104

H 2 O

N 2 O

SO 2

CS 2

CO 2

CH 4

OCS H^2

O 2

Hg

COO 3

Fig. 3.3Variability of trace and other components in the atmosphere as a function of

residence time. Large coefficients of variation indicate higher variability. From Brimblecombe

(1986). With kind permission of Cambridge University Press.