Electric mobility analyzer

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

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). 

Electrical mobility analyzers Several types of instruments for measuring particle sizes in the atmosphere depend on the mobility of charged particles in an electric field (e.g., see Yeh (1993) and Flagan (1998) for a review and history of the development of this field). The electrical mobility analyzer developed by Whitby and co-workers at the University of Minnesota, in particular, has been used extensively to measure particles in the range 0.003 to 1 yum (Whitby and Clark, 1966 Eisele and McMuriy, 1997). 

There are a number of techniques for generating aerosols, and these are discussed in detail in the LBL report (1979) and in volumes edited by Willeke (1980) and Liu et al. (1984). We briefly review here the major methods currently in use these include atomizers and nebulizers, vibrating orifices, spinning disks, the electrical mobility analyzer discussed earlier, dry powder dispersion, tube furnaces, and condensation of vapors from the gas phase. 

Next, the sampling flow was guided to different measurement instruments. The concentration number and the size distribution of submicron particles (0.01-0.5 pm) was obtained with a TSI Scanning Mobility Particle Sizer (SMPS) consisting of a differential electrical mobility analyzer coupled with a condensation nuclei counter. Previously a preconditioning gas system was used. This basically consists of a cyclon,... 

The ex pans ion-type CPC is cyclic in operation, and this may pose a problem in measuring concentrations from steady flow devices such as the electrical mobility analyzer and diffusion battery discussed later. Continuous-(low CPCs have been developed in... 

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).

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. 

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. 

The formation of aerosol particles from gaseous components is appropriately investigated under laboratory conditions. In so-called aerosol chambers an artificial atmosphere is created to which small quantities of appropriate trace gases (e.g. SO, NO, H,0, NH, and organics) is added. It is also possible to use ambient air purified from particulate matter. The chamber may be illuminated to initiate photochemical processes, and the behaviour of particles formed is studied by the methods outlined in Subsection 4.1.2, e.g. by electrical mobility analyzers (Whitby et aL 1972). 

Aitken counter in combination with electric mobility analyzer — set of double-stage impactors these data were obtained from measurements on board ships (Jaenicke, 1978a Haaf and Jaenicke, 1980). A Sea-salt particles observed by sodium resonance fluorescence (Radke, 1981) and-----by electron microscopy (Meszaros and Vissy, 1974). 

Electrical mobility analyzers, like the differential mobility analyzer, classify particles according to their electrical mobility Be given by (9.30). The electrical mobility equivalent diameter Dem is defined as the diameter of a panicle of unit density having the same electrical mobility as the given particle. Particles with the same Dem have the same migration velocity in an electric field. Particles with equal Stokes diameters that cany the same electrical charge will have the same electrical mobility. 

FIGURE 15.10 Diagram of a simple electrical mobility analyzer. 

---