ATMOSPHERIC SULFUR AND ITS LINKS TO THE BIOTA 173 



tion scavenging, and natural emission of sulfur compounds. All these processes 

 would have to be incorporated into any model attempting to relate observed 

 acidity in precipitation to remote sources since on a regional or continental scale 

 these processes would be likely to be of importance. 2 ' Information on the 

 alkalinity of aerosols in the region would also be required. 



There will no doubt be much further interest in regional or continental 

 air-pollution models and in other means of relating sources and remote receptor 

 regions as more evidence accumulates showing their need. In this regard, Pearson 

 and Fisher 2 and Likens, Bormann, and Johnson 8 have recently reported 

 findings of acidity in precipitation in the northeastern United States. The 

 findings are similar to those described by Engstrom for Scandinavia, and there is 

 also some evidence of increases in acidity in precipitation in the region with 

 time. Likens and coworkers note that, in addition to anthropogenic sulfur 

 compounds, anthropogenic nitrogen compounds may be involved in the 

 production of acid precipitation. 



Schmidt and Velds f ' have added a note of caution in connection with the 

 process of associating changing pollutant concentrations with changes in 

 anthropogenic emission rates. They showed that falling concentrations of S0 2 in 

 the vicinity of Rotterdam over the course of a few years could better be ascribed 

 to short-term meteorological variations than to the known decreasing emission 

 rate in the area. The successful assessment of man's role in setting off the chain 

 of events leading to high levels of excess acidity in precipitation on a regional or 

 continental scale will require much further work defining the processes involved. 

 In view of the possible ecological consequences discussed by Engstrom and 

 Likens et al. and in view of the fact that relations between nations are involved, 

 the effort should be more than justified. 



EMISSION OF BIOGENIC HYDROGEN SULFIDE 



This is the least well-known component of the atmospheric sulfur budget. In 

 fact, there appears to be no direct experimental evidence that may be used to 

 estimate the magnitude of this emission. It is then not surprising that estimates 

 of the magnitude of the emission in five studies of the sulfur cycle made during 

 the past decade range over almost a factor of 3. Putting this estimate on a sound 

 footing is crucial to the understanding of the sulfur cycle and the importance of 

 man's role in that cycle. This is so since anthropogenic and biogenic sources are 

 evidently the only major net sources. If, for instance, the actual amount of H 2 S 

 emitted is considerably less than the least estimate to date, then emission from 

 man-made sources becomes virtually the single driving force for the global sulfur 

 cycle. 



A second reason exists that provides strong motivation for obtaining data on 

 H 2 S emission rates. Until recently, H 2 S was the only sulfur compound 

 mentioned in the literature on the sulfur cycle as a volatile component in the 



