Electrical Measurement of Aerosols

Whitby, K. T. (1976). Electrical measurements of aerosols. In Fine Particles Aerosol Generation, Measurement, Sampling and Analysis (B. Y. H. Liu, ed.). Academic Press, New York. 

Haaf, W. (1980). Accurate measurements of aerosol size distribution II, construction of a new plate condensor electric mobility analyser and first results. J. Aerosol Sci. 11, 201-212. 

Knutson, E.O., Whitby, K.T., 1975. Accurate measurement of aerosol electric mobihty moments. J. Aerosol Sci. 6, 453-460. 

The activity concentration of radon and of the free and aerosol attached radon daughters i/ere measured simultaneously. During these measurements the aerosol particle concentration i/as registered by means of a condensation nuclei counter (General Electric). 

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. 

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. 

Chemical Amplification. The measurement of a small electrical signal is often accomplished by amplification to a larger, more easily measured one. This technique of amplification can also be applied to chemical systems. For peroxy radicals, Cantrell and Stedman (117) proposed, as a possible technique, the chemical conversion of peroxy radicals to N02 with amplification (i.e., more than one N02 per peroxy radical). This method has also been used for laboratory studies of H02 reactions on aqueous aerosols (21). The following chemical scheme was proposed as the basis of the instrument ... 

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. 

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

First, aerosol particles are important from the point of view of atmospheric electricity. A fraction of the air molecules is electrically charged (small ions), as a result of ionizing radiation. Measurements show (Bricard and Pradel, 1966) that in 1 cm3 of surface air about 10 ion-pairs are formed each second. Eight out of ten are produced by the radioactivity of the air and soil, while the other two are produced by cosmic radiation. The electrical properties of the air are determined by the electrical mobility (B) of ions formed ... 

Hogan, A. W., Mohnen, V. A. and Schaefer, V. J., 1973 Comments on Oceanic aerosol levels deduced from measurements of the electrical conductivity of the atmosphere . J. Atmosph. Sci. 30, 1455-1460. 

Situations in which a charged particle moves in an electric field are important in several gas-cleaning devices and aerosol measurements. If a particle has an electric charge q in an electric field of strength E, an electrostatic force... 

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