166 HILL 



was determined by measurement of the 35 S activity in the leaves. Degree of 

 stomatal opening was determined with a pressure porometer. 



Uptake rates varied markedly with condition of stomata. Rates with open 

 stomates were at least six times greater than those with closed stomates. The 

 stomates were open at humidities higher than about 80%. Below this level there 

 still was a variation with humidity, and this variation was taken to reflect 

 absorption of SO2 by a film of water on the leaf surfaces. Although the external 

 (fluid dynamic) resistance was appreciable even with open stomata, the major 

 resistance was always the internal leaf resistance. The average uptake rate with 

 open stomates corresponds to a deposition velocity of 0.2 cm/sec. If the leaf 

 surface area per unit area of ground is taken into account, then the deposition 

 velocity for a canopy of barley becomes 1.5 cm/sec. 



Hill 1 1 has made measurements of the uptake of several pollutants by alfalfa 

 and oat canopies in a chamber with a floor area of 1.4 m 2 . Measurements of 

 effects of wind speed, light intensity, canopy height, and pollutant concentra- 

 tion were made. For SO2 the uptake rates in alfalfa, at a light intensity 

 corresponding to approximately half of midday sunlight, varied linearly with 

 concentration over the range 140 to 700 jug/m 3 , corresponding to a deposition 

 velocity of 2.5 cm/sec. In a field experiment with the same plants, carried out at 

 the National Reactor Testing Station in Idaho using 35 S-tagged S0 2 , deposition 

 velocities found were 20% lower, representing encouragingly close agreement 

 between chamber and field experiments. 



On the basis of the evidence just cited, nighttime uptake of S0 2 by 

 vegetation might be expected to be much reduced from daytime uptake. 

 However, Martin and Barber 12 have reported data which suggest that other 

 phenomena may be responsible for maintaining high rates at night. These 

 authors were interested in monitoring the concentration of S0 2 in the vicinity 

 of a power plant. The sampler intake, probe was inadvertently located near a 

 hedge that evidently was a sufficiently effective sink for S0 2 to give significantly 

 lower S0 2 readings than would have been otherwise expected. The hedge was 

 found to be an effective sink at night as well as in the daytime, and the 

 nighttime losses were greater at higher relative humidities, suggesting that uptake 

 by dew was important. 



Data on rates of S0 2 uptake by soils are even scantier than those for uptake 

 by vegetation. Laboratory measurements have been made on uptake by soil 

 samples, 13 but these are not readily related to field conditions. Measurements 

 exist for building materials, including limestone, 14 and Liss 1 5 has made 

 estimates of specific uptake rates in natural water bodies. For the range of pH 

 values from 4 to 9, normally encountered in the environment, Liss found the 

 major resistance to S0 2 uptake to be in the gas phase. He showed this to be true 

 whether the air and water were quiescent or in turbulent motion. Presumably 

 this would be true for uptake in films of moisture on soils. Thus, to the extent 

 that soil uptake involves absorption in water films, knowledge of the 

 atmospheric resistance may be adequate for the estimation of soil uptake rate. 



