ATMOSPHERIC SULFUR AND ITS LINKS TO THE BIOTA 



165 



atmosphere, the vegetation canopy, the boundary layer adjacent to the leaf 

 surface, the stomatal opening into the interior of the leaf, and the water-film 

 lining of the walls of the cells in the mesophyll. For soils the deposition velocity 

 reflects the atmospheric resistances plus the resistance for absorption into a 

 water film on the surface or for adsorption onto a solid surface. The atmospheric 

 resistances are dependent on such variables as wind velocity and atmospheric 

 stability, whereas the interior leaf resistances vary with degree of stomatal 

 opening and hence with factors that affect stomatal opening, such as light 

 intensity, humidity, soil moisture, and temperature. Many other factors are 

 probably also important in determining leaf resistance. Canopy resistance is a 

 function of wind velocity and canopy structure. 



To date, estimates of the global rate of S0 2 removal by vegetation have been 

 based on the implicit assumption that the major resistance to mass transfer is in 

 the atmosphere 3 or that the effective deposition velocity is that corresponding 

 to uptake rates of shallow vegetation canopies in daylight. 2 



Experimental studies on uptake by vegetation are scarce, but the available 

 data suggest that, in the preparation of global estimates, biometeorological 

 factors should be given more emphasis. The principal results of three pertinent 

 investigations are summarized in Table 2 and discussed in the following 

 paragraphs. 



Katz and Ledingham, 9 cited by Robinson and Robbins, 2 placed fumigation 

 chambers over alfalfa in the field and measured the decrease in S0 2 

 concentration in mixtures of 2300 yiig S0 2 /m 3 of air passed through the 

 chambers. Maximum uptake by the plants and soil corresponded to a deposition 

 velocity of 1.3 cm/sec. At night the absorption rate dropped to about 10% of 

 the maximum, and for shaded plants the rate was 20% of the maximum. 



Spedding 1 has measured S0 2 uptake by barley leaves at concentration 



levels found in rural air (10 to 20jUg/m 3 ) and in urban air (100 to 150/ig/m 3 ). 



Barley plants at the three-leaf stage were exposed to a continuous flow of 



S-tagged S0 2 in air with a linear velocity of 2 cm/sec, and the rate of uptake 



TABLE 2 

 SULFUR DIOXIDE UPTAKE BY VEGETATION 



