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((Short description|Process by which aerosol particles collect onto solid surfaces))
((more citations needed|date=August 2012))

In the [[physics]] of [[aerosol]]s, '''deposition''' is the process by which aerosol [[Particulates|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 [[Aerosol impaction|impaction]] processes, while [[Brownian motion|Brownian diffusion]] has the greatest influence on small particles.<ref>((cite book  | last = Seinfeld  | first = John  |author2=Spyros Pandis  | title = Atmospheric Chemistry and Physics: From Air Pollution to Climate Change  | edition = Second  | publisher = John Wiley & Sons, Inc.  | year = 2006  | location = Hoboken, New Jersey  | isbn =  0-471-72018-6 ))</ref>  This is because very small particles coagulate in few hours until they achieve a diameter of 0.5 [[micrometre]]s.  At this size they no longer coagulate.<ref>((cite journal | last = Mishchuk | first = Nataliya A. | title = Chapter 9 - Coalescence kinetics of Brownian emulsions  | edition = D.N. Petsev | publisher = Elsevier  |  journal = Interface Science and Technology | volume = 4 | year = 2004 | pages = 351–390 | doi = 10.1016/S1573-4285(04)80011-5 | isbn =  9780120884995 ))</ref> This has a great influence in the amount of [[PM-2.5]] present in the air.

''Deposition [[velocity]]'' is defined from ((math|1='''F''' = '''v'''''c'')), where ((math|'''F''')) is [[flux density]], ((math|'''v''')) is deposition velocity and ((mvar|c)) is [[concentration]]. In gravitational deposition, this velocity is the [[settling velocity]] due to the [[gravity]]-induced [[drag (force)|drag]].

Often studied is whether or not a certain particle will impact with a certain obstacle. This can be predicted with the [[Stokes number]] ((math|1=Stk = ''S'' / ''d'')), where ((mvar|S)) is stopping distance (which depends on particle size, velocity and drag forces), and ((mvar|d)) is [[Characteristic length|characteristic size]] (often the [[diameter]] of the obstacle). If the value of ((math|Stk)) is less than 1, the particle will not collide with that obstacle. However, if the value of ((math|Stk)) is greater than 1, it will.

Deposition due to [[Brownian motion]] obeys both [[Fick's law of diffusion|Fick's first and second laws]]. The resulting deposition flux is defined as <math display=inline>J = n\sqrt{\frac{D}{\pi t))</math>, where ((mvar|J)) is deposition flux, ((mvar|n)) is the initial [[number density]], ((mvar|D)) is the diffusion constant and ((mvar|t)) is time. This can be integrated to determine the concentration at each moment of time.

== Dry deposition ==
[[Image:impaction scrub.gif|thumb|right| '''Figure 1''' – Impaction]]
[[Image:diffusion scrub.gif|thumb|right| '''Figure 2''' – Diffusion]]

'''Dry deposition''' is caused by:
* [[aerosol impaction|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 [[sedimentation]] – 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]]. Turbulent [[eddy (fluid dynamics)|eddies]] in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
* Other processes, such as: [[thermophoresis]], [[turbophoresis]], [[diffusiophoresis]] and [[electrophoresis]].

== Wet deposition ==

In '''wet deposition''', [[Precipitation (meteorology)|atmospheric hydrometeors]] (rain drops, snow etc.) scavenge aerosol particles. This means that wet deposition is gravitational, Brownian and/or turbulent coagulation with [[water|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 [[Parametrization (atmospheric modeling)|parametrised]].

== See also ==
* [[Condensation in aerosol dynamics]]
* [[Particle collection in wet scrubbers]]
* [[Van der Waals force]]

==References==
((Reflist))<!--added above categories/infobox footers by script-assisted edit-->

((Authority control))

[[Category:Particulates]]
[[Category:Aerosols]]

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