162 HILL 



atmosphere, SO2 is thought to be oxidized to sulfate via a number of 

 mechanisms, including photochemical reactions in air and catalytic reactions on 

 particles or in solution in water droplets. Although much careful laboratory 

 study has been given to these reactions, considerable uncertainty remains 

 concerning the relative importance of the different reactions in the atmosphere. 

 In addition to disappearance via chemical transformation within the atmosphere, 

 S0 2 may be removed by absorption at the atmosphere— earth interface. Sites for 

 absorption include the surfaces of soil, vegetation, and water bodies. Measure- 

 ments of the concentration of SO2 in air at sites remote from industrialized or 

 urban areas have yielded values in the range < 1 to 5 jUg/m . Concentrations as 

 high as several thousand jUg/m 3 have been reported in heavily industrialized 

 areas. The residence time of S0 2 in the atmosphere is highly dependent on the 

 presence of other pollutants and hence may be a function of locale. In urban 

 areas it may be measured in hours, whereas in rural regions it may range up to a 

 week. 



Sulfates in the atmosphere are injected there as sea-salt particles from sea 

 spray or they are produced in the atmosphere as the product of the oxidation of 

 SO2 and, earlier, H 2 S. Particles generated from sea spray are generally thought 

 to be somewhat larger in size than those of anthropogenic or biogenic origin. As 

 such they are more readily subject to removal near their point of origin by 

 precipitation scavenging and dry deposition. Sea-spray sulfate is thus involved in 

 a cycle over the oceans which is almost closed. Ninety percent of it returns to 

 the oceans, and 10% is carried over land. 5 Sulfate particles of anthropogenic or 

 biogenic origin are smaller in size than particles of marine origin and are subject 

 to transport over longer distances before removal by precipitation scavenging 

 and dry deposition. The amount of such sulfate, called excess sulfate, is 

 calculated by subtracting the sea-salt sulfate present from the total. Sea-salt 

 sulfate is calculated as the product of the chloride content of the aerosol and the 

 sulfate-to-chloride ratio in seawater. The tropospheric sulfate aerosol is found in 

 concentrations in the range 0.5 to 5 Hg/m 3 in regions remote from industrial 

 emissions. The residence time of the aerosol is estimated to be of the order of 

 several days to 1 week. 



Removal of sulfur from the atmosphere by precipitation scavenging occurs 

 via two processes: rainout and washout. Rainout refers to in-cloud removal by 

 such mechanisms as condensation of droplets on sulfate particles or absorption 

 of S0 2 by cloud droplets with subsequent oxidation to sulfate. Washout refers 

 to removal by precipitation as it falls below clouds. According to a theoretical 

 study by Beilke and Georgii, 6 in polluted atmospheres where below-cloud S0 2 

 concentrations are high, washout of S0 2 can contribute the major portion of the 

 sulfate found in precipitation, whereas in rural unpolluted atmospheres rainout 

 of sulfate particles can be dominant. 



An important point to note in connection with removal of sulfur from the 

 atmosphere in precipitation is that such precipitation may be acid in nature. 

 Hydrogen ions are produced when SO2 is dissolved in water and when it is 



