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). The certainty of using a Vg which is representative of small particles typically 1 pm 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 ^^^Cs, The dry deposition velocities of naturally-occurring ^Be were also determined and used as a 'marker' for small particle transport processes. Resuspension factors K (m ^) for radiocaesium and Plutonium were also derived. Automated individual particle characterisation (AlP) 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 deposition collectors. Enhanced a-emitting hot-spots from surface soils and material deposited to Frisbee collectors at 1 m above ground level were isolated and characterised for size and shape using nuclear track detector film, LR-115. Dry particle deposition velocities for ^^^Cs, 239+240py and were commonly in excess of 1E-03 m s"^ 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 ms"^ for both radiocaesium and plutonium. High derived values of dry deposition velocity for radiocaesium and plutonium were to some extent influenced by air sampling artefacts. Air sampling artefacts result in an under-estimation of airborne activity for radioactivity associated with particle diameters >10 pm. The derived values of dry particle deposition velocity for ^Be were consistent with literature values of sub-micron sized particles with evidence of a seasonal spring maximum. Resuspension factors K (m'^) for radiocaesium and plutonium agreed well with literature values of weathered weapons fallout values which ranged from 2E-08 to 5E-11 (m"^) 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 Cs and Pu with wind speed, wind direction and precipitation rates were not found. Size-specific air sampling show that the majority of the plutonium was predominantly associated with the >11 lam 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 fromthe nuclear weapons programme of the late 1950's-earty 1970's. Radiocaesiumactivities were equally distributed between the < 10 ixm and > 11 ^m aerosol fractions for two out of the four runs. This suggests that historically deposited ''^^Cs and current emissions contributed to measured airborne activities. Enhanced a-emitting hot-spots 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 mis-leadingly 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 s 50 |im precludes their collection by the Pmio air sampler. The main conclusions of this work indicates the very high values of Vg sometimes measured in the field are strongly influenced by large resuspended soil particles and associated air sampling artefacts. The inclusion of large particles within the deposition flux is confirmed by size selective mass based Vg using scanning electron microscopy
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:285387 |
| Date | January 1998 |
| Creators | Evans, Ellis Induro |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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