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Naif: The emissions are expressed as sulfur

Naif: The emissions are expressed as sulfur

The next important sulfur source is the biosphere. A large part of gaseous sulfur of biological origin is hydrogen sulfide. This gas is liberated into the air because of the reduction of sulfate in anaerobic swamps, muds and eutrophic waters by microorganisms (Jaeschke et ai., 1978). This H2S may escape into the air when the anaerobic zone is close to the atmosphere. It is also shown by measurements (Rasmussen, 1974) that the release of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) is also possible due to the activity of bacteria and fresh water green and blue-green algae. Some other reduced sulfur species (e.g. carbon disulfide, see Lovelock, 1974) were also proposed to explain the biological part of the sulfur cycle. Unfortunately, the global flux produced by micro-biota is not well known. Friend (1973) gives a value of 62 x 1061 yr"1 for continental biospheric production rate. He obtains this figure by balancing the pedospheric sulfur budget. He calculates the global oceanic flux by difference, requiring the atmosphere to be balanced. By using this procedure Friend's calculation results in an oceanic production rate of 51 x 106 t yr" l. Thus, the total biological release is 106 x 1061 yr" \ calculated as sulfur.

The main problem with the calculation of biological source intensity by difference is the fact that the figure obtained varies as a function of other terms in budget equation. Thus, Granat ei ai. (1976) calculated only 32 x 106 t yr"1 (expressed in sulfur) for the natural source strength since they used smaller deposition values than Friend. It follows from this latter figure that, in contrast to Friend's conclusion, the activity of man now produces more atmospheric gaseous sulfur than natural sources do. Considering the importance of this question it is obvious that an acceptable determination of global biological sulfur release, independent of the budget, would be of crucial interest.

The third sulfur source is provided by the formation of sea salt particles at the surface of the ocean (see Section 4.2). According to Eriksson (1960) the sulfur mass in sea salt particles produced yearly is 44 x 106 t. This figure seems to be rather well established, and for this reason, we shall not discuss it in more detail.

Finally, we must quantily the annual production rate due to volcanos. According to Friend (1973), volcanic activity produces 0.8 km3 of magma per year, which is equivalent to a mass of 2200 x 106 t yr" '. The composition of gases released by volcanic eruptions is roughly the following: 95 % HzO, 4 % C02 and 1 % SOz. Since the mass of H20 vapour is 5 °/„ of that of the magma emitted, a 2 x 1061 yr"1 global volcanic sulfur emission can be calculated. This figure is in good agreement with the estimate of other workers (Kellogg ei al.. 1972;Granatef al. 1976). We can conclude that sulfur production by volcanos is quantitatively small as compared to the intensity of other sources. We must emphasize, however, that a significant fraction of this sulfur quantity, emitted into the atmosphere in a very short time25, can reach the stratosphere to contribute to the stratospheric sulfate layer (see Section 4.3). This means that we cannot neglect volcanic activity when studying the pathways of sulfur in the atmosphere.

Table 12 lists the strength of different sulfur sources as proposed by Friend (1973) and Granat a al. (1976). One can see from Friend's data that around 1/3 of atmospheric sulfur is provided by man's activity. Taking into account that

Table 12

Absolute and relative intensities of different sulfur sources according to Friend (1973) and Granat et al. (1976)

Friend

Source

[lO'tyr '] ["„ of I ol ai J [10" I yr 1 ] ["„of total]

Anthropogenic Biological (cont.) Biological (oceanic) Sea-salt

Volcanic activity

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