Deposition (aerosol Physics) - Dry Deposition

Dry deposition is caused by:

• 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).
• 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.
• Diffusion or Brownian motion. This is the process by which aerosol particles move randomly due to collisions with gas molecules. Such collisions may lead to further collisions with either obstacles or surfaces. There is a net flux towards lower concentrations.
• 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.