Aerosol Characterization Techniques

There are many ways to describe the size of an aerosol particle. For example it may be described by its longest dimension or by a sphere of equivalent volume. It may be described by its light scattering properties or by the way it behaves in an airstream. In a very real sense there is no such thing as the correct size or diameter of an aerosol particle. There are as many correct diameters as there are ways of measuring it, and it is up to the researcher to measure a size parameter relevant to the particular application under investigation. From a deposition perspective, it is the inertial behavior of the particle in an airstream that defines how and where it will deposit (Chap. 2). This characteristic diameter is known as the aerodynamic diameter and is the characteristic diameter that is normally 

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Where d is the aerodynamic diameter, is the particle density, and dp is its physical diameter. For a sphere, dp is the sphere s diameter for an irregular particle, dp will depend on particle shape. By definition, a water droplet with density of 1 g/cnP will have the same aerodynamic and physical diameter. It is the aerodynamic size distribution which is measured as the in vitro surrogate for inhalation aerosols. 

Aerosols generally consist of particles or droplets with a range of sizes. It is advantageous to describe these size distributions by distribution functions. Many functions are in use (4) however, it is the log-normal distribution that is generally used to describe inhalation aerosols. Mathematically, this is described by the function 

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In Vitro Aerosol Characterization Techniques Inertial Impaction... 

Measurements of the air mobility spectrum seem to add considerable information toward an understanding of aerosol formation and growth at sizes below a few nanometers. Hence, to characterize nucleation mechanisms more precisely, such data should be included in experimental designs. Lagrangian aerosol sampling techniques would also be favored, since this approach can yield data on microphysical evolution without the complicating effects of a changing air mass. Further laboratory studies should be undertaken to quantify the thermodynamic data that define ion properties under tropospheric conditions, at ion sizes and compositions relevant to aerosol nucleation. The sparseness of such data imposes a limitation on our ability to quantify ion-based nucleation mechanisms [19,33],... 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Hidy, G. M. Characterization of Aerosols in California (ACHEX). Vol 2. Experimental Methods Analytical Techniques. Final Report. (Prepared for California Air Resources Board) Thousand Oaks, Calif. Rockwell International, 1974. 

A considerable effort has gone into the application of statistical techniques to the analysis of aerosol data for the extraction of source contributions. The use of novel statistical methods has been stimulated by uncertainties in the data collected in field measurements and in source characterization in some cases not all of the sources are known. 

The symposium was divided into four subject areas, and this volume follows that general format. The first group of chapters reviews and describes many of the recent modeling efforts. The next section is devoted to source characterization studies, while the third group includes chapters concerned with carbonaceous aerosols—both source apportionment and measurement techniques. The final section describes the results of several field studies in areas of the United States and China where wind-blown dust is a serious problem. 

Heightened interest on air reactions that can be associated with C cycle and climate changes include NOM compounds. Therefore, there are demands for characterization and reaction mechanisms of NOM in aerosols from different origins, namely, urban, rural, from biomass burning, and others. On that account, spectroscopic techniques, combined with adequate sample preparation methods, could bring additional insights into aerosol research studies. 

Both liquid and solid material can be suspended in a gas by a variety of mechanisms. Aerosols produced under laboratory conditions or by specific generating devices may have very uniform properties that can be investigated relatively easily by physical and chemical instrumentation. Natural aerosols found in the atmosphere are mixtures of materials from many sources that are highly heterogeneous in composition and physical properties. Their characterization has required the application of a variety of measurement techniques and has been a major activity in modern aerosol science. 

To characterize adequately the dynamic properties of a chemically reactive aerosol, a very large amount of information is required. However, aerosol properties generally are determined in only a limited way because of limitations of available techniques. With air pollution monitoring and the driving force of progress in the development of theory, heavy emphasis has been placed on the size distribution and its moments, as well as the chemical composition of particles and the suspending gas. 

Although DLS is most often used to size solid colloidal particles, the technique has also been applied to characterize aerosols [78,86,87], emulsion droplets [88,89], amphiphilic systems [90-92], and macromolecular solutions [12,16,93]. Another common application is the study of the fractal structure and kinetics of colloidal aggregation [94-102], More information about dynamic light scattering and its applications can be found in Refs. 23. 103 (104), and 105, in reviews, Refs. 11, 13, 36, 37, 49, 50, and 106, and in collections of papers Refs. 12. 14. 16. 93 (107), 105, and 108-114. 

Kaye, B.H. Characterization of Powders and Aerosols Wiley-VCH Weinheim, 1999. The Use of Fourier Techniques to Characterize the Shape of Profiles in Ch. 2, Direct Measurement of Larger Fineparticles and the Use of Image Analysis Systems to Characterize Fineparticles, 21-58, and Ch. 7, Light Scattering Methods for Characterizing Fineparticles, 205-232. 

Kaye, B.H. Alliet, D. Switzer, L. Turbitt-Daoust, C. The effect of shape on intermethod correlation of techniques for characterizing the size distribution of a powder II. Correlating the size distribution as measured by diffractometer methods, TSI-amherst aerosol spectrometer, and coulter counter. Part. Part. Syst. Charact. 1999, 16, 266-272. 

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