In this chapter we will consider the sources, transformations, transport, and sinks of chemicals in the natural troposphere. Our emphasis will be on gases; aerosols will be considered in more detail in Chapter 6.

Sources

The principal natural sources of gases in the troposphere are the biosphere, the solid Earth, the oceans, and in situ formation in air from other chemical species. These sources are discussed, in turn, in the following subsections.

Biological

Even though the biosphere contains only a small fraction of the total reservoirs of most chemicals on Earth, it plays a major role in determining the abundances and transport of many gases in the atmosphere. The smells associated with flowers and other forms of vegetation provide direct evidence that biota emit chemicals into the air. Some important biological sources of trace gases that enter the atmosphere are:

• Photosynthesis in plants (Reaction (1.1)), which is responsible for virtually all of the oxygen in the atmosphere.

• Respiration (the reverse of Reaction (1.1)), which releases CO2 into the air. This can be seen, on a seasonal basis, in Figure 1.1 where the decline of CO2 in the summer months is due to its uptake by plants during photosynthesis. The rise of CO2 in winter and early spring (Fig. 1.1) is due to respiration, and the decay of leaf litter and other dead plant material. This “breathing” of the atmosphere can also be detected in diurnal fluctuations in CO2; CO2 concentrations in forests can be ~35ppmv higher at night than in the day.

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