The deposition velocity of elemental iodine onto vegetation, measured by field experiments at the Julich Nuclear Research Center, can be described by a semi-empirical equation as proportional to the dry weight of vegetation per unit area, to the relative atmospheric humidity, to the friction velocity and to a biological quality factor. The fallout deposition onto moist grass surfaces is greater by more than a factor of two than the deposition onto dry grass surfaces. On clover, the deposition of elemental iodine is greater by about a factor of two than onto grass. The deposition velocity of elemental iodine onto dry grass surfaces, averaged over the grazing period, was estimated to be 2 cm/sec in the Federal Republic of Germany. However, taking into account the fact that the surface of the vegetation is periodically moist because of dew and rain, the average value of the deposition rate must be assumed to be increased to about 3 cm/sec. The deposition velocity of aerosols is lower by about a factor of 20 than that of elemental iodine. The deposition velocity of methyl iodide, measured in the laboratory, is only about 0.5% of that of elemental iodine. By field experiments the biological half-life of iodine on grass was found to be 7.5 d.
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