Aerodynamic size distributions

Aerodynamic Size Distributions of Naturally-Radioactive Aerosols. Measurements of radionuclide distributions using cascade impactors indicate that Be-7 and Pb-210 are associated with larger aerosols than Pb-212 and Pb-214 (Robig et al., 1980 Papastefanou and Bondietti, 1986). Measurements of Pb-210 associations over oceans indicated activity median aerodynamic diameters (AMAD) near 0.6 pm (Sanak et al., 1981). The impactor measurements of Moore et al. (1980) on Pb-210, Bi-210, and Sr-90 sizes in continental air indicated that about 80% of the activity from all three nuclides was associated with aerosols below 0.3 pm. That work also determined that the mean age of aerosol Pb-210 was about a week. Knuth et al. (1983) compared Pb-210 and stable Pb sizes at a continental location and found that 78% of the Pb-210 found below 1.73 pm was smaller than 0.58 pm. Young (1974) reported that the most of the Be-7 in the atmosphere was associated with submicron aerosols. 

This paper summarizes part of the results of an investigation designed to characterize the aerodynamic size distributions of natural radioactivity and to evaluate the results in the context of sulfate distributions and recent advances in the understanding of aerosol growth mechanisms. This paper, while emphasizing our results on Pb-212 and Pb-214, also summarizes our initial data for longer-lived radionuclides. 

Pb-212 and S0% Distributions. The aerodynamic size distributions of Pb-212 and S0 were quite different, reflecting the different dependencies of surface area and volume on aerosol diameter (Friedlander, 19/7). Pb-212, like the other... 

The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35-SoJ , and stable SO4 were measured using cascade impactors. Pb-212 and Pb-214, measured by alpha spectroscopy, were largely associated with aerosols small than 0.52 11m. Based on over 46 low-pressure impactor measurements, the mean activity median aerodynamic diameter (AMAD) of Pb-212 was found to be 0.13 11m, while for Pb-214 the AMAD was larger—0.16 lim. The slightly larger size of Pb-214, confirmed with operationally different impactors, was attributed to a-recoil-driven redistribution of Pb-214 following decay of aerosol-associated Po-218. A recoil model was presented that explained this redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SoJ ... 

In general, particles or droplets in the size range 5-10 wm tend to deposit in the nasal passages. Although the extent and site of particle deposition can be estimated from a knowledge of the aerodynamic size distribution of the aerosol, the situation can be complicated by the fact that the size of the particle can increase (and possibly its density decrease) as a result of water condensation, due to the humidity change upon entering the nasal cavity. 

The aerodynamic size distribution is a key parameter affecting the regional distribution, and hence the absorption, of macromolecular drugs from the lung. The synchronization of optimum breathing and aerosol delivery appears to be a prerequisite for efficient and reproducible delivery the key parameters to control are listed in Table 1. 

Cascade impactors and cyclones have been used in order to determine the aerodynamic size distribution. Impactors allow the classification of particles with an aerodynamic diameter Dp between 0.1 pm < Dp < 5 pm, while cyclones work in the 2 to 20 pm range. 

The aerodynamic size distributions of particles contained on each stage of the impactor can be calculated providing that the densities of the particles are known. However, it is reasonable to expect that there will be a distribution of particle densities as a result of sample origin, which tends to broaden the particle distribution. This empirical calculation was checked experimentally. 

In the applications of gas-solid flows, measurements of particle mass fluxes, particle concentrations, gas and particle velocities, and particle aerodynamic size distributions are of utmost interest. The local particle mass flux is typically determined using the isokinetic sampling method as the first principle. With the particle velocity determined, the isokinetic sampling can also be used to directly measure the concentrations of airborne particles. For flows with extremely tiny particles such as aerosols, the particle velocity can be approximated as the same as the flow velocity. Otherwise, the particle velocity needs to be measured independently due to the slip effect between phases. In most applications of gas-solid flows, particles are polydispersed. Determination of particle size distribution hence becomes important. One typical instrument for the measurement of particle aerodynamic size distribution of particles is cascade impactor or cascade sampler. In this chapter, basic principles, applications, design and operation considerations of isokinetic sampling and cascade impaction are introduced. 

Where d is the aerodynamic diameter, is the particle density, and dp is its physical diameter. For a sphere, dp is the sphere s diameter for an irregular particle, dp will depend on particle shape. By definition, a water droplet with density of 1 g/cnP will have the same aerodynamic and physical diameter. It is the aerodynamic size distribution which is measured as the in vitro surrogate for inhalation aerosols. 

The calculation of average, volume-equivalent, diameters was based upon total suspended particulate mass and the total number of particles, the latter determined by a condensation particle counter. Aerodynamic size distributions were determined via sedimentation. The operational assumption in this technique is that an insignificant... 

The two main determinants for medicine deposition in the respiratory tract are the aerodynamic size distribution of the aerosol and the manoeuvre with which the aerosol is inhaled. They govern the mechanisms that are respraisible for particle deposition in the lungs. By varying the inhalation manoeuvre, not only the distribution in the airways for the same aerosol is changed in many cases also the amount and properties of the delivered fine particle dose are affected. The complex interplay between inhalation manoeuvre, aerosol properties and site of deposition has led to many misconceptions regarding the best inhaler choice for individual patients and the way these inhalers need to be operated to achieve optimal therapy for the patient. In this chapter the medicine deposition mechanisms for inhaled aerosols are explained as functions of the variables involved. In addition, the working principles of different inhaler types are described and it is discussed how their performance depends on many inhalation variables. Finally, some persistent misconceptions in the literature about the most preferable dry powder inhaler properties and performance are umaveUed. 

The powder formulation contains the active substance in the correct aerodynamic size distribution, which for most currently marketed formulations is either obtained by micronisation or by spray drying. Both techniques produce polydisperse particles and their mass median aerodynamic diameter is preferably in the range between 1 and 5 pm, depending on the precise target area. Particles within this size range are extremely cohesive, whereas the powder masses to be measured are miniscule and mostly less than 5-500 micrograms for the active substances used in asthma and COPD treatment. Such small quantities of micronised powders cannot be delivered in a reproducible way without... 

The subject of this particular volume relates to aerosol particle physics including aerosol characterisation, the formation mechanism, the aerodynamic size distribution of the activity and aerosol residence time, instrumentation techniques, aerosol collection and sampling, various kinds of environmental (atmospheric aerosols), particularly radioactive aerosols and the special case of radon decay product aerosols (indoors and outdoors) and the unattached fl ac-tion, thoron decay product aerosols, the deposition patterns of aerosol particles in the lung and the subsequent uptake into human subjects and risk assessment. 

Aerodynamic size distribution of radionuclide-associated aerosol particles (radioactive aerosols)... 

The aerodynamic size distribution of radionuclide-associated aerosol particles is, as mentioned in Section 2, a surface distribution, and so it is trimodal, the first mode being the so-called Aitken nuclei mode, the second the accumulation mode and the third the coarse particle mode. Analytically, these modes can be summarised as follows ... 

Fig. 2.1. (a) Aerodynamic size distribution of Be ambient aerosols, (b) Relative activity size distribution of Be in outdoor air. 

The aerodynamic size distributions of Pb and SO were quite different, reflecting the different dependencies of surface area and volume on aerosol diameter (Friedlander, 1977). Lead-212, like the other radionuclides, becomes associated with atmospheric aerosol particles by condensation or coagulation processes which are surface-area related. Sulfate, on the other hand, is the main solute in the accumulation mode aerosol so that its steady-state distribution is proportional to volume, even though condensation of H2SO4 may dominate its initial aerosol association. The difference also reflects residence times of 16.7 h for Pb -i- Bi and of about a week for sulfate. 

In addition to " Pb, Pb would also undergo a-recoil following the decay of " Po. Since the " Po is separated from " Pb only by jS-decays, the model parameters derived above can be used to calculate the aerodynamic size distribution of Pb which would result if condensation processes (and recoil) alone affect radioactivity sizes. 

Fig. 2.21. Aerodynamic size distribution and the fitted log-normal distribution from the cascade impactor measurement of particles resuspended from the fixed air filter samplers (FAS) filter.

Plutonium is so toxic that processing and fabrication are always done in sealed cells or glove boxes, but accidental dispersion of aerosol occurs from time to time. Cheng et al. (2004) used a Lovelace Multi-jet cascade impactor for collecting aerosols during the release of an undetermined amount of Pu02 from a glovebox system, in a room within a Plutonium Facility (PF-4) at Los Alamos National Laboratory (LANL), Los Alamos, NM. They examined the plutonium particle activity size distribution. A plot of the aerodynamic size distribution and... 

Figure 4.1 shows the mass aerodynamic size distribution of sulfate aerosols obtained by a low-pressure cascade impactor (LPI) (Bondietti and Papastefanou, 1993). In Figure 4.1 a is the frequency distribution of sulfate aerosols, in which are indicated the Aitken nuclei mode (I), the accumulation mode (II) and the coarse mode (HI), while in Figure 4.1b is the log-normal distribution of sulfate aerosols. This distribution was selected from 12 measurements made... 

Bondietti, E.A., Papastefanou, C. (1989). Large particle nitrate artifacts in the aerodynamic size distributions of ambient aerosols. J. Aerosol Sci. 20, 667-670. 

The use of an impactor with a single circular orifice for each jet provides a convenient method for determining the aerodynamic size distribution of an easily assayed aerosol, such as of a radioactive aerosol. This type of impactor is preferred when small sample rates and short sampling periods are desired since the stage characteristics can be accurately prescribed and particle bounce or re-entrainment can be minimised with the use of an appropriate liquid film on the collectors. 

Raabe, O.G. (1979). Design and use of the Mercer-style impactor for characterization of aerosol aerodynamic size distributions. In Aerosol MeasuremerU. University Presses of Florida, Gainesville, FL, pp. 135-140. 

The aerodynamic size distribution was analyzed using a Next-Generation Pharmaceutical Impactor (MSP Corporation, Shoreview, USA) in combination with a Copley Critical Flow Controller TPK and a Copley High-Capacity Pump HCP 5 (Copley Scientific Ltd., Colwick, Nottinghamshire, UK) [8],... 

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