Parameterisation of radionuclide deposition in atmospheric long-range transport modelling

Radionuclides are released into the atmosphere in the form of gases or aerosols. The aerosols have different particle sizes (0.01–60 μm). This paper analyses parameterisations of the removal processes by dry and wet deposition of radionuclides to the ground, and recommends some parameterisations and their realisation for the DERMA model as an example. It is shown that the washout coefficient strongly depends on the particle size. This dependence, however, is not included in most atmospheric dispersion models. Therefore, a revised formulation of the wet deposition parameterisation for particles of different size is suggested. The effect of gravitational settling is included in the dry deposition parameterisation by a combination of Stokes' law with the Cunningham correction for small particles and an iterative procedure solving the differential equation for the terminal settling velocity for large particles. Comparisons of simulation results for the Algeciras accident, Spain, with measurement data from the European monitoring network are presented and discussed.