Aerosol instruments distribution

Evaporation of liquid drops is equally important. For example, in the application of a pesticide by spraying, it is desired that evaporation be minimized to increase the amount of pesticide reaching the plants. Yet in the production of such foodstuffs as powdered milk or powdered coffee, product quality is improved when evaporation proceeds as quickly as possible. In sampling aerosols, evaporation or condensation may alter aerosol size distribution and affect operation of the sampling instrument. In this case it is desired that static conditions be maintained if at all possible. 

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 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). 

Studies of aerosol size distributions in power plant plumes clearly show that gas-Co-panicle conversion is an important source of submicron aerosol. Condensable material is formed primarily by the oxidation of SOi to sulfates and NOj, to nitrates, both usually present as ammonium salts. It is somewhat easier to analyze plume aerosol dynamics than urban aerosol behavior because (in selected cases) the plume aerosol originates from a single source and is sufficiently well-defined to follow many kilometers downwind. By using ground-based mobile laboratories and/or suitably instrumented aircraft, the aerosol and associated gasc.s can be measured. 

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. 

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). 

Fig. 7-1. Left Idealized particle size distributions for the rural continental and the maritime aerosols. The distribution of sea-salt particles that contribute to the maritime aerosol is shown separately. The transition from the rural to the urban aerosol is indicated. Right Determination of remote tropospheric aerosol size distribution by a combination of instrumental techniques. [ Single-stage and free-wing impactors, O—O set of five double-stage impactors singleparticle optical scattering analyzer these data were obtained at the observatory lzana, Tenerife,...

After inertial separation of the particles within these instruments, it is necessary to quantify the amount of drug in each of the size fractions in order to derive an aerosol size distribution. This is usually performed by chemical assay for drug substance and may entail a variety of analytical techniques. However, it is important that drug substance be assayed, because most pharmaceutical aerosols contain excipients and the distribution of the drug and excipients will not necessarily be uniform and in equal proportion aaoss the entire size distribution. 

See instruction manual for fibre optic Doppler anemometer, published by SIRE (Scientific Instrument Research Establishment) Ltd., Southhill Chislehurst, Kent, England. W. H. Yanta, Measurement of Aerosol Size Distributions with a Laser Doppler Veloci-meter , National Bureau of Standards Special Publication 412, Aerosol Measurement, proceedings of a seminar on aerosol measurement held at National Bureau of Standards, Gaithersburg, Maryland, May 7, 1974. 

The electrical aerosol analyzer and the optical counter are used to measure particle size distributions. Describe the size range and resolution characteristics of each of these instruments. 

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). 

The composition distribution of the particles produced in a laboratory pulverized coal combustor will be explored in this paper using aerosol classification techniques capable of resolving the composition distribution to 0.03 ym diameter. Unlike previous attempts to measure the composition distribution, the particles were classfied directly, without having to resort to resuspension, using calibrated instruments. Experiments were conducted in a laboratory combustor in which operating parameters can be varied over a wide range. Data are presented which demonstrate that the composition of fine particles varies substantially with combustion conditions and does, under some conditions, differ considerably from that of the bulk ash. 

On-line aerosol measurements were made using a Thermo-Systems, Inc., Model 3030 Electrical Aerosol Size Analyzer (EAA). This instrument uses the electrical mobility of the particles to measure the size distribution in the 0.01 to 0.5 ym range. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

We have operated the University of Washington MKV impactor as a low-pressure impactor to provide for chemical analysis, four discretely sized fly-ash fractions in the sub-half-micrometer- diameter aerosol accumulation region. Instrumental neutron activation analysis provided the sensitivity to determine accurately the concentrations of 28 major, minor, and trace elements with sufficient precision to reveal fine structure in the elemental distributions that might be missed by techniques of lesser accuracy and precision. 

Calibration of the instruments to measure the size distribution and chemical composition requires methods of generating aerosols of well-defined sizes and known composition. Generating aerosols of known... 

Experimental evaluation of various widely used commercial instruments has been undertaken by several investigators, including Liu et al. (1974), who used the vibrating orifice technique to generate spherical aerosols with narrow size distributions. Similarly, Pinnick and Auvermann (1979) generated droplets of a solution which were dried to make solid particles for evaluating the response of... 

Particle Measurements. A variety of instruments is available for measuring the number density and size distribution of particles sampled from airborne platforms. This discussion is restricted to instruments that measure particles smaller than 50 xm (cloud droplets and aerosol particles) because these particles are of most interest to atmospheric chemists. 

Particle Number Concentration and Size Distribution. The development of aerosol science to its present state has been directly tied to the available instrumentation. The introduction of the Aitken condensation nuclei counter in the late 1800s marks the beginning of aerosol science by the ability to measure number concentrations (4). Theoretical descriptions of the change in the number concentration by coagulation quickly followed. Particle size distribution measurements became possible when the cascade impactor was developed, and its development allowed the validation of predictions that could not previously be tested. The cascade impactor was originally introduced by May (5, 6), and a wide variety of impactors have since been used. Operated at atmospheric pressure and with jets fabricated by conventional machining, most impactors can only classify particles larger... 

A related technique that is suitable for measurement of aerosols at lower mass loadings is the aerodynamic particle sizer (3, 10). In this instrument the aerosol is rapidly accelerated through a small nozzle. Because of their inertia, particles of different aerodynamic sizes are accelerated to different velocities, and the smallest particles reach the highest speeds. The particle velocity is measured at the outlet of the nozzle. From the measurements of velocities of individual particles, particle size distributions can be determined. The instrument provides excellent size resolution for particles larger than about 0.8 xm in diameter, although sampling difficulties limit its usefulness above 10 xm. 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

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