Particle size aerodynamic diameter

A round jet impacior is to be designed to sample a flowing aerosol. For a maximum jet velocity of 200 m/sec and a flow rate of 1 liter/min, determine the minimum particle size (aerodynamic diameter) that can be collected. Assume that the pressure throughout the impactor is approximately atmospheric and the temperature is 20 0. Be sure to account for the slip correetton factor (Chapter 2). 

Fig. 2 Total and regional deposition of particles in the respiratory tract dependent on log particle size (aerodynamic diameter) during quite breathing of an healthy individual (based on ICRP 1994)...

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

St. Louis Sample Collection. Ambient aerosols were collected in St. Louis in 6-h Intervals with a TWOMASS automated sequential tape sampler. This sampler fractionated the aerosol into two size classes, fine particles having aerodynamic diameters less than 3pm, and coarse particles with diameters greater than 3pm. It was equipped with a beta-attenuation mass monitor to determine fine-particle mass (11). Only the fine particle filter was examined in this study. Pallflex E70 glass-fiber filter tape with a detachable cellulose backing (Pallflex Inc. Putnam, CT) was used with this sampler. An aerosol sampler operating from the same inlet manifold as the... 

The model has been used to predict the sampling efficiency of the VE for a wide range of MMAD and GSD values typical of what might be encountered in cotton textile processing. These parameter values are for the actual size distribution of particles in the sampled air, and not for those collected on the membrane filter. These results are summarized in Table II. A remarkable feature of this model is that it predicts that the VE will collect significant amounts of particles with aerodynamic diameters greater than 30 pm. 

An optimum aerodynamic particle diameter for lung delivery is considered to be 3 pm. Particles having aerodynamic size above 5 pm typically deposit in the upper part of the lung, which is not an efficient site for drug adsorption. Particles with aerodynamic diameter smaller than 1 pm can be expelled during exhalation. To deliver powders efficiently via inhalation, particles with defined size must be obtained and delivered. ... 

Ultrafine particles have been defined as those, which are smaller than 0.1 pm. Another classification is into submicrometre particles, which are smaller than 1 pm, and supermicrometre particles, which are larger than 1 pm. The terminology that has been used in the wording of the ambient air quality standards, and also for characterisation of indoor and outdoor particle mass concentrations, includes PM2.5 and PM fractions and the total suspended particulate (TSP). PM2.5 (fine particles) and PM, are the mass concentrations of particles with aerodynamic diameters smaller than 2.5 and 10 pm, respectively (more precisely the definitions specify the inlet cutoffs for which 50% efficiency is obtained for these sizes). TSP is the mass concentration of all particles suspended in the air. There have been references made in the literature to PMj or PMq 1 fractions, which imply mass concentrations of particles smaller than 1 and 0.1 pm, respectively. These terms should be used with caution, as particles below 1 pm, and even more those below 0.1 pm, are more commonly measured in terms of their number rather than their mass concentrations, and therefore these terms could be misleading. 

Particles with aerodynamic diameters between 2 and 3 pm are ideal for deep lung delivery (76). This aerodynamic diameter depends primarily on the geometric diameter and bulk density (77). Particles with bigger size ( 10 pm) and lower bulk density aerosolize better and avoid lung macrophage clearance. 

In the case of toxic particulates in air, particles with larger sizes (aerodynamic diameters of 5-30 [xm) are caught in the upper airways, while particles with smaller sizes, of 1-5 p,m and under 1 pm, can penetrate into the lower airways and blood vessels, respectively. 

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. 

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