is often assumed in theoretical modeling that particles become less avail- able for resuspension with time. The assumption in these models is that pollutant particles become fixed or attached to soil particles and subse- quently “migrate” into the ground surface. weathering. | - 10 This process is called P | TT | TY | | TT T i yd TT TT | 4 misec xO ALL 4 1310201 f resuspension literature indicates that airborne radioactivity concentrations decrease with a weathering half-life of 35 days (Wilson et_al., 1961}. In | I WIND SPEED INCREMENT, — win : "RESPIRABLE"” rT r The independence of wind-caused resuspension rates with time is a significantly different observation than some others have made. Radioactive TT he 0 451094 — 9" 2ga F 5 4 4 contrast, tf there is a weathering half-life for the controlled tracer experiments described above, the weathering half-life must be on the order of years. Some differences in reported weathering might be explained by how air samples were collected. In early work, air concentrations were measured continuously. In contrast, in these tracer experiments air concentrations and hence resuspension rates were measured as a function of wind speed. Even these differences in determining weathering half-lives iPlustrate how poorly weathering is understood. Average wind-caused tracer resuspension rates are reported for both respi- rable and nonrespirable partictes in Figure 11. In these cases, respirable refers to all particles collected within cascade impactors while nonrespi- rable refers to particles collected within cowls. Nonrespirable particle resuspension rates were nearly independent of time and were of the order of 10-'' fraction resuspended/sec. Resuspension rates for respirable particles ranged from about 10-!1 to 10-7 fraction resuspended/sec. These resuspension rates did not decrease with time. For the first two sampling periods, resuspension was measured for all wind speeds. In succeeding experiments, resuspension rates were meaThe upper or solid sured only for wind speeds above | and above 4 m/sec. line portion of the respirable particle curve corresponds to resuspension rates calculated for the wind sampling periods. These periods correspond to wind speeds above 1 and above 4 m/sec. The lower limit of the respirable particle curve corresponds to resuspension rates calculated by assuming resuspension time corresponding to the total time that cascade impactors were in the fietd (i.e., time included for winds less than 1 and Tess than 4 m/sec), <= - _ a P L % wt 4 4 LOWER LIMIT FROM TOTAL s = | TUME IN FRELD 2 10 4 4 w 2 vt i 4 _ 4 Oe * ’ ae 100 -ll - H} Pe. . me ‘ Po ba y ‘ ‘ . o \ ra ae s s _| q ae ~\ 4 -] NON-RESPSRABLE 4 ue J HI 102? Initial Generalized Wind ResuspensionRate Correlation 0 rit 1972 J A ia or oer fort ai | J 1974 ] 0 DATE A J 1975 0 1976 Guidelines for estimating resuspension rates are needed based upon existing experimental resuspension rate data. An initial correlation was developed from data reported for uranine particles resuspended from a smooth surface (Sehmel, 1975b), ZnS from an asphalt surface (Sehmei and Lloyd, 1972), sub- micrometer molybdenum tracer from a vegetated desert soil (Sehmel and Lloyd, 1976b), and for DDT from a forest (Orgili et al., 1976). Each of these surfaces has a much different estimated surface roughness height, Z9, ranging FIGURE 11. Vegetated “Average” Wind Caused Resuspension Rates (Lightly Desert on Hanford Reservation) O1 200