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Nolv: 1 ppm: parts per million: I ppm- 10* ?„by volume:' /<g «10~ft g: STP: Standard temperature and pressure; 3 excluding CH4 and halocarbons.

The main constituents are designated by an asterisk

Nolv: 1 ppm: parts per million: I ppm- 10* ?„by volume:' /<g «10~ft g: STP: Standard temperature and pressure; 3 excluding CH4 and halocarbons.

The main constituents are designated by an asterisk

It should be mentioned that in the atmospheric reservoir gases other than those listed in Table 1 can be found. For the sake of simplicity the atmospheric cycle of these gases will not be presented. However, the effect of different halogen compounds, including anthropogenic chlorofluorocarbons, on the stability of atmospheric ozone layer will be discussed, in relation to the ozone budget.

The atmospheric gases can also be classified on the basis of their relative quantity. The gaseous components with concentrations larger than or equal to about 1% are called main constituents (oxygen, nitrogen and argon), while gases of smaller concentration constitute the category of trace gases. It should be noted that the air also contains minute suspended liquid and solid particles of different composition. Thus the atmosphere is a dispersed system called an aerosol. The quantity of these air-borne particles is very small compared to the mass of the air (see Chapter 4). For this reason the trace gases and aerosol particles have a common name: trace constituents.

The aim of this book is, first of all, to present the atmospheric cycle of the trace constituents. We will discuss in more detail the trace substances (Chapter 3) with relatively short residence time (<10 yr). The study of these compounds is particularly interesting since their sources and sinks as well as their concentrations are very variable in space and time. They undergo several physical and chemical transformations in the atmosphere. Among these transformations the processes leading to the formation of aerosol particles have unique importance. The aerosol particles control the optical properties of the air, the formation of clouds and precipitation and, together with some gases, the radiation and heat balance of the Earth-atmosphere system. Because of their importance the physical and chemical characteristics of aerosol particles will be summarized in a separate chapter (see Chapter 4).

A very significant peculiarity of the atmosphere is that so-called self-cleaning processes take place within the air. Due to these processes even those trace constituents which have no other chemical or biological sinks are relatively quickly removed from the air. Hence these removal mechanisms are of great importance for the control of the atmospheric pathways of some trace gases and aerosol particles (see Chapter 5).

The study of trace constituents is also very important in order to estimate the effect of the activity of mankind on their atmosheric concentration levels. Taking into account their relatively low concentrations, an anthropogenic modification of their level is much easier than in the case of the main constituents. These alterations in the atmospheric composition raise the possibility of inadvertent climate modification since, as we shall see (see Chapter 6), there is a well determined relation between the composition of the atmosphere and the Earth's climate.

However, the problems connected .with trace constituents cannot be evaluated fully without knowledge of the chemistry and budget of the main components and without a discussion of the formation of the present "permanent" chemical composition. For this reason, Chapter 2 is devoted to a brief presentation of the evolution of the Earth's atmosphere during geological times. The evolution and composition of our atmosphere will be discussed in connection with the composition of the atmospheres of other inner planets (Mars, Venus), which consist mainly of carbon dioxide. Therefore, a discussion of the variations of this gas in the Earth's atmosphere will also be presented in this chapter.

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