Deposition, in physics, is a process where aerosol particles collect or deposit onto solid surfaces. It can be divided into two sub-processes: dry and wet deposition. Deposition decreases the concentration of aerosol particles in the air. The rate of deposition, or the deposition velocity is slowest for particles of intermediate size, because mechanisms for deposition are most effective for either small or large particles.

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 settling velocity due to the gravity and drag.

It is often studied whether or not the particle will impact an 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; d is characteristic size, often the diameter of the obstacle. If Stk is smaller than 1 the particle will not collide with that obstacle, and if it is much larger than 1 it will.

Deposition due to Brownian motion obeys Ficks first and second law, and the resulting depostion flux is: J=n*(D/πt)¹/², where J is deposition flux, n is concentration at the beginning, D is 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:

• Gravitational sedimentation - particles fall down due to gravitation.
• Impaction - when interfacing an obstacle, particles are not able to follow curved streamlines of the flow due to their inertia; the larger the mass of the particle, the greater the displacement from the streamline.
• Interception - small particles follow the streamlines, but if they flow very close to an obstacle, they may collide (e.g. a branch of a tree).
• Brownian diffusion - aerosol particles move randomly due to collisions with gas molecules; this may lead to collisions with obstacles or surfaces, and 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 include thermophoresis, diffusiophoresis and electrophoresis.

Wet deposition

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 - falling rain droplets collide with particles. This is also called "below-cloud scavenging".
• In-cloud scavenging - aerosol particles collide with cloud droplets (special case: inside fog) or clouds may intercept with terrain (eg. onto a mountain).
• Snow scavenging - Falling snow removes the material below it.

see also Condensation in aerosol dynamics

Particulates | Deposition