Interception by foliage

Measurements of activity in grass and soil in areas where no rain fell at the relevant time have been used to estimate dry deposition after the Chernobyl accident. In Denmark and in southern England, vg for137Cs was about 0.5 mm s 1 (Roed, 1987 Clark Smith, 1988). In Stockholm, however, where the Chernobyl fallout arrived several days earlier, and the particle size was larger, the dry deposition velocity of caesium was 5 mm s 1 (Persson et al., 1987). Refractory elements such as 95Zr had dry deposition velocities about 20 mm s 1. 

Whether fallout is by washout or dry deposition, a certain fraction of the deposited activity is intercepted by foliage. Foliar deposition, followed by uptake by grazing animals, is an important pathway into food chains. The leaf area of herbage eaten by cattle and sheep is large. Also, animals eat herbage as it is, whereas humans usually wash leaf vegetables and discard outer leaves, pods and husks. 

Herbage can be considered as a coarse filter interposed between atmosphere and ground. If a proportion p of the activity in fallout is intercepted by foliage, it is reasonable to write 

Mixed grasses 89Sr Spray (0.171m-2) 3.33 0.56 Milbourn Taylor (1965) 

Mixed grasses 85Sr Spray (1.2 lm 2) 2.30 0.08 Chadwick Chamberlain (1970) 

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The interception by foliage of activity in rain has been studied experimentally by Hoffman et al. (1989). Polystyrene microspheres, of diameter 3, 9 and 25 pm were labelled with 141Ce, 95Nb and 85Sr respectively. Artificial rain, with the particles in suspension, was applied to plots of clover, fescue and mixed herbage, and the fractional interception of the activity by the foliage was measured. 

The greatest amounts of fallout from Chernobyl were found in those areas where heavy rain fell as the plume arrived, and the fractional interception by foliage appears to have been low in these circumstances. Near Aberdeen the fallout of 137Cs was 2.7 kBq m-2, mostly concentrated in a heavy rainstorm on 3 May 1986 (Martin et al., 1988). Activity of 137Cs in grass, extrapolated back to the time of deposition, was 800... 

Table 2.15. Interception by foliage of particulate activity deposited in rain (analysis of results of Hoffman et al., 1989)...

Smith, S., T. Reagan, G.H. Willis, J.L. Flynn, and D.C. Rester. 1989. Fenvalerate interception by and dissipation from sugarcane foliage as affected by application technology. Bull. Environ. Contam. Toxicol. 42 30-36. 

FIGURE 10.8 Changes in the percentage of photosynthetically active radiation intercepted by the foliage of Columbia and Violet de Rennes at 51°58 N. (After Spitters, C.J.T. et al., in Topinambour (Jerusalem Artichoke), Report EUR 11855, Grassi, G. and Gosse, G., Eds., Commission of the European Communities, Luxembourg, 1988, pp. 37-43.)... 

Adsorption and persistence in plants can be modified by other chemicals or by selected carriers, although mechanisms to account for these phenomena are unclear. The application mixture influences adsorption and persistence of fenvalerate. For example, interception and persistence in sugarcane were increased when fenvalerate was applied in a 25% water/75% soybean oil mixture vs. water or soybean oil alone (Smith et al. 1989). Also, biocidal properties of fenvalerate residues on cotton foliage were increased up to 100% due to enhanced persistence of fenvalerate in the presence of toxaphene (Brown et al. 1982). 

In a modeling study of Jerusalem artichoke productivity, foliage dry matter content was found to be a linear function of annual intercepted radiation (Allirand et al., 1988). Yearly productivity variations in France, in terms of biomass, were also explained in great measure by differences in the absorption of solar radiation (Barloy, 1988b). 

Cloud droplets are typically far more acidic than precipitation droplets collected at the ground. In essence cloud drops are small and have not been subjected to the dilution associated with growth to the size of raindrops, snowflakes, and so on, nor the neutralization as sociated with the capture of surface-derived NH3 and alkaline particles held in layers at lower altitudes. Interception of these droplets therefore provides a route by which concentrated solutions of sulfate and nitrate can be transferred to foliage in high-elevation areas that are exposed to clouds. Only limited areas of the eastern part of the United States are frequently exposed to such deposition, but for these sensitive areas cloud interception is an important acid deposition pathway. 

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