M M

where M is the quantity of the material in the atmospheric reservoir, while F and R are its formation (source strength) and removal (sink) rates, respectively. Since it can be demonstrated that under equilibrium conditions the residence time is equal to the turn-over time (Bolin and Rodhe, 1973), only the more common residence time is used in the following.

The concept of the residence time gives a good basis for classifying the atmospheric gases. Thus on the basis of the differences in residence time Junge (1963) proposed the following classification:

(1) highly variable gases with a residence time of several days or weeks (generally less than 1 year);

(2) variable gases with a residence time of the order of years, and

(3) quasi-permanent gases the residence time of which is much longer (generally longer than 103 years).

It is to be noted that the relative and absolute quantities of the present quasipermanent gases were variable during geological times. This means that their concentration can only be considered loosely as permanent if we study the atmospheric phenomena on a time scale short compared to the age of the Earth. Even at the present time only the noble gases (except helium) are strictly permanent since they have practically no sources and sinks. In this sense we cannot speak about their atmospheric cycles.

The residence time of atmospheric gases has a very important characteristic: the longer the residence time of a given constituent the smaller the variability of its concentration in time and space. This rule based on atmospheric observations was recently verified theoretically (Junge, 1974).

Table 1 gives the concentration and residence time of atmospheric gases on the basis of the data of different authors. In the table the concentrations of quasipermanent and variable gases are expressed partly in parts per hundred (percent by volume) and partly in parts per million (ppm). In the case of some highly variable gases the concentrations are also expressed in /ig/m3 STP because these units are widely used in atmospheric chemical measurements. (For the conversion of ppm concentrations into /ig/m3 values see Table 4, p. 30).

Table 1

Composition of unpolluted air near the Earth's surface and residence time (t) of different gases in the atmosphere. In the case of permanent gases the data were taken fromGluekauf (1951) and Junge (1963 and 1974) Concerning data sources for other gases the reader is referred to the corresponding section of the book

Table 1

Composition of unpolluted air near the Earth's surface and residence time (t) of different gases in the atmosphere. In the case of permanent gases the data were taken fromGluekauf (1951) and Junge (1963 and 1974) Concerning data sources for other gases the reader is referred to the corresponding section of the book

Gas

Formula

/„ by volume

ppm1

/igm J STP2

0 0

Post a comment