21^ TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



For comparable wind-speed increments, tracer resuspension rates were nearly 

 independent of time the tracer was on tlie ground surface. However, it is often assumed in 

 theoretical modeUng that particles become less available for resuspension with time. The 

 assumption in these models is that pollutant particles become fixed or attached to soil 

 particles and subsequently "migrate" into the ground surface. This process is called 

 weathering but is poorly understood. 



The independence of wind-caused resuspension rates with time is a significantly 

 different observation than some others have made. Literature on radioactive resuspension 

 indicates that airborne radioactivity concentrations decrease with a weathering half-life of 

 35 days (Anspaugh et al., 1969; Wilson, Thomas, and Stannard, 1961). In contrast, if 

 there is a weathering half-life for the controlled tracer experiments described above, the 

 weathering half-life must be on tlie order of years. Some differences in reported 

 weathering might be explained by how air samples were collected. In work reported by 

 others, air concentrations were measured continuously. In contrast, in our tracer 

 experiments air concentrations and hence resuspension rates were measured as a function 

 of wind speed. Even these differences in determining weathering half-lives illustrate how 

 poorly weathering is understood. 



Average wind-caused tracer resuspension rates are reported for both respirable and 

 nonrespirable particles in Fig. 28. In these cases respirable refers to all particles collected 

 within cascade impactors and nonrespirable refers to particles collected within cowls. 

 Nonrespirable particle resuspension rates were nearly independent of time and were of 

 the order of 10^ ^ ^ fraction resuspended per second. 



Resuspension rates for respirable particles ranged from about 10~^ ^ to 10~^ fraction 

 resuspended per second. 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 measured only for wind speeds above 1 and above 

 4 m/sec. The upper, or solid Une, portion of the respirable particle curve corresponds to 

 resuspension rates calculated for the wind samphng 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 that resuspension time 

 corresponds to the total time that cascade impactors were in the field (i.e., time included 

 for winds less than 1 and less than 4 m/sec). 



Initial Generalized Wind-Resuspension-Rate Correlation 



Guidelines based on exisfing experimental resuspension-rate data are needed for 

 estimating resuspension rates. An initial correlation (Sehmel, 1975; 1977b) was developed 

 from data reported for uranine particles resuspended from a smooth surface, ZnS from an 

 asphalt surface (Sehmel and Lloyd, 1972), submicrometer molybdenum tracer from a 

 vegetated desert soil (Sehmel and Lloyd, 1976a), and DDT from a forest (Orgill, Sehmel, 

 and Petersen, 1976; Orgill, Petersen, and Sehmel, 1976). Each of these surfaces has a 

 much different estimated aerodynamic surface-roughness height, Zq, ranging from 

 4 X 10~^ cm for the smooth surface to 1 m for a forest. Roughness height is calculated 

 from the log-Unear velocity profile and is the height at which the extrapolated velocity 

 profile reaches zero velocity. 



Ranges of measured average resuspension rates were correlated (Sehmel, 1975; 

 1977b) as a function of measured or estimated surface-roughness heiglits (zo)in Fig. 29. 

 Resuspension rates range seven orders of magnitude from 10~'° to lO"'' fraction 



