effects and effects on particle momentum under these conditions (Droppo, 1976). An additional factor affecting particle deposition includes convective currents established by heat and water fluxes originating at foliar surfaces, This would again lead to a necessary modification of predictive models for the deposition process. Under aerodynamically controlled conditions, however, measured deposition rates on plant leaves agree closely with the values predicted theoretically from measured aerosol parameters (Craig et al., 1976), suggesting that either aerodynamic perturbations under field conditions or subsequent retention characteristics are the primary variables of concern. As regards retention, areas requiring attention include particle bounce and resuspension from the foliar surface as a function of absolute particle size and entrapment of particles at the surface. Although there is little information on this problem, the microtopegraphy of the leaf surface (roughness) and its chemical characteristics with respect to wetness, stickiness and charge (Holloway, 1971) must have a profound effect on the efficiency of retention at the time of impaction, These effects may be more closely related to other physicochemical properties of the particle than to its simple solution solubility. THE PROBLEM OF RETENTION OF PARTICULATES ON FOLIAR SURFACES The retention of particles on foliar surfaces is dependent on many parameters associated with the foliar surface and physical aspects of the particle itself. Leaf factors which can affect the efficiency of particle entrapment include components of the leaf which affect roughness (Holloway, 1971), namely, venation, surface features of epidermal cells, nature of the cuticle surface, nature and frequency of trichomes and the microstructure of surface wax. Each of the microtopographical features of the leaf may contribute to the entrainment and retention of particulates. Other factors affecting retention include surface stickiness (presence of organic and inorganic secretion), leaf wetness and charge attraction between particulate and surface waxes and components. In addition, retention is very dependent on particle size, particle density, wind speed within the boundary layer, and solubility when considering a particular element comprising a particle, Available information on foliar retention is sometimes disconcerting and contradictory when trying to reconcile the retention and behavior of relatively insoluble particulates with early fallout data on soluble or volatile fiassion products. Early fallout work, with respect to fission products, has been reviewed by Chamberlain (1970) and Russell (1965). In general, these reviews indicate a retention half-time For soluble fission products of 10-14 days, with losses resulting from reentrainment of carrier particles and sloughing of surface wax (Moorby and Squire, 1963), and loss through rainfall (Middleton, 1959). Except for radioiodine (Markee, 1971), such a short retention halftime is probably characteristic only of large aerosol particles, as illustrated below. Several studies have approached the problem of particulate retention using a simulated quartz fallout containing adsorbed fission products. Witherspoon and TayLor (1969) found that 88-177 um diameter ump) 1 parti-~ cles were more effectively entrained by pine foliage than oak over a 33day period. Although wind resuspension accounted for a 90% reduction in number of particulates in oak leaves after only one hour, as compared with a 10% reduction in pine, the first rainfall (t_ + 1 day) accounted for a 15% reduction of particle activity remaining at one hour. A similar study by Witherspoon and Taylor (1970) presented data for the retention of 4488 and 88-175 ym (MMD)1 particles by various agricultural plants. These studies indicate that average wind speeds of 0.5 mph over the initial 12hour period following contamination are more effective in removal of smaller particles, while average wind speeds of 1.1 mph over a 12-36 hour period resulted in a higher loss of larger particles. Subsequent to ty + 6 days, varying amounts of rainfall occurred; these resulted in a marked reduction in retention for both particle size ranges. The resuspension behavior of these particles is in keeping with theoretical and empirical measurements on the inertial forces within the boundary layer required to resuspend spores (Aylor, 1975, 1976; and Aylor and Parlange, 1975). Subsequent studies of Witherspoon and Taylor (1971) with 1-44 um (MMD)! simulated fallout particles reported longer weathering half-lives for 1-44 ym particles than those reported earlier with 44-88 and 88-175 um particles. Loss rates were also less affected by time or rainfall after a residence time of seven days. This would suggest that particle size does in fact play an important role in the extent of foliar retention. Although these studies aid in our understanding of the interception and retention of larger particles, analogous to close in fallout, questions arise as to the behavior of submicron fallout particles. Both Iranzo (1968) and Romney ef al. (1975) have reported that Pu-containing material resuspended in contaminated field situationa is difficult to remove from contaminated foliage with as much as 50% being tenaciously held on foliar surfaces. This would suggest that there are factors affecting retention other than the passive association of particles with relatively flat foliar surfaces with only inertial forces influencing their removal or resuspension as with larger particles (> 10 um). Wedding et al. (1975) have shown that deposition of 6.77 + 0.02 ym (AMAD) uranium particles is related to surface leaf roughness. By analogy, the leaf roughness factors affecting deposition should also affect retention. The effect of wind and rainfall of foliarly deposited PbC1, particles (1-3 um, MMD) was evaluated by Carison et al. (1975). These studies showed Pb particulates to remain fixed to leaves under controlled conditions for up to four weeks following fumigation; reentrainment wind speeds up to 6.7 m sec”) were ineffective in removal of surface deposits. Losses due to simulated rainfall were proportional to amount of rainfall; mists were more effective than droplets in removing foliarly deposited lead, with only 15 and 5% of the foliar deposits, respectively, being leachable. Limited data are available on the retention faces; these result from laboratory studies exposure chamber for contamination of plant 1976). Figure 1 describes the leachability THMD assumed, particles physically measured. 335 of plutonium on foliar suremploying a low windspeed canopies (Cataldo et al., of two forms of 738py dioxide