Modeling of Sea-to-Land Transfer of Marine Aerosol
In this application FACSIMILE is being used to simulate the cumulative mass transfer of radionuclides from the Eastern Irish Sea to land due to marine aerosols. A number of the physical mechanisms are generic to the transport of various species by aerosols; for example the occupational risk due to aerosols containing DNA or pathogens.
Radionuclides discharged into the sea may return to land in the marine aerosol. The extent and significance of this transfer around the Sellafield area has been assessed using FACSIMILE.
Measurements of radioacdvity in Cumbria have shown the existence of a maritime effect in which radionuclides, originally discharged into the sea from Sellafield, are resuspended from the sea surface and returned to the land in the marine aerosol, as shown schematically in the Figure. The transfer causes enhanced deposition of some radionuclides along the coast and 239+24OPu deposits ten times 1-dgher than those expected from nuclear weapons fallout have been measured.
The calculated concentrations of radioactivity in the coastal grid cells of a separate marine model, also modelled using FACSIMILE (Marine Dispersion Application Sheet) have been used as a source term for aerosol generation from the Eastern Irish Sea. Continuous measurements of annual deposition due to marine aerosols have been made at Eskmeals, a site 20km south of Sellafield and 300m inland, since 1978. The values were measured at varying distances from the coast. Deposition at Eskmeals is dominated by large spray droplets produced in the surf zone, but air concentrations also contain a component of small spray droplets produced off-shore and capable of travelling long distances. The distance travelled by the individual droplets is strongly determined by seasonal meteorological conditions such as temperature and wind velocity.
The FACSIMILE model, which simulates this phenomenon, contains expressions for the suspension, transport and final deposition of the aerosol. The model uses 2-D K-theory for simulating the advection/diffusion of the aerosol. The lateral advection in the x-z slice is determined by adding the contribution from all wind directions whilst the vertical motion of the aerosols is primarily determined by diffusional terms. Final deposition of the aerosol is modelled by three mechanisms: dry deposition, gravitational settling and washout.
The simulation is run to integrate over time the accumulation of aerosol and the entrained radionulides at various distances inland.
FACSIMILE is a powerful tool for this sort of problem for three particular reasons:
- it simplifies the coding of the problem and conditions
- it is purpose designed for modelling physical/chemical systems with time dependent interactive mechanisms with advection
- it is computationally efficient.