The concentration of trace substances in the atmosphere is very variable in space and time and their residence time (see Table 1 ) is only some years, weeks or even less. The only exceptions are the noble or rare gases which, with the exception of helium and radon, have no sources and sinks so that we cannot speak of atmospheric cycles in this case. For this reason they are considered permanent constituents and will be omitted from further consideration.

The problems related to the water cycle will also not be considered in spite of the fact that, taking into account its quantity and atmospheric effects, water is one of the most important trace materials. This omission is explained by a historical precedent. The study of the atmospheric cycle of the water as well as the measurement of its concentration were included in the past in the program of other branches of atmospheric science. Thus, the formation of clouds and precipitation, the subject of the cloud physics (e.g. Mason, 1957, Fletcher, 1962), will only be discussed in relation with the wet removal of aerosol particles and water-soluble gases.

Furthermore it seems appropriate not to discuss radioactive materials here, but to leave them to a separate volume. The atmospheric fate of radioactive aerosols and gases is similar in some respects to that of other trace substances (e.g. dry and wet removal). However, the presentation of their formation and decay would render the present volume too diffuse. Thus, results gained by radioactive tracers will only be mentioned in Chapter 5 dealing with the removal processes. Concerning atmospheric radioactivity, the reader is referred to other textbooks (Junge, 1963; Cad le, 1966; Israël and Israël, 1973).

It should be mentioned in this introduction that atmospheric concentrations of trace constituents are sometimes given by the authors in different units. For this reason we have to discuss this problem in some detail. In Table 1 the concentration of trace gases is expressed either in ppm or in /ig/m3 STP. The ppm is a so-called volume mixing ratio1 which is equal to unity if the concentration of a certain gas in the air is 1 cm3 m~3.

1 The more exact abbreviation is ppmv (v: volume). In this book for the sake of simplicity we always use ppm for ppmv.

In the case of smaller concentrations pphm (parts per hundred million) and ppb (parts per billion; where billion = 109) can also be used. Another possibility to express the atmospheric level of trace gases and aerosol particles is the "mass concentration", which gives the mass of a substance per unit volume of air. The dimension of the mass concentration is g cm 3 or more frequently fig m"3. This quantity is a function of the temperature and pressure since the air volume in the denominator depends upon these physical parameters. Thus, it is reasonable to express its values in fig m " 3 STP (standard temperature and pressure). In this book we will use concentrations expressed mainly in ppm and in fig m"3 STP. Table 4

Table 4

Conversion factors for concentraions expressed in jig m"3 STP and ppm for different trace gases (Junge. 1963)

Conversion factor (/)

Table 4

Conversion factors for concentraions expressed in jig m"3 STP and ppm for different trace gases (Junge. 1963)

Conversion factor (/)

Gas -1

ppm = |/(gm'J

1 ¿ig m '=/ x 10 4 ppm


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