inlet of the cowl either settled on the cowl floor or were drawn up into the impactor. Particles settling on the cowl floor will be called “nonrespfrable" in this report. Respirable particles entering the particle cascade impactor were separated into nominal aerodynamic diameter ranges of 7, 3.3, 2.0 and 1.7 ym, which are impactor stage 50% cutoff diameters for unit-density spheres. Smaller particles were collected on an impactor backup filter. RESULTS AND DISCUSSION This site was called sampling site A. Sampling site B was along the eastern cattle fence, and sampling site AB was between sites A and B. The distance from site A to site AB was 392 m, and from site AB to site B was 277 m. Airborne plutonium at these sites was sampled and analyzed as a function of sampling height, particle size, and wind speed. For comparison, a particle cascade impactor sample was simultaneously collected at a background site 13 km away in the mountains. Fallout levels of plutonium-239 entering the area were estimated from the cascade impactor operated in the mountains. There was no detectable plutonium-239 activity on the 7, 3.3, and 1.1-um impactor stages, and there was no radiochemical result for the 2-um stage. The only detectable background plutonium activity was on the backup filter which nominally collects submicrometer particles. Airborne plutonium-239 concentration at the back- ground station was 4 + 3.5 aCi/m3, which corresponds to 0.7 + 0.62 pCi/g of Particle resuspension rates were determined for CaMo0q, ZnS, and DOT tracer airborne soil on the backup filter. counting limits. Rocky Flats, and only total airborne radionuclide concentration was deter- Airborne plutonium-239 concentrations at the three Rocky Flats sampling stations were reported in aCi/m3 of air and pCi/g of airborne soil. The maximum airborne plutonium-239 concentration was 3700 aCi/m?. The maximum plutonium-239 concentration on the airborne soil was 50 pCi/g total airborne sotl and 69 pCi/g for the respirable fraction of airborne soil collected on the 2-um particle impactor stage. All airborne plutonium-239 concentrations were significantly less than maximum permissible concentrations of soluble plutonium-239 in air for occupational exposure in a 40-hr work week (2 x particles. In contrast to resuspension rates, only airborne concentrations as a function of wind speed were determined for plutonium resuspension at mined for the forest fire smoke. be discussed separately. Results for each set of experiments will RADIOACTIVE PARTICLES Airborne plutonium concentrations in July 1973 at Rocky Flats, Colorado were measured as a function of respirable particle diameter, wind speed, and sampling site, Total radionuclide concentration in smoke from a forest fire near Mt. St. Helens, Washington was measured on September 6, 1975. Plutonium Resuspension at Rocky Flats Plutonium resuspension at Rocky Flats was investigated experimentally. In early work, an empirical resuspension model was developed by Sehmel and Orgill (1973) based on published weekly plutonium concentrations at Health and Safety Laboratory sampling station S-8 along the site's eastern security fence. These plutonium data were analyzed in terms of the meteorology occurring during sampling times. Collected airborne plutonium was related to hourly average wind speeds and wind directions. Model results indicated that airborne plutonium concentrations increased as the 2.1 power of wind speed. Subsequently, the model was used to predict airborne concentrations for a succeeding time period. These results showed a wide difference between predictions and experimental results. The interpretation of these differences was that the plutonium resuspension source characteristics changed as a function of time (Sehmel and Orgill, 1974). Battelle-Northwest experimental measurements of plutonium resuspension at Rocky Flats were made in July 1973 (Sehmel and Lloyd, 1975a, 19764}. Airborne plutonium concentrations were measured at three sampling sites east of the plant. The first sampling site was along the eastern security fence, Error limits are the 20 radiochemical 106 aCi/m3) or nonoccupational exposure in a 168-hr week period (6 x 105 aCi/m3) (International Commission on Radiological Protection, 1959). The functional relationship between airborne plutonium resuspension concentrations and wind speed could not be developed as unequivocally as initially anticipated (Sehmel and Lloyd, 1976a}). This was due in part to the inadvertent loss of about a fifth of the collected filter samples during radiochemical analysis. Unfortunately, most samples from the hfgher wind speeds were lost. Nevertheless, some incremental wind speed results were obtained. Airborne plutonium concentrations were a function of both sampling height and particle diameter, Airborne concentrations in aCi/m? are shown for site AB in Figure 2 for each stage of the particle cascade impactors. In contrast to simple modeling concepts, airborne concentrations did not always decrease with an increase in height. There were unexpectedly high plutonium239 concentrations at this site for several particle diameters and heights. Plutonium was assoctated with particles collected on each stage of the particle cascade impactors. Since there was no plutonium in the upper stages of the impactor at the background mountain site, the plutonium-239 found in the upper stages of the impactors at the Rocky Flats sampling sites indicates that resuspension was occurring. Resuspension of submicrometer particles also occurred at Rocky Flats. 187 186