' Electrometer >1014A
Total flow particles will strike the extraction port, providing a narrow range of particle sizes at the aerosol outlet. This monodisperse aerosol exiting the mobility analyzer is then directed to a measuring device such as a condensation particle counter (CPC; see later) or a Faraday cup electrometer (e.g., Winklmayr et al., 1991). A modified version that extends the range down to 1 nm, approaching molecular ions, has also been developed (Rosell-Llompart et al., 1996).
Results of particle size distributions in air using different methods tend to be in reasonably good agreement when the different sampling times are taken into account. For example, Hoff et al. (1996) made particle size measurements in a rural area in Ontario, Canada, using a DMA, an eight-stage impactor and light scattering instruments. The number size distributions obtained using each technique were in excellent agreement, assuming for the impactor samples that the composition of the particles had a density of 2 g cnr3.
Variable high voltage
Variable high voltage
To aerosol sensor
— Aerosol inlet 85Kr charger
FIGURE 11.65 (a) Electrical aerosol analyzer (adapted from Whitby and Clark, 1966). (b) Schematic diagram of differential mobility analyzer (adapted from Yeh, 1993).
A widely used form of the electrical mobility analyzer now in use is called a differential mobility analyzer (DMA), which operates on the same principle. Figure 11.65b is a schematic diagram of a DMA. Charging is carried out using a radioactive source such as 85 Kr, 90 Sr, 21 "Po, or 24'Am to produce ions of both signs that become attached to the particles. In this case, an extraction port is located in the center rod. Smaller particles of opposite electrical charge to the center rod and having higher electrical mobilities strike the central rod before this port and are removed; larger particles are carried beyond the extraction port and out with the major flow. Only a narrow range of
(4) Diffusion separators As discussed in Chapter 9.A.3, small particles with diameters < 0.05 /¿m undergo diffusion via Brownian motion sufficiently rapidly that this can be used to separate particles. Thus the aerosol can be passed through a tube in which the smaller particles diffuse more rapidly to the walls and are removed there, leaving the larger, more slowly diffusing particles to pass through. Variation of residence time in the tubes by varying the flow rates and tube lengths leads to different size cutoffs (but not high resolution). Hence size fractionation of small particles can be achieved using such diffusion separators. The particles exiting the tube can be measured using techniques such as the CPC. The design and testing of a typical diffusion battery-CPC apparatus are described by Raes and Reineking (1985).
(5) Aerodynamic particle size This technique is based on measuring the velocity lag of particles in accelerating air flows (Wilson and Liu, 1980; Baron et al., 1993). A laser beam is split into two coherent beams using a beam splitter, and these two beams are then focused onto a point, forming an interference pattern. When a particle passes through this interference pattern, it scatters light, with the scattered light intensity oscillating as the particle passes through the interference fringes. The frequency of the oscillation of the scattered light multiplied by the spacing of the fringes gives the velocity of the particle perpendicular to the fringes. From the particle velocity, the size can be obtained.
(6) Condensation particle counter (CPC) Very small particles in the Aitken range act as condensation nuclei for the formation of larger particles in a supersaturated vapor. If these very small particles are injected into air that is supersaturated with water or another vapor such as an alcohol, the vapor condenses on them to form droplets. In the condensation particle counter (CPC), supersaturation of the air containing these particles is achieved by passing them through a higher temperature region saturated with the vapor and then into a lower temperature region in which the alcohol vapor condenses on the nuclei to form larger droplets. These can be counted as is done in absolute nuclei counters, for example, by measuring the pulses of scattered light by a single droplet as it passes through the viewing volume. Alternatively, the particles can be measured using techniques such as total light extinction or scattering. In this case (sometimes called photoelectric nuclei counters), calibration against some other reference is required. CPCs are applicable in the size range from ~3 to fOOO nm. Note that a fO-nm particle contains ~104 molecules, whereas a 2.7-nm particle contains only ~102 molecules (Eisele and McMurry,
1997). Hence these very small ultraiine particles are approaching molecular clusters.
Measurement of ultrafine particles, those < 10 nm in diameter, has become of increasing interest due to their importance in acting as cloud condensation nuclei (see Chapter 14) and in elucidating rates and mechanisms of homogeneous nucleation. Methods based on the foregoing principles have been developed and applied for these very small particles (e.g., see Stolzen-burg and McMurry, 1991; McDermott et al., 1991; Wiedensohler et al., 1993, 1994; Saros et al., 1996; Marti et al., 1996; and Weber et al., 1998). For example, CPCs have been used in conjunction with pulse height analysis to measure particles with diameters in the 3- to 15-nm range. Higher supersaturations of the alcohol vapor are required to grow the smaller particles into the light-scattering range, and hence these particles travel further in the measuring system before activation occurs. As a result, for particles less than 15 nm, the final size of the light-scattering particles formed from smaller particles is smaller as well, whereas above 15 nm, the final particle size is relatively independent of the initial particle size (Saros et al., 1996). For particles in this smaller size range, the height of the detector pulses produced by the scattered light decreases with particle size. Thus, for ultrafine particles, pulse height analysis provides a means of determining particle size (e.g., Stolzenburg and McMurry, 1991; Saros et al., 1996; Marti et al., 1996; Weber et al.,
(7) Summary In summary, no one technique is capable of measuring the size distribution of atmospheric
Optical microscope •
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