164 HILL 



No data are available on which to base estimates of biogenic emissions. 

 Instead, transfer rates for this source are determined by difference. Thus, 

 assuming that the sulfur burden of the atmosphere is not changing on the time 

 scale of 1 year, the biogenic emission rate is set equal to the source deficit 

 usually found. More discussion of this point will be given below in the section on 

 biogenic emission. 



Anthropogenic and biogenic emissions represent the major net emissions 

 since sea-spray sulfate largely returns to the oceans. 



Precipitation scavenging and dry deposition are by far the largest removal 

 processes. Precipitation scavenging is estimated to account for 80% of the sulfate 

 removal processes. Removal of SO2 by soils and vegetation accounts for about 

 14% of the sulfur-removal processes over land according to Kellogg et al. These 

 same authors find insufficient evidence to decide whether the oceans are a sink 

 for SO2 or a source of that compound. 



Global models for the atmospheric sulfur budget have so far avoided the 

 necessity of making estimates of the rates of chemical transformations within 

 the atmosphere by dealing only with the inputs and outputs of the element 

 regardless of chemical form. When and if such rates can be adequately 

 characterized, then separate budgets can be prepared for H 2 S, S0 2 , and sulfates, 

 and the resulting models may be more useful for predictive purposes. Without 

 making this differentiation, global budgets are perhaps most useful in pointing 

 out areas in which further investigation is required. 



A further limitation on the usefulness of global sulfur models is the large 

 spatial variation in source distribution in combination with atmospheric 

 residence times that are short compared to global atmospheric mixing times. 

 These factors tend to make regional and even local models of more practical 

 interest. 



ABSORPTION OF SULFUR DIOXIDE 

 BY VEGETATION AND SOILS 



We now turn to a more detailed consideration of the links between the 

 atmosphere and the biota, and we discuss first the absorption of S0 2 by 

 vegetation and soils. The point to be made here concerns the basis for estimating 

 global removal rates and the need for more data describing uptake by these 

 processes. 



The rate of removal of S0 2 by vegetation or soils may be expressed as a 

 concentration driving force divided by an overall resistance. If the sink is 

 regarded as perfect, i.e., if the concentration at the surface is zero, then the 

 concentration in the bulk air becomes the driving force, and the reciprocal of the 

 overall resistance is called the deposition velocity, the effective velocity at which 

 a molecule of S0 2 approaches the absorption site. In the case of vegetation, the 

 deposition velocity is the reciprocal of the sum of the resistances due to the lower 



