The calculation of dry particle deposition velocity (Vg) for small particles in field studies is subject to much variability between theoretical and measured values. This work will assess some of the factors which may influence the calculation of Vg. The other key parameter which is used to define the near-surface exchange of material is the resuspension factor (K) and some of the variables in the derivation of K will also be assessed. The certainty of using a Vg which is representative of small particles typically 1 um requires clarification.

During September 1993-1994, four size-selective air and deposition experiments were carried out at an on-site and off-site sampling location close to the BNFL Sellafield nuclear reprocessing plant in Cumbria UK. Dry and total (wet & dry) particle deposition velocities were determined for 137Cs, 239+240Pu and 238Pu. The dry deposition velocities of naturally-occurring 7Be were also determined and used as a ‘marker’ for small particle transport processes. Resuspension factors K (m-1) for radiocaesium and plutonium were also derived. Automated individual particle characterisation (AIP) using scanning electron microscopy coupled to energy-dispersive x-ray analysis (SEM-EDXA) was used to determine the particle size distribution and associated elemental composition of material deposited to Frisbee collectors. Enhanced alpha-emitting hotspots from surface soils and material deposited to Frisbee collectors at 1 m above ground level were isolated using nuclear track detector film, LR-115.

Dry particle deposition velocities for both 137Cs, 239+240Pu and 238Pu were commonly in excess of 1E-03 m s-1 by more than two orders of magnitude. Total deposition velocities (dry and wet) were even higher with values ranging from 0.2 m s-1 - 1 m s-1 for both radiocaesium and plutonium. Derived high values of dry deposition velocity for radiocaesium and plutonium were to some extent influenced by air sampling artifacts. Air sampling artifacts result in an under-estimation of airborne activity for radioactivity associated with particle diameters > 10 um. Values of Vg for 7Be were consistent with literature values of sub-micron sized particles with evidence of a seasonal spring maximum. Resuspension factors K (m-1) for radiocaesium and plutonium agreed well with literature values of weathered weapons fallout values which ranged from 2E-08 to 5E-11 (m-1) for plutonium and radiocaesium respectively.

Differences in dry particle deposition velocities for radiocaesium and plutonium between the on-site and off-site locations varied by no more than a factor of three. Correlations between dry particle deposition velocities for radiocaesium and plutonium with wind speed, wind direction and precipitation rates were not found. Size-specific air sampling show that most of the plutonium was associated with the > 11 um aerosol size fraction. The association between activity and large particle size suggest this material was probably attached to large soil-derived particles. Plutonium isotope data indicate this material originated from the nuclear weapons programme of the late 1950’s-early 1970’s. Radiocaesium was equally distributed between the < 10 um and > 11 um aerosol fractions for two out of the four runs. This suggests that historically deposited 137Cs and current emissions contributed to measured airborne activities. Enhanced alpha-emitting hotspots were isolated from bulk surface soils and in material deposited to Frisbee collectors. Rare particles such as these with atypical activities of Pu may lead to misleadingly high deposition fluxes. The calculation of Vg in the field is therefore sensitive to the presence of these particles because they are not representative of the aerosol flux and their size > 50 um precludes their collection by the Pm10 air sampler. Hotspot particles were typically rod-shaped and approximately 70 um long. Plutonium isotope data suggest these particles were discharged in the late 1950s-early 1970’s.

The main conclusion of this work indicate that the very high values of Vg sometimes measured in the field are strongly influenced by large resuspended soil particles and consequent air sampling artifacts. The inclusion of large particles within the deposition flux is confirmed by a novel size selective mass-based Vg using scanning electron microscopy.