Aerosol instruments

Figure 2. Size and time scales covered by current aerosol instruments.

Aerosol Instrument Classification. Friedlander (34) classified the range of aerosol instrumentation in terms of resolution of particle size, time, and chemical composition. This classification scheme is illustrated in Figure 3. The ideal instrument would be a single-particle counter-sizer-analyzer. Operating perfectly, this mythical instrument would fully characterize the aerosol, with no lumping of size or composition classes, and would make such measurements sufficiently rapidly to follow any transients occurring in the aerosol system. 

Figure 3. Classification of aerosol instruments in terms of their inherent time, size, and composition resolution. (Adapted from reference 34.)...

Amott W. P., MoosmuUer H., Rogers C. F., Jin T. F., and Bruch R. (1999) Photoacoustic spectrometer for measuring light absorption by aerosol instrument description. Atmos. Environ. 33, 2845—2852. 

Commercial instruments are available for a variety of applications in aerosol instrumentation, production of materials from aerosols, contamination control, etc. (ISO/CD 15900 2006, Determination of Particles Size Distribution—Differential Electrical Mobility Analysis for Aerosol Particles). 

The following sections focus on the principles of collection and characterization of pharmaceutical aerosols. Instruments have been selected to illustrate the methods that they typify. 

In the sections that follow, mathematical methods for characterizing aerosol size and chemical properties are discussed. These are primarily of a definitional nature and are needed to provide a common basis for discussing the broad range of aerosol properties and behavior. However, aerosol characterization does not provide, directly, information on the mechanisms of aerosol formation, or temporal and spatial changes in the aerosol—that is, aerosol transport processes and aerosol tlynatnics. These and related topics are covered in later chapters. Advances in aerosol instrumentation have made it possible to measure many of the most important parameters necessary to characterize aerosols (Chapter 6). However, much rejnains to be done in developing aerosol instrumentation for research as well as industrial and atmospheric applications. 

For particles smaller than about 0.05 rm, the diffusion charging theory discussed above breaks down the calculated number of charges per particle decreases to a value less than unity which is physically unacceptable. Instead, only a fraction of such particles acquire unit charge for values of the product in the range usually of interest in such applications as electrostatic precipitation and aerosol instrumentation (< 10 ions sec cin - ). 

The steady-state distribution is independent of the ionic concentration. However, the rate of approach to the steady state depends on the ionic concentrations and other properties of the system. The net result can be summarized as follows for the atmosphere. Ions are steadily generated by cosmic rays and radioactive decay processes. These attach to particle surfaces where they are neutralized at a rate equal to their rate of formation. The particle charge distribution is determined by the steady state relationship between particles separated by one charge. In the atmosphere, the equilibration process takes about 30 min. The rate of equilibration can be increased by increasing the ion concentration using a bipolar ion generator. Radioactive ion sources such as are often used in electrical aerosol instrumentation (Chapter 6). 

For example, many methods are available for the chemical analysis of deposited aerosol particles. Individual particles can be analyzed as well as heavier deposits. A serious gap in aerosol instrumentation is the lack of instrument.s for on-line measurement of aerosol chemical constituents without removing them from the gas. Very large amounts of information on multicomponent, polydi.sperse aerosols would be generated by an instrument capable of continuously sizing and chemically analyzing each particle individually, thereby permitting the determination of the size-composition probability density function, g (Chapter I), From this function, in principle, many of the chemical... 

Willekc, K and Baron, P. A. (Eds,) (1993) Aerosol Measurement, Van Nostrand Reinhold, New York. This isa valuable state-of-the-art compilation of information on aerosol instrumentation written by specialists in the lield. It includes applications in different fields such as air pollution monitoring, industrial hygiene, commercial production of particles, and dean-room monitoring,... 

Several other chapters have been substantially rewritten to reflect the sharpened focus on aerosol dynamics. For example, the chapter on optical properties has been expanded to include more applications to polydisperse aerosols. It help.s support the chapter that follows on experimental methods in which coverage of instrumentation for rapid size distribution measurements has been augmented. Methods for the rapid on-line measurement of aerosol chemical characteristics are discussed in the chapters on optical properties and experimental methods. This chapter has been strongly influenced by the work of the Minnesota group (B. Y. H. Liu, D. Y. H, Pui, P, McMuny, and their colleagues and students) who continue to invent and perfect advanced aerosol instrumentation. Discussions of the effects of turbulence have been substantially expanded in chapters on coagulation and gas-to-particle conversion. 

The indoor facility comprises a 20 x20 x40 thermally insulated enclosure that is continually flushed with purified air at a rate of 1000 L min and is located on the second floor of a laboratory building specifically designed to house it. Located directly under the enclosure on the first floor is an array of gas-phase continuous and semi-continuous gas-phase monitors. Within the enclosure are two 90 m (6.1 m x 3.1 m x 5.5 m, Siuface area to volume = 1.35 m ) 2 mil FEP Teflon film reactors, a 200 kW Argon arc lamp, a bank of 72 W 4-ft blacklights, along with the light and aerosol instrumentation. A schematic of the enclosure is provided in Figure 1. 

In the fourth type of identification the chemical composition of particles is studied in situ. By suitable chemical aerosol instruments the concentration and the size distribution of certain elements can be continuously monitored. The flame photometry of sodium containing particles (e.g. Hobbs, 1971) is a good example for such a method. Recently flame photometric detectors have also been developed to measure aerosol sulfur in the atmosphere (e.g. Kittelson et at., 1978). 

For a spherical particle of nonunit density the classical aerodynamic diameter is different from its physical diameter and it depends on its density. Aerosol instruments like the cascade impactor and aerodynamic particle sizer measure the classical aerodynamic diameter of atmospheric particles, which is in general different from the physical diameter of the particles even if they are spherical. 

One of the stated goals of the NOSH Consortium is to evaluate commercial aerosol instrumentation with the purpose to report on the instrument specifications, ease of use, applicability to different operating conditions, and types or form of data generated to the membership, and to discuss these results with particular emphasis in communicating issues or sensitivities that are relevant to occupational safety and health issues. Major focus areas were in comparing commercial classification and detection instmmentation (1) nano-DMA and long-DMA, (2) ELPI and DMA, and (3) CNC and AE. 

Additionally, the NOSH Consortium was able to achieve the vast majority of its objectives, including the definition of synthesis methods for stable sources of aerosol nanoparticles of Si02, Ti02, citric acid, silver, and PSL. The PSL particles were particularly important as a calibration standard for several aerosol instruments. The aerosol instrument characterization was successfully completed for numerous aerosol instruments, including the long- and nano-differential mobility analyzers, electrostatic-based impactor, aerosol chargers and neutralizers, and optical and electrostatic detectors. 

The dichotomous sampler is a virtual impactor with two emerging aerosol flows. The air beyond the virtual interface is withdrawn at a slow rate, thus rendering this impactor a two-flow or dichotomous impactor. In both flows aerosol particles can be collected on a filter or analysed in a real-time aerosol instrument. A continued suspension of the size-fractionated particles in the effluent air flows is the principal advantage of these virtual or dichotomous impactors (Chen et al., 1985). 

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