Deposition of decay products on surfaces

Decay products are formed in air by the decay of their precursors. They are removed by three processes  

Ventilation should strictly be considered as exchange between indoor and outdoor air, but the concentrations indoors are usually much higher, and no great error is made in discussing indoor concentrations if the input from outside is neglected. 

Attachment of decay products to nuclei greatly affects the process of deposition, because the Brownian diffusivity of nuclei is typically about four orders of magnitude less than the molecular diffusivity of unattached decay products. The lifetime of decay products in air before deposition on surfaces is shorter if the air is clean than if it is dirty. 

Air movement indoors is much slower than outdoors, but it is usually enough to ensure that concentrations are fairly uniform in a room. Convection from heating appliances gives air speeds typically in the range 0.05-0.5 m s-1 (Daws, 1967). However, to undergo deposition, vapour molecules or particles must be transported across the boundary layer, typically a few millimetres thick, of almost stagnant air over surfaces. This may be achieved by sedimentation, molecular or Brownian diffusion, or under the action of electrostatic or thermophoretic forces. 

The velocities of deposition of attached and unattached decay products are the fluxes divided by the concentrations 

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