FIGURE 11.64 Energy-dispersive spectroscopy light-element spectra acquired from (a) one of the small microcrystallites attached to NaCl after exposure to gaseous HNO, (see Fig. 11.63C) and (b) an adjacent area of the NaCl crystal (adapted from Allen et al., 1996).

the atomic scale have been used extensively in many laboratory studies to map atomic and molecular structures. In STM, which is used with electrically conducting materials, a probe with a very fine metal tip scans over the surface of the sample, which is held at a potential relative to the tip. When the tip is very close to the surface, there is a tunneling current between the tip and the sample, whose magnitude depends exponentially on the distance between the tip and the sample. The tip is kept at a constant distance from the sample using a feedback circuit to measure and maintain the tunneling current at a constant value. The tip thus moves up and down with the surface topography. For insulating surfaces, AFM accomplishes similar topographical mapping. In this case, the force acting between the surface and a tine tip attached to a cantilever is kept constant and the deflections of the cantilever required to do this are monitored, for example using optical means.

AFM has been used in only a few studies to explore the sizes and morphology of airborne particles (e.g., Friedbacher et al., 1995; Posfai et al., 1998). In this case, atomic scale resolution is not used, but rather much lower resolution that provides information on particle sizes and shapes in the micron and submicron size range under ambient conditions. This has the advantage that effects due to the application of vacuum to the particles do not occur, as is the case for TEM (vide supra). AFM combined with TEM has been applied by Posfai et al. (f 998), for example, to explore the loss of water from particles upon exposure to vacuum conditions.

(3) 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 ¡xm (Whitby and Clark, 1966; Eisele and McMurry, 1997).

Figure 11.65a illustrates the principles of the electrical aerosol analyzer. The essential components are the aerosol charger, the mobility analyzer, and the detector (shown in Fig. 11.65a as the current-collecting filter). The air containing the particulate matter is first introduced into the aerosol charger, where a corona discharge generates positive ions for particle charging. The positively charged particles are introduced as a thin layer around the outside of the tubular mobility analyzer. Clean air flows down the central portion of the tube between the layer of ambient aerosol at the walls and the charged collection rod in the center of the tube. A negative voltage is applied to the collection rod, causing the positively charged particles to move from the outer wall through the clean air to the collection rod.

The particles with the highest mobilities reach the collection rod first and are removed from the gas stream; those that do not reach the rod before the flow passes out of the region of the electric field pass through to a detector and are measured. Increasing the voltage on the collection rod increases the number of charged particles that reach it before passing out of the field and hence decreases the number reaching the detector. The relationship of the particle count at the detector to the voltage in the analyzer is thus dependent on the particle mobility in the analyzer, which depends on particle size. Thus size distributions can be obtained by studying the detector output as a function of collection rod voltage. The detector may be a current-sensing device, as in Fig. 11.65a, or other type such as a condensation nuclei counter (vide infra).

Details of the calibration, use, performance, and artifactual problems are given in a proceedings entitled Aerosol Measurement (Lundgren et al., 1979); this also shows data for the mobility distribution for monodisperse aerosols.

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