Deposition (aerosol physics)

Deposition velocity is defined from F = vc, where F is flux density, v is deposition velocity and c is concentration. In gravitational deposition, this velocity is the settling velocity due to the gravity-induced drag.

Often studied is whether or not a certain particle will impact with a certain obstacle. This can be predicted with the Stokes number Stk = S / d, where S is stopping distance (which depends on particle size, velocity and drag forces), and d is characteristic size (often the diameter of the obstacle). If the value of Stk is less than 1, the particle will not collide with that obstacle. However, if the value of Stk is greater than 1, it will.

Deposition due to Brownian motion obeys both Fick's first and second laws. The resulting deposition flux is defined as $J=n\sqrt{\frac{D}{\pi t}}$, where J is deposition flux, n is the initial number density, D is the diffusion constant and t is time. This can be integrated to determine the concentration at each moment of time.

Dry deposition

Dry deposition is caused by:

• Impaction. This is when small particles interfacing a bigger obstacle are not able to follow the curved streamlines of the flow due to their inertia, so they hit or impact the droplet. The larger the masses of the small particles facing the big one, the greater the displacement from the flow streamline.
• Gravitational sedimentation – the settling of particles fall down due to gravity.
• Interception. This is when small particles follow the streamlines, but if they flow too close to an obstacle, they may collide (e.g. a branch of a tree).
• Turbulence. Turbulent eddies in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
• Other processes, such as: thermophoresis, turbophoresis, diffusiophoresis and electrophoresis.