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gives the numerical factors to recalculate one into the other in case of different gases. It goes without saying that pphm and ppb values can easily be calculated from concentrations expressed in ppm.

In some references the concentration of a trace material is given in mass mixing ratio. The dimension of this concentration is g g~ \ g kg"1 or fig kg "', where 1 /jg = 10"6 g. By taking into account the air density, these figures can easily be converted to mass concentrations. Concerning this dimension, it is to be noted that mg kg"1 values can also be called ppmm (m: mass), which is widely used in precipitation chemistry studies (see Chapter 5) since for water 1 kg~ 1 litre.

It may be helpful for the reader that an indication is given here of the organization of the remainder of this book. In this chapter we will discuss the atmospheric cycle of all trace constituents including the particulate matter formed by transformation of gaseous compounds (organic vapours, gaseous nitrogen and sulfur compounds) by chemical and physical processes in the air. Later we shall present separately (see Chapter 4), in a coherent way, the physical and chemical properties, as well as the behaviour of atmospheric aerosol particles. Furthermore, in addition to chemical reactions in the air and to biological sinks discussed in this chapter, the so-called dry and wet removal processes also play an important part in the control of the atmospheric fate of some constituents (especially those nitrogen and sulfur compounds soluble in water). Although these sink terms will also be given in relation to the cycle of nitrogen and sulfur compounds, the readier is referred to Chapter 5 for further details.

Finally, it should be emphasized that in the present volume we do not deal with the local air pollution2 of cities and industrialized areas. This is done by textbooks pn air pollution meteorology. Great attention will be devoted, however, to global air pollution, which denotes the anthropogenic modification of the atmospheric composition on a much larger scale. Generally, this will be done by the consideration of a single lumped anthropogenic source which can be compared to the strength of natural sources. This procedure makes possible to discuss in some detail the global atmospheric equilibrium of different trace materials.

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