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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 micrometres. At this size they no longer coagulate. This has a great influence in the amount of PM-2.5 present in the air. ''Deposition velocity'' is defined from , where is flux density, is deposition velocity and is concentration. In gravitational deposition, this velocity is the settling velocity due to the gravity-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 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 obeys both Fick's first and second laws. The resulting deposition flux is defined as , 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: * 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). * 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. 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. Turbulent 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Deposition (aerosol physics)」の詳細全文を読む スポンサード リンク
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