276 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



speed was subsequently increased from 30 to 40 mph, resuspension per pass also 

 increased. Apparently increased air turbulence at the base of the cheat grass increased 

 resuspension rates. 



Pedestrian Resuspension. Resuspension caused by a man walking along the ZnS 

 tracer lane of tlie asphalt road was also measured (Sehmel, 1977b; Sehmel and Lloyd, 

 1972). The man walked across the tracer area in a leisurely fashion; the stride and paces 

 per second were not measured. For tracer on a 3-m-wide road lane, the reported 

 resuspension rate was the fraction of particles resuspended each time the person walked 

 down the length of tlie tracer lane. With wind speeds of 3 to 4 m/sec, pedestrian-caused 

 resuspension rates were from 1 x 10~^ to 7 x lO""* fraction resuspended per pass along 

 the tracer lane. This pedestrian-generated resuspension was greater than wind resuspen- 

 sion during the experiment. 



Wind-Caused Resuspension. Experimental values of wind-caused resuspension rates 

 of tracer particles from environmental surfaces have not been experimentally determined 

 from mass balance techniques other than for the present data (Orgill, Petersen, and 

 Sehmel, 1976; Orgill, Sehmel, and Petersen, 1976; Sehmel, 1975; 1977b; Sehmel and 

 Lloyd, 1972; 1976a; 1976c). Some data were initially obtained using S-^tm mass-median- 

 diameter (MMD) ZnS particles and average wind speeds from 1 to 5 m/sec. More 

 extensive data as a function of wind speed were obtained using submicrometer CaMo04 

 particles. Average resuspension rates for ZnS particles were measured for resuspension 

 from an asphalt surface (Sehmel and Lloyd, 1972) and a cheat grass surface (Sehmel. 

 1976c). For average wind speeds of 1 to 4 m/sec, wind resuspension rates from an asphalt 

 surface ranged from 5 x 10 ^ to 6 x 10^^ fraction resuspended per second. For average 

 wind speeds of 1 to 5 m/sec, wind resuspension rates from a cheat grass surface ranged 

 from 5 X 10"^ to 6 X 10"^ fraction resuspended per second. 



Wind-caused resuspension was measured for submicrometer CaMo04 particles 

 deposited in a liglitly vegetated area on the Hanford area. Tracer particles were deposited 

 in a circular area of 23-m radius around a centrally located air-sampling tower. 

 Resuspended particles were measured at the tower as a function of wind-speed increments 

 for respirable particle diameters and at all wind speeds for nonrespirable particles. 

 Respirable particles were collected within particle cascade impactors (Fig. 3), and 

 nonrespirable particles were collected by impaction and gravity settHng within cowls. 

 Resuspension rates for each size range airborne were calculated by assuming that the 

 entire tracer source was also in tliat same size range. 



Wind-caused resuspension rates for the tracer— host soil particles as resuspended are 

 sliown in Fig. 27 as a function of wind speed. Resuspension rates ranged from about 

 10 '^ to 10"^ fraction resuspended per second. Different functional dependencies of 

 resuspension rates on wind speed can be obtained from these data, depending on which 

 set of wind-speed increments is used. During the January to February period, air sampling 

 was for large wind-speed increments; in subsequent experiments wind-speed increments 

 were smaller. The straight lines shown in Fig. 27 were drawn through all data points. In 

 these cases resuspension rates increased with the 1.0 to 4.8 power of wind speed. 

 However, when only data points for smaller wind-speed increments were used, 

 wind-caused resuspension rates increased with wind speed to the 4.8 power for 7-, 3.3-, 

 2.0-, and l.l-/.tm-diameter particles as well as for the smaller particles collected on the 

 cascade impactor backup filter. 



