Equivalent aerodynamic diameter

Besides mass concentration, atmospheric particles are often characterized by their size distribution. Aerosols are typically sized in terms of the aerodynamic equivalent diameter (dae) of the particle, usually expressed in micrometer (pm) or nanometer (nm) (Mark, 1998). Atmospheric particles are usually nonspherical and with unknown density. Therefore, the r/ae of a particle is usually defined as the diameter of an equivalent unit density sphere (p = 1 gctrf3) having the same terminal velocity as the particle in question (Mark, 1998 Seinfeld and Pandis, 1998). 

Can failures occur from time to time. The release of fission products from them depends on the temperature and type of fuel. If the fuel is uranium metal, as in the Windscale and Magnox reactors, and the can fails, the uranium will oxidise in air or C02. In laboratory experiments, the mass median aerodynamic equivalent diameter (MMAD) of the particles produced by oxidation of uranium increased from about 40 ptm when the temperature of oxidation was 600°C to 500 jum at 1000°C (Megaw et al., 1961). At high temperature, a coherent sintered oxide layer formed on the uranium and this hindered the formation of particles. 

Biological, chemical, and physical effects of airborne metals are a direct function of particle size, concentration, and composition. The major parameter governing the significance of natural and anthropogenic emissions of environmentally important metals is particle size. Metals associated with fine particulates are of concern particles larger than about 3-fjim aerodynamic equivalent diameter are minimally respirable, are ineffective in atmospheric interactions, and have a short air residence time. Seventeen environmentally important metals are identified arsenic, beryllium, cadmium, chromium, copper, iron, mercury, magnesium, manganese, nickel, lead, antimony, selenium, tin, vanadium, and zinc. This report reviews the major sources of these metals with emphasis on fine particulate emissions. 

Atmospheric particles have spherical equivalent diameters (Dp) ranging from 1 nm to 100 pm. Plots of particle number concentration (as well as surface area and volume) as a function of particle size usually show that an atmospheric aerosol is composed of three or more modes, as illustrated in Figure 1. By convention, particles are classified into three approximate categories according to their size Aitken (or transient) nuclei mode (Dp <0.1 pm), accumulation mode (0.1 < Dp < 2.5 pm), and coarse mode (Dp > 2.5 pm) (Seinfeld and Pandis 1998). Particles smaller than 2.5 pm are generally classified as fine. The terms PM2.5 and PMio refer to particulate matter with aerodynamic equivalent diameters under 2.5 and 10 pm, respectively. These terms are often used to describe the total mass of particles with diameters smaller than the cutoff size. 

Airborne PM samples with an aerodynamic equivalent diameter of less than 10 microns (PMio) were collected with a high-volume sampler (DHA-80, Digitel) on cellulose nitrate filters with a diameter of 150 mm and a pore size of 3 pm. The total volume of air sampled for the collection of PM was about 4,500 m for the rural locations and 1,400 m for the sampling location in the city of Frankfurt am Main. 

Milling. Both processes involve milling. The milling requirement of PUO2 particulates for the ceramic process is less than 20 pm, which is comparable to the 10 pm nominal requirement for the MOX fuel process. For plutonium oxide, particulate less than about 3 pm (corresponding to an aerodynamic equivalent diameter of 10 pm) are respirable. Thus under similar conditions, the potential inhalation dose associated with a spill accident of plutonium oxide powder for the ceramic process is no worse than that for the MOX fuel process. 

Particles up to about 10 pm aerodynamic equivalent diameter (AED) in size are respirable and can reach deeper regions of the lung, where clearance times may be long. Particles between 10 pm and 100 pm AED are of little concern for the inhalation pathway, but they can contribute to other exposure pathways after deposition. Particles greater than 100 pm AED deposit very quickly. While this could lead to a localized contamination in the immediate vicinity of the accident, it would not represent a significant mechanism for internal exposure. 

The impaction of spherical particles depicted in Figure 9.1 depends on the fluid properties and the particle diameter and density as described by the Stokes number. Those particles with an aerodynamic diameter larger than a well-defined cutoff size will collide with an impaction plate due to their larger inertia. Smaller particles will follow the flow field and pass by the plate. The impaction of nanoflbers, however, depends on their length in addition to diameter and density. Cheng et al. [14] have outlined a theoretical approach to this and confirmed their theory via experimental studies. For nanofibers, the aerodynamic equivalent diameter is defined as... 

Inhalation is the most important route of exposure to fibers. Only some fibers can be inhaled and deposited in the respiratory tract. The respirability of fibers is largely determined by two variables density and cross-sectional area length plays only a minor role in determining respirability. Fibers with aerodynamic equivalent diameters between 5 and 10 pm can deposit in the larger airways, and those less than 5 pm can reach the terminal bronchioles and alveoli. This latter aerodynamie diameter is equivalent to a fiber diameter of approximately 3 pm however, density will also affect this value. 

Criteria pollutants are air pollutants emitted from numerous or diverse stationary or mobile sources for which National Ambient Air Quality Standards have been set to protect human health and public welfare. The original list of criteria pollutants, adopted in 1971, consisted of carbon monoxide, total suspended particulate matter, sulfur dioxide, photochemical oxidants, hydrocarbons, and nitrogen oxides. Lead was added to the list in 1976, ozone replaced photochemical oxidants in 1979, and hydrocarbons were dropped in 1983. Total suspended particulate matter was revised in 1987 to include only particles with an equivalent aerodynamic particle diameter of less than or equal to 10 micrometers (PM10). A separate standard for particles with an equivalent aerodynamic particle diameter of less than or equal to 2.5 micrometers (PM25) was adopted in 1997. 

The most commonly used equivalent diameters are aerodynamic (mainly... 

Thus, Eqs. (2) and (3) enable us to convert the volume-equivalent particle size distribution into aerodynamic-equivalent distribution for any flow regime. The characteristic mean diameters are dy (volume geometric) and (volume aerodynamic). Although dp is numerically close to the mass-median aerodynamic diameter (MMAD) often used for the aerosols, it is better defined for asymmetrical distribution, so often observed for respiratory powders. 

The aerodynamic diameter is one of the most common equivalent diameters. It can be defined as the diameter of a unit den.sity sphere with the same terminal settling velocity as the particle being measured. The aerodynamic diameter is commonly used to describe the mt)lioii of particles in collection devices such as cyclone separators and impactors. However, in shear flows, the motion of irregular particles may not be characterized accurately by the equivalent diameter alone because of the complex rotational and translational motion of inegular particles compared with spheres. That is, the path of the irregular particle may not follow that of a particle of the same aerodynamic diameter. It is of course possible that there may be a. sphere of a certain diameter and unit density that deposits at the same point this could be an average point of deposition because of the effects of turbulence or the. stochastic behavior of irregular particles. 

Classification of a particle cloud into discrete sizes using cascade impaction may be interpreted as measuring aerodynamic (equivalent spherical particle) diameter. Several impaction stages (cascade impactor) are used in the classification of a poly disperse cloud (see Figure 35). 

For larger particles or nanoparticle aggregates, SMPS measurements can be coupled with an aerodynamic particle sizer (APS). For spherical particles, it is easy to relate the measured diameters from the SMPS and APS because no corrections need to be made for shape and volume, but for irregularly shaped particles the APS reports an aerodynamic diameter. Da, by comparing the settling velocity to a spherical particle with a density of 1 g cm to compute the particle size. A volume equivalent diameter, D g, which is defined as the volume of a sphere with the same volume as a particle with an irregular shape, is used to relate the aerodynamic diameter from the APS with mobility diameter, D , from the SMPS (46) ... 

For irregularly shaped particles, the diameter of a particle may be estimated using one of several methods Feret diameter, Martin diameter, projected-area diameter, equivalent diameter, or aerodynamic diameter. The two most commonly used diameters, the Feret and Martin diameters, are relative to a randomly chosen reference line, typically the bottom edge of the viewing frame, where the Feret... 

For non-spherical particles, microscopies and light scattering can provide shape information, as discussed in Section 2.3. Shape information can also be obtained from the influence of particle shape on, for example, sedimentation. One such me asure is the dynamic shape factor (j), which is given by the square of the equivalent diameter divided by the aerodynamic diameter x = X 1-00 for a... 

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