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Deposition (Aerosol physics)
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 and wet deposition. The rate of deposition, or the deposition velocity, is slowest for particles of an intermediate size. This is because mechanisms for deposition are most effective for either very small or very large particles.
Additional recommended knowledge
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:
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:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Deposition_(Aerosol_physics)". A list of authors is available in Wikipedia.|