ATMOSPHERIC SULFUR AND ITS LINKS TO THE BIOTA 167 



The foregoing discussion suggests that biometeorological factors should be 

 given further consideration in making estimates of vegetation uptake rates but 

 that much more information is needed before meaningful refinement of present 

 estimates can be undertaken. The following are but a few of the factors that 

 need study: diurnal and seasonal variation, effect of shading, kind of vegetation, 

 growth stage, variation within and between species and within individual plants, 

 canopy uptake compared with uptake by individual leaves, and stresses of 

 various kinds. 



Refinement of present global estimates may not have a major effect on the 

 overall atmospheric budget, although such refinement is desirable in principle. 

 The information that would result from further research on uptake by 

 vegetation would be perhaps more practically useful in connection with 

 evaluation of regional and local budgets and in understanding the relation of 

 uptake to injury. Injury has so far not been noted at background concentrations 

 but has been found in susceptible plants at concentrations well below those 

 presently found in urban atmospheres. 



SULFATE REMOVAL BY PRECIPITATION 

 SCAVENGING 



As mentioned earlier, almost all the sulfur in the atmosphere is believed to 

 return to the earth's surface via precipitation scavenging and dry deposition of 

 sulfates, with precipitation scavenging accounting for about 80% of the total of 

 the two processes. Also, the material removed in precipitation is often acid in 

 nature. This latter feature represents the point of interest in this discussion. 



Over the past few decades, acidity in association with high sulfate 

 concentrations has been noted in soils and groundwaters in many places in the 

 United Kingdom and in the United States and Canada. 1 6 In many instances the 

 locales in which the acidity was found were in the immediate vicinity of smelters 

 or other types of large emitters of SO2. Likewise, acidity in precipitation has 

 been noted during the same time period in close proximity to many industrial 

 centers in various parts of the world. 1 ** ' l 8 In these cases the transit time from 

 the locale of S0 2 emission to the locale of detection of acidity in soil, 

 groundwater, or precipitation was short compared to the lifetime of S0 2 in the 

 atmosphere. 



During the past 15 years, evidence has been accumulating to show that 

 acidity in precipitation has become significant at relatively great distances from, 

 but still downwind of, major industrial centers. The transit time to such 

 distances, which range up to 1000 km or more, is of the order of a day to a few 

 days, which is comparable to the lifetime of SO2 in the atmosphere. During the 

 15-year period, the acidity has steadily increased, roughly in parallel with 

 industrial growth. This evidence has been well documented by Engstrom 1 9 for 

 parts of Sweden and Norway. 



