In the

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

of aerosols, deposition is the process by which aerosol

particles In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

collect or deposit themselves on solid surfaces, decreasing the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number 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 Sedimentation is the deposition of sediments. It takes place when particles in suspension 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 ...

(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 0.5

micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

s. At this size they no longer coagulate. This has a great influence in the amount of

PM-2.5 Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...

present in the air. ''Deposition

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

'' is defined from , where is flux density, is deposition velocity and is

. In gravitational deposition, this velocity is the

settling velocity Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid (air is the most common example). It occurs when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of gravit ...

due to the

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

-induced drag. Often studied is whether or not a certain particle will impact with a certain obstacle. This can be predicted with the Stokes number , where is stopping distance (which depends on particle size, velocity and drag forces), and is characteristic size (often the

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

of the obstacle). If the value of is less than 1, the particle will not collide with that obstacle. However, if the value of is greater than 1, it will. Deposition due to

Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of random fluctuations in a particle's position insi ...

obeys both Fick's first and second laws. The resulting deposition flux is defined as

J = n\sqrt

, where is deposition flux, is the initial number density, is the diffusion constant and is time. This can be integrated to determine the concentration at each moment of time.

Dry deposition

Dry deposition is caused by: * 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. * Gravitational

– 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). *

Turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

. Turbulent

eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid b ...

in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations. * Other processes, such as:

thermophoresis Thermophoresis (also thermomigration, thermodiffusion, the Soret effect, or the Ludwig–Soret effect) is a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temper ...

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. Caporaloni et al. (1975) first fo ...

diffusiophoresis Diffusiophoresis is the spontaneous motion of colloidal particles or molecules in a fluid, induced by a concentration gradient of a different substance. In other words, it is motion of one species, A, in response to a concentration gradient in an ...

and electrophoresis.

Wet deposition

In wet deposition, atmospheric hydrometeors (rain drops, snow etc.) scavenge aerosol particles. This means that wet deposition is gravitational, Brownian and/or turbulent coagulation with water droplets. Different types of wet deposition include: * Below-cloud scavenging. This happens when falling rain droplets or snow particles collide with aerosol particles through Brownian diffusion, interception, impaction and turbulent diffusion. * In-cloud scavenging. This is where aerosol particles get into cloud droplets or cloud ice crystals through working as cloud nuclei, or being captured by them through collision. They can be brought to the ground surface when rain or snow forms in clouds. Within aerosol computer models aerosols and cloud droplets are mostly treated separately so that nucleation represents a loss process that has to be parametrised.

References

{{Reflist Particulates Aerosols

Dry deposition

Wet deposition

See also

References