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Manuscript Title: Code for calculating the vertical distribution of radon isotopes and their progeny in the atmosphere.
Authors: A. Lupu, V. Cuculeanu
Program title: VERDI
Catalogue identifier: ADPB_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 141(2001)149
Programming language: C++.
Computer: Pentium III (Coppermine), 750 MHz.
Operating system: Linux (Kernel 2.2.12-20/RedHat 6.1), Windows 98 (4.10.1998).
RAM: 1M words
Word size: 32
Keywords: Geophysics, Diffusion equation, Backward time centred space scheme, Radon, Radon progeny, Aerosol, Dry deposition, Rainout, Turbulent transport.
Classification: 13.

Nature of problem:
Radon (222Rn) and thoron (220Rn) are inert radioactive gases produced in the Earth's crust by decay of 226Ra and 224Ra, respectively. Transported through soil, these gases enter the atmosphere where they undergo meteorological processes. Their decay products react with trace gases and water vapors. These freshly formed clusters attach to existing aerosol particles. As a consequence of radioactive decay, a cluster may desorb from its host by recoil. All progeny, whether attached or not, are removed from the atmosphere by dry deposition, rainout and washout. Assuming horizontal homogeneity and gradient transport theory valid, processes taking place in the atmosphere are modeled into a set of coupled one-dimensional time-dependent diffusion equations which, in the mean, can provide a working approximation of the boundary layer concentration activities of radon isotopes and their decay products as a function of time.

Solution method:
The set of coupled diffusion equations is differenced by using a backward time centred space scheme. In order to have sufficiently fine resolution near the surface, a logarithmic scale is chosen for the vertical coordinate. The resulted tridiagonal system is solved at each timestep by the method of backsubstitution.

Restrictions:
When condition of homogeneity does no longer hold (e.g., near large bodies of water, where advection may have important effects on the activity concentrations), the one-dimensional model will fail. Moreover, due to limitations of gradient transport theory, the model cannot adequately describe non-local events (e.g., free convection).

Running time:
Running times for the test runs described in Appendix A are: ~0.05 s per nuclide for the steady-state cases and ~3.6 s per nuclide and hour of simulation for the time-dependent case on a Pentium III 750 MHz PC running Linux.