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In thephysics ofaerosols,deposition is the process by which aerosolparticles collect or deposit themselves on solid surfaces, decreasing theconcentration of the particles in the air. It can be divided into two sub-processes:dry andwet deposition. The rate of deposition, or thedeposition 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 throughsedimentation (settling) orimpaction processes, whileBrownian diffusion has the greatest influence on small particles.^[1] This is because very small particles coagulate in few hours until they achieve a diameter of 0.5micrometres. At this size they no longer coagulate.^[2] This has a great influence in the amount ofPM-2.5 present in the air.

Depositionvelocity is defined fromF =vc, whereF isflux density,v is deposition velocity andc isconcentration. In gravitational deposition, this velocity is thesettling velocity due to thegravity-induceddrag.

Often studied is whether or not a certain particle will impact with a certain obstacle. This can be predicted with theStokes numberStk =S / d, whereS is stopping distance (which depends on particle size, velocity and drag forces), andd ischaracteristic size (often thediameter of the obstacle). If the value ofStk is less than 1, the particle will not collide with that obstacle. However, if the value ofStk is greater than 1, it will.

Deposition due toBrownian motion obeys bothFick's first and second laws. The resulting deposition flux is defined as$J=n\sqrt{\frac{D}{\pi t}}$, whereJ is deposition flux,n is the initialnumber density,D is the diffusion constant andt is time. This can be integrated to determine the concentration at each moment of time.

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.
• Gravitationalsedimentation – 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. Turbulenteddies in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
• Other processes, such as:thermophoresis,turbophoresis,diffusiophoresis andelectrophoresis.

Inwet deposition,atmospheric hydrometeors (rain drops, snow etc.) scavenge aerosol particles. This means that wet deposition is gravitational, Brownian and/or turbulent coagulation withwater 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 thatnucleation represents a loss process that has to beparametrised.

• Condensation in aerosol dynamics
• Particle collection in wet scrubbers
• Van der Waals force

• Particulates
• Aerosols

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