Vehicular Resuspension Resuspension was a1so measured when a 3/4-ton truck was driven on the A 3/4-ton truck and a car were driven over ZnS tracer particles placed upon a lane of asphalt road. Resuspended tracer was measured to determine resuspension rates (Sehmel, 1973a). Results are shown in Figure 5 for particle resuspension rates at vehicle speeds of 5, 15, 30, and 50 mph. The resuspen- sion rate is the fraction of particles resuspended from the tracer lane each time the vehicle was driven down the road (fraction resuspended/pass). When a car was driven through the tracer lane, resuspension rates increased with the square of car speed from about 10-4 to 10-2 fraction resuspended/pass. This means that these resuspension rates were proportional to car-generated turbulence, When the car was driven on the lane adjacent to the tracer jane, resuspension rates were lower for each speed but increased with speed from about 10-5 to 10-3 fraction resuspended/pass. t TIT wr? As shown in Figure 6, particle resuspension rates decreased as a function of time. For these data, the tracer had been on the road for four days. Vehicle-generated resuspension rates increased ‘from about 10-5 to about 10-3 fraction resuspended/pass as vehicle speed increased from 5 to 50 mph. For both vehicles, resuspension was greater when the vehicle was driven through the tracer lane than when driven on the lane adjacent to the tracer lane. was also measured (Sehmel, 1976). Results are shown in Figure 8 along with resuspension rates from the asphatt road. Truck-caused resuspension in the cheat grass area was always less than that on the asphalt road. This decrease is attributed to the protective action of cheat grass in hindering truck-generated turbulence in reaching the ground and resuspend- T TRUCK DRIVEN-THROUGH TRACER Resuspension rates were also a function of the time tracer particles were on the asphalt road. Resuspension caused by truck passage through a cheat grass area (Figure 7) CAR DRIVEN -THROUGH TRACER: Ne wt CAR ORIVEN-BY po TRACER 104% Leelee ta ing tracer, a ewewe FRACTION OF PARTICLES RESUSPENOED FROM THE ROAG PER VEHICLE PASS TTT ITT tracer lane, Resuspension rates for truck passage increased from about 10-3 to 10-2 fraction resuspended/pass. Since resuspension rates were higher, truck-generated surface stress turbulence appears to have been much greater than for car-generated turbulence. For vehicle speeds above 30 mph, resuspension rates for car and truck passage are comparable. Resuspension from the cheat grass area decreased for truck speeds from 5 to 30 mph, This decrease iS attributed to the experimental truck speed sequence. The initial truck speed was 5 mph. Apparently, the relatively larger resuspension rate at 5 mph was caused by the most readily resuspended particles being removed from the cheat grass. In succeeding experiments at increasing truck speeds up to 15 mph, and possibly 30 mph, all readily resuspended tracer was removed from the cheat grass foliage. When truck 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. The results above indicate that particle resuspension rates are a function of both vehicle speed and the relative location of the material being resuspended. 10? yo tl 1 ee 10 a tl 10 Tracer particles at the base of the cheat grass are more protected from resuspension forces and are less readily resuspended than material on the cheat grass upper foliage. VEHICLE SPEEDO, MPH Pedestrian Resuspension FIGURE 5. Rates of Particle Resuspension Caused by Vehicle Passage Over an Asphalt Road 192 Resuspension caused by a man walking along the ZnS tracer Jane of the asphalt road was also measured (Sehmel and Lloyd, 1972). The man walked across the tracer area ina leisurely fashion; the stride and paces per second were not measured. For tracer on a 3-m-wide road Jane, the reported resuspension rate was the fraction of particles resuspended each time the person walked 193