In aerosol physics, Deposition
is the process by which aerosol
particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: dry
deposition. The rate of deposition, or the deposition velocity
, is slowest for particles of an intermediate size. Mechanisms for deposition are most effective for either very small or very large particles. Very large particles will settle out quickly through sedimentation (settling) or impaction processes, while Brownian diffusion has the greatest influence on small particles. This is because very small particles coagulate in few hours until they achieve a diameter of 0.3 micrometers. At this size they don't coagulate any more. This has a great influence in the amount of PM-2.5
present in the air.
Deposition velocity is defined as F = v*c, 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 and 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*(D/πt)¹/², where J is deposition flux, n is the initial concentration, D is the diffusion constant and t is time. This can be integrated to determine the concentration at each moment of time.
Dry deposition is caused by:
- Gravitational sedimentation. This is where particles fall down due to gravitation.
- 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. Turbulentic eddies in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
- Other processes, such as: Thermophoresis, diffusiophoresis and electrophoresis.
In wet deposition, there are always some atmospheric hydrometeors which scavenge aerosol particles. This means that dry deposition is gravitational coagulation with water droplets. Different types of wet deposition include:
- Precipitation scavenging. This is where falling rain droplets collide with particles. This is also called "below-cloud scavenging".
- In-cloud scavenging. This is where aerosol particles collide with the water droplets in clouds. A common example of this type of deposition is inside fog. Clouds may also intercept with terrain (e.g. onto a mountain).
- Snow scavenging. This is where falling snow "removes" the material below it.