Polydisperse aerosols

Aerosol, polydisperse An aerosol with a geometric standard of deviation of size-distribution greater than 1.5. 

The kinetic theory and the diffusion theory may be used for certain aerosol distributions. However, these two theories begin to deviate from the hybrid theory as the aerosol polydispersity increases. Use of either the kinetic theory or the diffusion theory may therefore result in large errors. 

Mewhinney JA, Muggenburg BA. 1977. Patterns of dose after inhalation of monodisperse or polydisperse aerosols. In Boecker BB, Hobbs CH, Martinez BS, eds. Inhalation Toxicology Research Institute annual report Lovelace Biomedical and Environmental Research Institute. Albuquerque, NM Inhalation Toxicology Research Institute, 44-47. 

Figures 3 and 4 show the variation of the average attachment coefficient with CMD. It can be seen that for particles of CMD less than 0.06 ym and Og = 2 the kinetic theory predicts an attachment coefficient similar to the hybrid theory, whereas for CMD greater than about 1 ym the diffusion theory and the hybrid theory give approximately the same results. For a more polydisperse aerosol (Og = 3) the kinetic theory deviates from the hybrid theory even at a CMD = 0.01 ym. The diffusion theory is accurate for a CMD greater than about 0.6 ym. These results are easily explained since as the aerosol becomes more polydisperse, there are more large diameter particles (CMD >0.3 ym) which attach according to the diffusion theory. In contrast, the kinetic theory becomes more inaccurate as the aerosol becomes more polydisperse.

Figure 5 shows the variation of the hybrid theory with CMD for various Og. It is obvious that assuming an aerosol to be mono-disperse when it is in fact polydisperse leads to an underestimation of the attachment coefficient, leading in turn to large errors in calculation of theoretical unattached fraction. 

Unattached fractions of RaA (at t = °°) for two mine aerosols and for a typical room aerosol are shown in Table III. It is usually assumed that the attachment of radon progeny to aerosols of CMD < 0.1 ym follows the kinetic theory. In Table III it is apparent that the hybrid and kinetic theories predict similar unattached fractions for monodisperse aerosols. However, for more polydisperse aerosols, the kinetic theory predicts lower unattached fractions than the diffusion theory and thus the diffusion theory is the more appropriate theory to use. It is also evident that the kinetic-diffusion approximation predicts unattached fractions similar to those predicted by the hybrid theory in all cases. 

Raabe, O.G., The Adsorption of Radon Daughters to Some Polydisperse Submicron Polystyrene Aerosols, Health Phys. 14 397 (1968a). 

Both from deposition studies and force balances it can be derived that the optimum (aerodynamic) particle size lies between 0.5 and 7.5 pm. Within this approximate range many different subranges have been presented as most favourable, e.g. 0.1 to 5 pm [24], 0.5 to 8.0 pm [25], 2 to 7 pm [26] and 1-5 pm [27-29]. Particles of 7.5 pm and larger mainly deposit in the oropharynx [30] whereas most particles smaller than 0.5 pm are exhaled again [31]. All inhalation systems for drug delivery to the respiratory tract produce polydisperse aerosols which can be characterized by their mass median aerodynamic diameter (MMAD) and geometric standard deviation (oq). The MMAD is the particle diameter at 50% of the cumulative mass curve. 

Arriagada FJ, Osseoasare K (1994) Silica Nanoparticles Produced in Aerosol Ot Reverse Microemulsions - Effect of Benzyl Alcohol on Particle-Size and Polydispersity. J Dispers Sci Technol 15 59-71... 

FIGURE 2.13 Calculated deposition of particles in various regions of the lung for polydisperse aerosol (crg = 2.5 see Chapter 9.A.2) (adapted from Yeh et al., 1996). 

The variation of scattered light intensity with 0 as typified by Fig. 9.19 clearly becomes more complex as the particle size increases, with sharp oscillations seen at a 10. However, recall that this is for a spherical homogeneous particle of a fixed size and for monochromatic light (e.g., a laser) when the particle is irregular in shape, these oscillations are far less prominent. This is also true for a group of particles of various sizes, that is, a polydisperse aerosol, where the overall scattering observed is the sum of many different contributions from particles of various sizes. Finally, nonmonochro-matic light and fluctuations in polarization also help to smooth out the oscillations. 

A monodisperse aerosol is one with a narrow size distribution, which, for log-normal-distributed particles, usually means a geometric standard deviation of about 1.2 or smaller. Monodisperse particles are expected to have simple shapes and uniform composition with respect to size. A polydisperse aerosol, on the other hand, is one containing a wide range of particle sizes, but which may otherwise be homogeneous in terms of the basic physical and chemical properties that are not related to size. The term heterodisperse is also used occasionally this describes aerosols varying widely in physical and chemical characteristics, as well as size. 

As discussed in detail by Raabe (1976), an investigator s use of the terms monodisperse and polydisperse aerosols may depend on the particular properties of importance in the study thus an aerosol may consist of particles of the same size, that is, be monodisperse with respect to size, but may vary in settling speed due to variations in density, that is, be polydisperse with respect to settling speed. 

These compressed air nebulizers produce polydisperse aerosols. After the aerosol is produced, the size distribution may change due to evaporation of liquid from the droplets. In addition, the particles may be electrically charged due to an ion imbalance in the droplets as they form if such charges become further concentrated due to evaporation, the particle may break up into smaller particles. Thus electrical neutralization of the aerosol, for example, by exposure to a radioactive source, is usually necessary to prevent electrostatic effects from dominating the particle motion, coagulation, and other behavior. 

However, care must be taken to ensure that seed nuclei are present on which condensation can occur. If a very clean system is used in which nuclei are not present, spontaneous nucleation may occur this process is such that nuclei do not appear uniformly either in space or in time, and the initial particle growth rate depends on the degree of supersaturation. As a result, a polydisperse aerosol is produced under these conditions. 

All these methods generally give (< 1 /xm) polydisperse aerosols of the solid particles and, unless rapid air dilution is provided, coagulation leads to large agglomerates of the small primary particles. 

Sinha, M. P., and S. K. Friedlander, Mass Distribution of Chemical Species in a Polydisperse Aerosol Measurement of Sodium Chloride in Particles by Mass Spectrometry, J. Colloid Inteface Set., 112, 573-582 (1986). 

DOP polydisperse aerosol generated by blowing air through liquid dioctyl phthalate (DOP) at room temperature. The approximate light-scattering mean droplet size distribution of the aerosol is ... 

A chromel-alumel thermocouple is used to monitor the temperature of the ceramic boat and also acts as a sensor for the furnace temperature controller. A constant flow of clean, dry N2 gas is maintained through the quartz tube. The metal vapor is carried out of the furnace with N2 and condensed to form a polydisperse aerosol. The primary aerosol is diluted by mixing with a filtered dry air stream. The diluted aerosol is then routed to the chamber as required and the excess aerosol is vented out through a glass fiber filter. 

A popular method for certifying the integrity of the filter installation uses a polydisperse aerosol, created by blowing air through liquid (e.g., poly-alpha-olefin) introduced into the upstream ductwork, followed by scanning the entire downstream side of the filter face and periphery with a probe nozzle of an aerosol photometer. This testing will identify leaks caused by damage due to... 

The Fundamentals of Acoustic Agglomeration of Small Particulates. Let us consider a polydisperse aerosol consisting of submicrometer and micron sized particles. The mean separation distance between particles would typically be about 100 micrometers. Brownian movement of the particles is caused by the collision of the thermally agitated air molecules with the particles. Also any convection currents or turbulence in the carrier gas will of course cause the particles to be partially entrained and moved in the air. If we next impose an acoustic field of acoustic pressure p, the acoustic velocity u will be given by... 

Experimental data regarding the coagulation coefficients of Pt aerosols, measured by Nolan and Keenan (15) are compared with different models in Table IV. The corrections for polydispersity in the above data, as reported by Mercer (16), are accounted for in the experimental values of the coagulation... 

The preparation of synthetic atmospheres for nonreactive gases and vapors is relatively straightforward, but the preparation of fumes, aerosols, and particulates is considerably more difficult. For purposes of industrial hygiene sampling, a polydisperse aerosol containing respirable-size particles is required. 

This nozzle has been used to generate polydispersed aerosols of various organic liquids as the bulk phase. Particles in the size range of submicrometers to 20 pm were obtained. The concentration varied by less than 10% over several hours as long as the air pressure was well regulated. 

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