Particle size distribution typical aerosols

The modal approach assumes a shape for the particle-size distribution, typically one or more lognormal distributions, and represents evolution of the size distribution as evolution of the parameters characterizing the distribution, i.e., the amplitude, mode radius, and variance for the lognormal distribution (Binkowski and Shankar, 1995 Wilck and Stratmann, 1997). This approach offers the possibility of representing aerosol microphysical properties in models with far fewer variables (modal parameters) than are required in the sectional method (numbers of particles in each bin). 

Spray pyrolysis routes have been extensively investigated to prepare Pt-based catalysts. Typically, a liquid feed of metal precursor and carbon is atomized into an aerosol and fed into a continuous furnace to evaporate and heat-treat to form a collectable powder. The method has good control over final aggregate particle size and metal particle size distributions, as well as producing powder without further isolation or separation. Hampton-Smith et al. have reviewed efforts of Superior MicroPowder (now Cabot Fuel Cells) in this area. ... 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

Figure 12.28 shows the particle surface area size distribution before the Mount Pinatubo eruption (Fig. 12.28a), inside the main aerosol layer several months after the eruption (Fig. 12.28b), and almost two years after the eruption (Fig. 12.28c). (See Chapter 9.A.2 for a description of how particle size distributions are normally characterized.) Prior to the eruption, the surface area distribution is unimodal, with typical radii of 0.05-0.09 /xrn and a number concentration of l-20 particles cm 1. In the main stratospheric aerosol layer formed after the eruption, the distribution is bimodal...

MDI products are subject to batch control and acceptance tests similar to those for other pharmaceutical dosage forms, that is, active drug identification, dose delivery, and dose uniformity. Additional special tests unique to inhalers, e.g., characterizing the particle size distribution of the delivered aerosol, are also applied. Typical tests are shown in Table 3. 

The characterization and quality control of the particle size distribution of the discharged aerosol has become one of the key tests applied to MDI and other inhaler products, and a wide variety of methods have been developed to make this possible. The available methods can be broadly split into two categories optical (typically laser) methods or methods based on inertial impaction. 

Aerosols present a special case in that the investigator needs to measure the mass concentration of the chemical, the chemical composition as a function of particulate size, and the particle-size distribution of the aerosol. No continuous sampling instruments are available to measure both particle-size and chemical concentration. Particle detection can be accomplished using both forward- and back-scatter detectors. A typical back-scatter allows for non-invasive determinations over a range from 6 to 10 000 mg m . In the test, the aerosol is drawn through an orifice and articles impact on a surface positioned between a source and a counter. 

A third technique used to prepare both Gd-doped CeOi and NiO/YSZ powders is that of aerosol flame deposition [134, 135]. In the case of NiO/YSZ powders used for the anodes (see Chapter 12), nanosized spherical particles were obtained and the particle size distribution could be limited by controlling the processing parameters [135]. Subsequently, the electrical conductivity of 70% NiO/YSZ was found to be lO Scm at 700°C, with the material exhibiting typically metallic behavior. However, the use of such materials in fuel cells was not reported. 

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. 

Mobility sizers collect, count and determine size distributions for aerosols. These can typically handle particle sizes in the range of about 0.02-1 pm. An example is the differential mobility analyzer, which is described later in this section. 

A plot of the attachment coefficient, p, of radon decay product ions is shown in Figure 2.3 (Chamberlain, 1991). The line is Equation (2.2) with the diffusion coefficient value, D = 7 X 10 m s Vm = 44 ms and a = 1. In the natural aerosol size distribution, typical of well-populated country districts, Junge s (1963) natural aerosol size distribution includes particles such as sea salt and resuspended dust which extend the distribution at the large-diameter end, the rate constant for attachment = 2.1 x 10 s , and since A = 1.7 x 10 m for the Junge s aerosol, the corresponding value of the attachment coefficient is P = 1.2x10 m s . Measured values for the attachment coefficient p for outdoor aerosols... 

Stage 10. A typical mass size distribution of aerosols as measured by the MOUDI cascade impactor in a coal mine using diesel-powered equipment where the concentration was much higher and the run times shorter (90 min) is shown in Figure 6.16. These data indicate a bimodal distribution with the lower and the upper modes consisting of diesel exhaust particles and coal and rock dust, respectively. 

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

In exposures of humans to artificially generated aerosols, where the information is to be relevant to ambient aerosols, several factors are important the particle diameter distribution must be fairly constant and fall within size ranges typical for the given compound in the ambient air, the chemical composition of the aerosol must be stable and predictable, and the electric charge distribution of the aerosol must simulate that of normal atmospheric aerosols. 

FIGURE 9.28 Size distribution of particle geometric cross section (A) as a function of geometric diameter for a typical rural aerosol (adapted from Hegg et at., f993). 

In a similar vein, the time scales to achieve equilibrium for inorganics have been examined by Meng and Seinfeld (1996), who show that small (submicron) particles can come to equilibrium with the gas phase in less than a few hours typically but that larger particles may not. The major factors determining the time needed to reach equilibrium are the aerosol size distribution,... 

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