Electrical aerosol size analyzer

On-line aerosol measurements were made using a Thermo-Systems, Inc., Model 3030 Electrical Aerosol Size Analyzer (EAA). This instrument uses the electrical mobility of the particles to measure the size distribution in the 0.01 to 0.5 ym range. 

Several methods were used to characterize aerosol distributions within the exposure chambers. The principal method employed a rectangular jet impactor which was inserted through a port in the chamber and used to inertially classify particles larger than 0.3 urn in diameter. As an additional index, total sulfuric acid mass use per total mass flow was calculated to give an approximation of H SO, mass per volume. Since a substantial part of the aerosol distribution in this system was below inertial impaction limits, an electrical mobility size analyzer (Thermo-... 

The electrical aerosol analyzer and the optical counter are used to measure particle size distributions. Describe the size range and resolution characteristics of each of these instruments. 

Effects of carrier gas flow rate, dilution flow rate, and the combustion boat temperature were studied by sampling the aerosol stream with an electrical aerosol analyzer to obtain the particle size distribution. Filter samples were taken for chemical analysis to determine mass concentrations. Aerosol samples were also collected in an electrostatic sampler for electron microscopic examination. 

Based on these results, generation temperatures of 895°C and 500°C were chosen for Pb and , respectively. The aerosol characteristics are given below in Table I. Electron microscopic examination of the aerosol samples showed spherical particles in the 0.01 to 1.0 ym size range—in agreement with the electrical aerosol analyzer data. 

The introduction of the electrical aerosol analyzer allowed direct observations of the evolution of the size distribution of the fine aerosol particles (41). The growing aerosol was found to develop what appeared to be an... 

In this section, we briefly review three types of instruments, the optical particle counter, electrical aerosol classifier, and diffusion battery. These system.s are based on very dilTerent physical characteristics of the aerosols. The optical counters respond to signals from individual particles. The electrical analyzers depend on the measurement of a current carried by a slreaJTi of cbrnged aerosol particles. The ditfusion battery also depends on the behavior of particle clouds. The system often used to cover the size range from about 10 nm to 10 /jm is a combination of (a) the electrical analyzer up to about 0.2 jum and (b) the optical particle counter over the rest of the range. 

Experiments on simultaneous coagulation and growth were made by Husar and Whitby (1973). A 90-m polyethylene bag was filled with laboratory air from which paniculate matter had been removed by filtration. Solar radiation penetrating the bag induced photochemical reactions among gaseous pollutants, probably SO2 and organics, but the chemical composition was not determined. The reactions led to the formation of condensable species and photochemical aerosols. Size distributions were measured in 20-min intervals using an electrical mobility analyzer. The results of one set of experiments for three different time,s are shown in Fig. 11.3. 

K. T. Whitby and W. E. Clarke, Electric Aerosol Particle Counters and Size Distribution Measuring System for the Range 0.015 to 1 Micron Size. Tellus 18 (1966), 573—585. Commercial information on the Whitby aerosol analyzer is available from Thermo Systems, Inc., 2500 Cleveland Avenue North, St. Paul, Minnesota, USA 55113. 

The device resembles a cylindrical differential mobility analyzer (DMA) in that a sample flow is introduced around the periphery of the annulus between two concentric cylinders, and charged particles migrate inward towards the inner cylinder in the presence of a radial electric field. Instead of being transmitted to an outlet flow, the sample is collected onto a Nichrome filament located on the inner cylinder. The primary benefit of this mode of size-resolved sampling, as opposed to aerodynamic separation into a vacuum, is that chemical ionization of the vapor molecules is feasible. Because there is no outlet aerosol flow, the collection efficiency is determined by desorption of the particles from the filament, chemical ionization of the vapor, separation in a mobility drift cell, and continuous measurement of the current produced when the ions impinge on a Faraday plate. 

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). 

Husar( 1971) studied the coagulation of ultrahne particles produced by a propane torch aerosol in a 90-m polyethylene bag. The size distribution was measured as a function of time with an electrical mobility analyzer. The results of the experiments are shown in Fig. 7.11 in which the size distribution is plotted as a function of particle diameter and in Fig. 7.12 in which is shown as a function of t) both based on particle radius. Numerical calculations were carried out by a Monte Carlo method, and the results of the calculation are also shown in Fig. 7.12. The agreement between experi ment and the numerical calculations is quite satisfactory. 

Figure 7.11 Coagulation of aerosol panicles much smaller than the mean free path. Size distributions measured with the electrical mobility analyzer (Husar. 1971).

The smaller aerosol particles can be captured from the air for subsequent counting and size measurement by means of so-called thermal precipitators. In these instruments, metal wires are heated to produce a temperature gradient. Aerosol particles move away from the wire in the direction of a cold surface, since the impact of more energetic gas molecules from the heated side gives them a net motion in that direction. The particles captured are studied with an electron microscope. Another possible way to measure Aitken particles is by charging them electrically under well-defined conditions. The charged particles are passed through an electric field and are captured as a result of their electrical mobility (see equation [4.6]). Since size and electrical mobility are related, the size distribution of particles can be deduced. These devices are called electrical mobility analyzers. 

One potential method for measuring the size of aerosol nanoparticles is a scanning mobility particle sizer (SMPS), consisting of a differential mobility analyzer (DMA) and a condensation particle counter (CPC). Aerosol particles enter the DMA where they are charged using a radioactive source and their size is classified based on the electrical mobility, Z, of the particles in the applied electrical field ... 

Figure 6.39. Schematic of nanoparticle growth via pulsed-laser vaporization with controlled condensation (LVCC), coupled to a differential mobility analyzer (DMA). A DMA is used to control the size of gas-phase synthesized nanoparticles by exploiting differences in the electrical mobility of nanoparticles under a flow of an inert gas. Reproduced with permission from Glaspell, G. Abdelsayed, V. Saoud, K. M. El-Shall, M. S. Pur. Appl. Chem. 2006, 75, 1667. Copyright 2006 lUPAC. The bottom image shows how gas-phase techniques may be used to synthesize Au/Ga core/shell nanoparticles with the assistance of multiple DMAs. Reproduced with permission from Karlsson, et al. Aerosol Sci. Technol. 2004, 38, 948.

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