Aerosol From Wikipedia, the free encyclopedia Dust is one sort of aerosol which can easily be seen An aerosol is a suspension of fine solid particles or liquid droplets in a gas.[1] Examples are clouds, and air pollution such as smog and smoke.[1] In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can. Aerosols have many technological applications including aerosol sprays; dispersal of pesticides; medical treatment of respiratory illnesses and in combustion technology.[2] Aerosol science covers a wide range of topics, such as generation and removal of aerosols, technological application and their impacts on the environment and people.[1] Contents [hide] 1 Definitions 2 Size distribution 3 Physics 3.1 Terminal velocity of a particle in a fluid 3.2 Aerodynamic diameter 3.3 Dynamics 3.3.1 Coagulation 3.3.2 Dynamics regimes 3.3.3 Partitioning 3.3.4 Activation 3.3.5 Solution to the General Dynamic Equation 4 Generation 5 Detection 5.1 In situ observations 5.2 Remote sensing approach 5.3 Size selective sampling 6 Applications 7 Atmospheric 8 See also 9 References 10 Works cited 11 Further reading 12 External links Definitions[edit] Photomicrograph made with a Scanning Electron Microscope (SEM): Fly ash particles at 2,000x magnification. Most of the particles in this aerosol are nearly spherical. An aerosol is defined as a suspension of solid or liquid particles in a gas. This includes both the particles and the suspending gas, which is usually air.[1] The name aerosol is thought to have been first used by F.G. Donnan during World War I to describe clouds of microscopic particles in air. This term was an analogy to a liquid colloid suspension called a hydrosol.[3] A primary aerosol has particles that are introduced directly into the gas and secondary aerosols are formed when gas-to-particle conversion occurs.[4] There are several measures of aerosol concentration. The most important in the area of environmental science and health is the mass concentration (M), defined as the mass of particulate matter per unit volume with units such as μg/m3. Also commonly used is the number concentration (N), the number of particles per unit volume with units such as number/m3 or number/cm3.[5] The size of particles has a major influence on their properties and the aerosol particle radius or diameter (dp) is a key property used to characterise aerosols. If all the particles in an aerosol are the same size it is known as monodisperse and this type of aerosol can be produced in the laboratory. Most aerosols however are polydisperse, i.e. they have a range of particle sizes.[6] While liquid droplets are nearly always spherical, solid particles have a variety of shapes and to understand their properties, a equivalent diameter is used. The equivalent diameter is the diameter of a regular particle which has the same value of some physical property as the irregular particle.[7] The equivalent volume diameter (de) is defined as the diameter of a sphere having the same volume as that of the irregular particle.[8] Also commonly used is the aerodynamic diameter. Size distribution[edit] The same hypothetical log-normal aerosol distribution plotted, from top to bottom, as a number vs diameter distribution, a surface area vs diameter distribution, and a volume vs diameter distribution. Typical mode names are shows at the top. Each distribution is normalised so that the total area is 1000. For a monodisperse aerosol, a single number - the particle diameter - suffices to describe the size of the particles. However, for a polydisperse aerosol, we describe the size of the aerosol by use of the particle-size distribution. This defines the relative amounts of particles present, sorted according to size.[9] One approach to defining the particle size distribution is to use a list of the size of all particles in a sample. However, this approach is awkward to use so other solutions have been found. Another approach is to split the complete size range into intervals and find the number of particles in each interval. This data can then be visualised using a histogram where the area of each bar represents the total number of particles in that size bin, usually normalised by dividing the number of particles in an interval by the width of the interval and by the total number of particles so that the total area is equal to one and the area of each bar is equal to the proportion of all particles in that size range.[10] If the width of the bins tends to zero we get the frequency function:[11] where is the diameter of the particles is the fraction of particles having diameters between and + is the frequency function Therefore the area under the frequency curve between two sizes a and b represents the total fraction of the particles in that size range:[12] It can also be formulated in terms of the total number density N:[13] If we assume the aerosol particles are spherical, we then find that the aerosol surface area per unit volume (S) is given by the second moment:[13] and the third moment gives the total volume concentration (V) of the particles:[13] It can also be useful to approximate the particle size distribution using a mathematical function. The normal distribution is not usually suitable as most aerosols have a skewed distribution with a long tail of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes which is clearly not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain pollen grains and spores.[14] A more widely chosen distribution is the log-normal distribution where the number frequency is given as:[14] where: is the standard deviation of the size distribution and is the arithmetic mean diameter. The log-normal distribution has no negative values, can cover a wide range of values and fits observed size distributions reasonably well.[15] Other distributions which can be used to characterise particle size are: the Rosin-Rammler distribution, applied to coarsely dispersed dusts and sprays; the Nukiyama-Tanasawa distribution, for sprays having extremely broad size ranges; the power function distribution, which has been applied to atmospheric aerosols; the exponential distribution, applied to powdered materials and for cloud droplets the Khrgian-Mazin distribution.[16] Physics[edit] Terminal velocity of a particle in a fluid[edit] For low values of the Reynolds number (
Posted on: Wed, 19 Jun 2013 05:04:56 +0000
Trending Topics
Recently Viewed Topics
© 2015