Aerodynamic particle diameter

Impactors utilize aerodynamic forces to separate the various particle size classes. This is consistent with the particles aerodynamic diameter. The operation... 

Particle aerodynamic diameter The diameter of a sphere of density 1 g cm that has the same terminal velocity due to gravitational force in still air at set conditions of temperature, pressure, and relative humidity as the particle in question. 

Four, five and six rings PAHs are mainly distributed into particle size smaller than 2,0 pm. The concentration of high molecular weight PAH increased while the particle aerodynamic diameter decreased. 

Three primary mechanisms govern the deposition of aerosols in the respiratory tract inertial impaction, sedimentation, and diffusion (Fig. 7). Early work by Landahl and coworkers showed that both sedimentation and inertial impaction in the mouth, throat, and lungs uniquely depend on the particle aerodynamic diameter [220], Deposition by diffiisional transport is independent of particle density and limited primarily to particles with geometric diameters smaller than 0.5 p,m [221],... 

Figure 8 Particle deposition in the human respiratory tract as a function of particle aerodynamic diameter. (Reprinted from Ref. 226. Courtesy of Medical Physics Publishing.)...

Figure 7. Deposition Velocity vs. Particle Aerodynamic Diameter, Type Surrogate Surface Filter Paper (23-28).

The ICRP 66 model employs 10 input parameters for the determination of regional deposition fraction. These input parameters were varied according to their characteristic distributions, in an effort to perform uncertainty analysis of regional deposition fraction. The parameter particle aerodynamic diameter d (Table 27.1), is dependent on the AMAD distribution of the aerosol, and cannot be varied with respect to the uncertainty analysis. The remaining nine parameters (Table 27.1) are independent and are varied to determine the uncertainty in regional deposition fraction estimates. The resulting estimates are fitted to known distributions and described by their characteristic parameters. Statistical analyses were performed on input... 

Regardless of the method used to generate the aerosol, efficient pulmonary deposition of the active agent is critically dependent on the aerodynamic diameter of the inhaled particle. Aerodynamic diameter is the physical property of a particle, which defines how it will behave in an airstream, and depends on the particle geometric size, density, and shape. An in-depth discnssion of how particle shape affects aerodynamic diameter is beyond the scope of this review therefore, the cited examples will assume a spherical particle. 

As a negative control, an aerosol bolus of monodisperse, radiolabeled particles (aerodynamic diameter d, 3 pm) was injected at the beginning of an inhalation. Then, the particles were transported deep into the lungs and were deposited in the alveolar region. The measurements showed that less than 5% of the particles were cleared within 24 hr and the retention decreased with a half-time of about 100 days. This was also shown by other investigators, who found that particles were deposited mainly in the alveolar region (2,38,39). 

When a distribufion of particle sizes which must be collected is present, the aclual size distribution must be converted to a mass distribution by aerodynamic size. Frequently the distribution can be represented or approximated by a log-normal distribution (a straight line on a log-log plot of cumulative mass percent of particles versus diameter) wmich can be characterized by the mass median particle diameter dp5o and the standard statistical deviation of particles from the median [Pg.1428]

From the standpoint of collector design and performance, the most important size-related property of a dust particfe is its dynamic behavior. Particles larger than 100 [Lm are readily collectible by simple inertial or gravitational methods. For particles under 100 Im, the range of principal difficulty in dust collection, the resistance to motion in a gas is viscous (see Sec. 6, Thud and Particle Mechanics ), and for such particles, the most useful size specification is commonly the Stokes settling diameter, which is the diameter of the spherical particle of the same density that has the same terminal velocity in viscous flow as the particle in question. It is yet more convenient in many circumstances to use the aerodynamic diameter, which is the diameter of the particle of unit density (1 g/cm ) that has the same terminal settling velocity. Use of the aerodynamic diameter permits direct comparisons of the dynamic behavior of particles that are actually of different sizes, shapes, and densities [Raabe, J. Air Pollut. Control As.soc., 26, 856 (1976)]. 

When the size of a particle approaches the same order of magnitude as the mean free path of the gas molecules, the setthng velocity is greater than predicted by Stokes law because of molecular shp. The slip-flow correc tion is appreciable for particles smaller than 1 [Lm and is allowed for by the Cunningham correc tion for Stokes law (Lapple, op. cit. Licht, op. cit.). The Cunningham correction is apphed in calculations of the aerodynamic diameters of particles that are in the appropriate size range. 

For determination of the aerodynamic diameters of particles, the most commonly apphcable methods for particle-size analysis are those based on inertia aerosol centrifuges, cyclones, and inertial impactors (Lundgren et al.. Aerosol Measurement, University of Florida, Gainesville, 1979 and Liu, Fine Paiiicles—Aerosol Generation, Measurement, Sampling, and Analysis, Academic, New York, 1976). Impactors are the most commonly used. Nevertheless, impactor measurements are subject to numerous errors [Rao and Whitby, Am. Ind. Hyg. A.s.soc.]., 38, 174 (1977) Marple and WiUeke, "Inertial Impactors, in Lundgren et al.. Aerosol Measurement and Fuchs, "Aerosol Impactors, in Shaw, Fundamentals of Aerosol Sci-... 

Because a filter sample includes particles both larger and smaller than those retained in the human respiratory system (see Chapter 7, Section III), other types of samplers are used which allow measurement of the size ranges of particles retained in the respiratory system. Some of these are called dichotomous samplers because they allow separate measurement of the respirable and nonrespirable fractions of the total. Size-selective samplers rely on impactors, miniature cyclones, and other means. The United States has selected the size fraction below an aerodynamic diameter of 10 /xm (PMiq) for compliance with the air quality standard for airborne particulate matter. 

Airborne particulate matter, which includes dust, dirt, soot, smoke, and liquid droplets emitted into the air, is small enough to be suspended in the atmosphere. Airborne particulate matter may be a complex mixture of organic and inorganic substances. They can be characterized by their physical attributes, which influence their transport and deposition, and their chemical composition, which influences their effect on health. The physical attributes of airborne particulates include mass concentration and size distribution. Ambient levels of mass concentration are measured in micrograms per cubic meter (mg/m ) size attributes are usually measured in aerodynamic diameter. Particulate matter (PM) exceeding 2.5 microns (/i) in aerodynamic diameter is generally defined as coarse particles, while particles smaller than 2.5 mm (PMj,) are called fine particles. 

The particles most likely to cause adverse health effects are the fine particulates, in particular, particles smaller than 10 p and 2.5 mm in aerodynamic diameter, respectively. They are sampled using (a) a high-volume sampler with a size-selective inlet using a quartz filter or (b) a dichotomous sampler that operates at a slower flow rate, separating on a Teflon filter particles smaller than 2.5 mm and sizes between 2.5 mm and 10 mm. No generally accepted conversion method exists between TSP and PM,o, which may constitute between 40% and 70% of TSP. In 1987, the USEPA switched its air quality standards from TSP to PMk,. PM,q standards have also been adopted in, for example, Brazil, Japan, and the Philippines. In light of the emerging evidence on the health impacts of fine particulates, the USEPA has proposed that U.S. ambient standards for airborne particulates be defined in terms of fine particulate matter. 

PM Impingement-plate tower collection efficiencies range from 50 to 99 percent, depending upon the application. This type of scrubber relies almost exclusively on inertial impaction for PM collection. Therefore, collection efficiency decreases as particle size decreases. Short residence times will also lower scrubber efficiency for small particles. Collection efficiencies for small particles (< 1 fim in aerodynamic diameter) are low for these scrubbers hence, they are not recommended for fine PM control. 

Panicles entrained in the airstream deposit along the airway as a function of size, density, airstream velocity, and breathing frequency. Sizes of rougjily spherical or irregularly shaped particles arc commonly characterized by relating the settling velociiy of the particle to that of an idealized spherical particle. For example, an irregular particle which settles at the same rate as a 5 pm spherical particle has a mean mass aerodynamic diameter (MMAD) of. 5 pm. Since spherical particle mass, is a function of particle diameter, J... 

FIGURE 5.28 Estimated overall airway deposition as a function of initial particle size and particle hygroscopicity for particles with mass median aerodynamic diameters (MMAD) between 0.1 and 10 p.m. ° Geometric dispersion, a measure of particle size distribution, principally affects only smaller MMAD,... 

Recently, much emphasis has been put on the harmful effects of small particles, i.e., particulate matter (PM), on human health. A number of standards have been established to characterize the PM fractions in the air and their effects on human health. A widely used PM standard in force in both Europe and the United States is based on the mass concentration of particles with a diameter of 10 gm or less (PMjo). However, recently the U.S. Environmental Protection Agency (EPA) proposed a new standard that is based on the aerodynamic diameter of 2.5 gm particles. This new standard emphasizes the significant impact of small particles on human health, especially on the respiratory and cardiovascular systems. 4 ... 

PMy 5 is the concentration of the fraction of particles where at least 50% (by weight) have an aerodynamic diameter less than 2.5 pm. 

Test bench methods for machines not too large for test cabins have been developed in order to obtain comparative results. In the case of particles, the tracer gas describes well the behavior of aerodynamic diameter particles less than 5 to 10 gm. For larger particles, correction factors should be used to modify the efficiency results obtained using the tracer gas technique. 

Aerosol dynamics are based on spherical particles, a premise which almost never exists in practice. However, if there is consistency in handling the aerosol dynamics calculations, the aerodynamic diameter (see Section 13.5.2.2) that is measured gives fairly accurate predictions of aerodynamic behavior. As a result, the difference between the real shape and size of the particles and the aerodynamic shape and size is unimportant for most practical purposes. 

Aerodynamic Diameter The aerodynamic diameter of a particle is defined as that of a sphere, whose density is 1 g cm " (cf. density of water), which settles in still air at the same velocity as the particle in question. This diameter is obtained from aerodynamic classifiers such as cascade impactors. 

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