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. 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

Stokes number

The Stokes number, named after Irish mathematician George Gabriel Stokes, is a dimensionless number corresponding to the behavior of particles suspended in a fluid flow...

 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

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 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

Brownian motion

Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

 obeys both Fick's first and second laws

Fick's law of diffusion

Fick's laws of diffusion describe diffusion and can be used to solve for the diffusion coefficient, D. They were derived by Adolf Fick in the year 1855.- Fick's first law :...

. 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

Dry deposition is caused by:

• Gravitational sedimentation
  Sedimentation
  Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
  
  . 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
  Brownian motion
  Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
  
  . 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
  Turbulence
  In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
  
  . Turbulentic eddies
  Eddy (fluid dynamics)
  In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...
  
   in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
• Other processes, such as: Thermophoresis
  Thermophoresis
  Thermophoresis, thermodiffusion, or Soret effect , is a phenomenon observed when a mixture of two or more types of motile particles are subjected to the force of a temperature gradient and the different types of particles respond to it differently. The term "Soret effect" normally means...
  
  , turbophoresis
  Turbophoresis
  Turbophoresis is the tendency for particles to migrate in the direction of decreasing turbulence level. The principle tends to segregate particles entrained in high velocity gases axially toward the wall region....
  
  , diffusiophoresis and electrophoresis
  Electrophoresis
  Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
  
  .

Wet deposition

In wet deposition, there are always some atmospheric hydrometeors which scavenge aerosol particles. This means that wet deposition is gravitational, Brownian and/or turbulent coagulation with water droplets

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. 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 cloud
  Cloud
  A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
  
  s. A common example of this type of deposition is inside fog
  Fog
  Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
  
  . Clouds may also intercept with terrain (e.g. onto a mountain).
• Snow scavenging. This is where falling snow
  Snow
  Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
  
  "removes" the material below it.
• Nucleation scavenging. This is not a physical scavenging process strictly speaking. It stands for the conceptual representation of aerosol activation to cloud droplets within aerosol computer models. Aerosols and cloud droplets are mostly treated separately within computer models so that aerosol activation to cloud droplets represents a loss process that can be assimilated with aerosol scavenging.