Aerosol science
Encyclopedia
Aerosol science is the rapidly expanding field of science which investigates the physical, chemical, and biological properties of aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

ized materials (airborne particles), which behave in ways that make them unique from even other forms of similar materials. Of particular interest are their measurement; methods of formation, transport and deposition
Deposition (Aerosol physics)
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,...

; and applications in medicine, industry, and environmental science
Environmental science
Environmental science is an interdisciplinary academic field that integrates physical and biological sciences, to the study of the environment, and the solution of environmental problems...

.

Size Distribution

Aerosols are characterized by a particle size distribution
Particle size distribution
The particle-size distribution of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size...

 function (PSD). Most natural aerosols have a lognormal distribution.

Aerosol Mechanics

  • Equation of Motion


where is particle velocity is fluid velocity is fluid viscosity is the mass of particle is particle diameter is the acceleration of gravity is Cunningham correction factor
Cunningham correction factor
The Cunningham correction factor or Cunningham slip correction factor is used to account for noncontinuum effects when calculating the drag on small particles. The derivation of Stokes Law, which is used to calculate the drag force on small particles, assumes a No-slip condition which is no longer...


Here, the buoyancy of air is neglected and Stokes drag
Stokes' law
In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a continuous viscous fluid...

 is assumed.
  • Particle relaxation time
    Relaxation time
    In the physical sciences, relaxation usually means the return of a perturbed system into equilibrium.Each relaxation process can be characterized by a relaxation time τ...

  • Particle stopping distance
  • 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...

     or Diffusion
    Diffusion
    Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...


Aerosol Dynamics

Aerosol dynamics is focus on the evolution of aerosol population.

Aerosol population is characterized by a particle size distribution
Particle size distribution
The particle-size distribution of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size...

 function (PSD). Most natural aerosols have a lognormal distribution.

Aerosol formation and growth consists of 3 processes:
  1. Nucleation
    Nucleation
    Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

  2. Coagulation
    Coagulation (disambiguation)
    Coagulation may refer to:* Coagulation, in colloid chemistry, a process in which dispersed colloidal particles agglomerate. See flocculation* Coagulation of blood* Coagulation , the coagulation of milk into curd by rennet or acid...

    /Agglomeration
  3. Condensation
    Condensation
    Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....



Aerosol dynamics is characterized by a single differential equation called as the Aerosol General Dynamic Equation (GDE).



where J is the nucleation rate, is the coagulation kernel and G is the growth rate.

The common methods to solve the GDE are:
  1. Moment method
  2. Modal/Sectional Method
  3. Quadrature Method of Moments.

Nucleation

Nucleation is the process of forming particles from a purely gaseous precursor phase.

There are two types of nucleation processes,
  1. homogenous nucleation
  2. heterogeneous nucleation.


Nucleation occurs for different reasons. Important among them, is the supersaturation of a vapor. When a hot vapor in a gas is cooled down, its supersaturation ratio increases, and it becomes supersaturated. Therefore, it "precipitates out" into particles. This process is called nucleation.

Coagulation

When particles are present in an aerosol they collide with each other. During that they may undergo coalescence or aggregation. This process leads to a change in the aerosol size distribution
function.

Condensation

Condensation is the process by which an aerosol particle grows with the accretion of monomers or individual molecules to an already existent particle.

Generation Instruments

  • Atomizer nozzle
    Atomizer nozzle
    An atomizer nozzle is an aspirator nozzle for producing a fine spray of a liquid based on the Venturi effect.-Principle of operation :When a gas is injected under pressure through a tube with a decreasing section, it speeds up, generating a pressure drop at the narrowest point .The reduced...

     or Nebulizer
    Nebulizer
    In medicine, a nebulizer is a device used to administer medication in the form of a mist inhaled into the lungs....

  • Electrospray
    Electrospray
    The name electrospray is used for a device that employs electricity to disperse a liquid or for the fine aerosol resulted in this process. The method is sometimes improperly called electrohydrodynamic atomization. High voltage is applied to a liquid supplied through an emitter...

  • Vibrating Orifice Aerosol Generator (VOAG)

Detection Instruments

  • Aerosol Mass Spectrometer (AMS)
  • Differential Mobility Analyzer (DMA)
  • Aerodynamic Particle Sizer (APS)
  • Wide Range Particle Spectrometer (WPS)
  • Micro-Orifice Uniform Deposit Impactor(MOUDI)
  • Condensation Particle Counter (CPC)
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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