160 HILL 



Today, identification of the phenomena just described with the existence of 

 sulfur compounds in the atmosphere and in the biota goes hand in hand 

 with recognition of the circulations of the element through these reservoirs and 

 of the existence of links between the reservoirs. A continuing study of the 

 circulations and their connections may be of great importance because of the 

 now significant perturbation of the natural circulations resulting from human 

 activity. 



In the following discussion the atmospheric circulation and its links to the 

 biota will be surveyed. Interest will be centered on characterizing and 

 understanding this circulation and its inputs and outputs, especially those 

 involving interaction with the biota. Effects on the biota will be mentioned in a 

 limited way. 



ATMOSPHERIC SULFUR 



The circulation of sulfur through the atmosphere is represented schemati- 

 cally in a highly simplified form in Fig. 1. Also shown are similar representations 

 of the circulation in the biota and of the connections between the two 

 circulations. Starting with the movement through the biota, hydrogen sulfide in 

 soil and water is oxidized to sulfate ion via spontaneous reaction or via the 

 action of colorless and photosynthetic bacteria. Elemental sulfur is produced as 

 an intermediate product in some of these processes. Reduction of sulfate to H 2 S 

 occurs via two routes: by assimilatory reduction by plants and microorganisms, 

 followed by nonspecific reduction by a wide variety of bacteria, and by 

 dissimilatory reduction involving anaerobic sulfate-reducing bacteria. 



Not all the H 2 S produced by these two paths is retained in soil or water. A 

 significant fraction is thought to be emitted into the atmosphere. Once in the 

 atmosphere, H 2 S is oxidized to S0 2 and ultimately to a sulfate compound. 

 Among the processes by which S0 2 and sulfates are removed from the 

 atmosphere are two which involve important interactions with the biota: (1) the 

 absorption of S0 2 by vegetation and soils and (2) the deposition of sulfur in 

 precipitation, largely as sulfates, on soils and in water bodies in which plant and 

 animal life exists. These two processes of removal from the atmosphere and the 

 process of emission of biogenic H 2 S are the links between the two reservoirs 

 which we will consider in some detail later on. 



Origin and Fate of Atmospheric Sulfur Compounds 



A more complete but still brief description will now be given of this topic, 

 based on the recent reviews of Robinson and Robbins 2 and Kellogg et al. 

 Sulfur occurs in the atmosphere principally in the three forms already 

 mentioned: H 2 S, S0 2 , and sulfates. Included as sulfates are sulfuric acid, 

 metallic sulfates, and ammonium sulfate. 



