Particulate measurements, optical

Black smoke (BS) is a particulate measure that typically contains at least 50% respirable particulates smaller than 4.5 mm in aerodynamic diameter, sampled by the British smokeshade (BS) method. The reflectance of light is measured by the darkness of the stain caused by particulates on a white filter paper. The result of BS sampling depends on the density of the stain and the optical properties of the particulates. Because the method is based on reflectance from elemental carbon, its use is recommended in areas where coal smoke from domestic fires is the dominant component of ambient particulates. 

Direct Mass Measurement One type of densitometer measures the natural vibration frequency and relates the amplitude to changes in density. The density sensor is a U-shaped tube held stationaiy at its node points and allowed to vibrate at its natural frequency. At the curved end of the U is an electrochemical device that periodically strikes the tube. At the other end of the U, the fluid is continuously passed through the tube. Between strikes, the tube vibrates at its natural frequency. The frequency changes directly in proportion to changes in density. A pickup device at the cui ved end of the U measures the frequency and electronically determines the fluid density. This technique is usefiil because it is not affec ted by the optical properties of the fluid. However, particulate matter in the process fluid can affect the accuracy. 

Weiss, R. E and A. P. Waggoner, Optical Measurements of Airborne Soot in Urban, Rural and Remote Locations, in Particulate Carbon Atmospheric Life Cycle (G. T. Wolff and R. L. Klimisch, Eds.), pp. 317-325, Plenum, New York, 1982. 

While inferring k from measurements of absorption by particulate samples may be valid for many kinds of sohds at visible wavelengths, it may not be valid in spectral regions where the optical constants rapidly vary, such as the infrared or far ultraviolet. 

To conclude, we venture to state that the problem of how to determine accurate optical constants from measurements on particulate samples, in contrast with homogeneous solids and liquids, has not been solved in general, even for single-component powders. This does not mean that there have not... 

In the study of atmospheric aerosols several techniques have been used to determine optical constants from measurements on particulate samples. And there have been many such measurements. Yet under pressure from funding agencies and from those waiting at computer terminals for optical constants of the complicated mixture that is the atmospheric aerosol, comparatively little effort has been expended on evaluating these techniques by applying them to particles of solids with known optical constants. 

In granular solids or in analysis of liquids or slurries in which a considerable amount of particulate material exists, the scattering effect attenuates the optical signal in addition to the absorption. Scattering back from the body of the sample toward the surface produces the intensity to be measured as diffuse reflectance. Scattering also controls the depth of penetration of the sample as well as does its absorptivity (10). The complexity of these two factors acting at once is difficult to predict a priori. This is another reason why the empirical method and the empirical equation coefficients produced by a training set are essential. 

The optical measurements presented in the previous chapters can be used to either characterize local, microstractural properties or as probes of bulk responses to orientation processes. In either case, it is normally desirable to make the connection between experimental observables and their molecular or microstractural origins. The particular molecular properties that are probed will naturally depend on the physical interaction between the light and the material. This chapter explores molecular models and theories that describe these interactions and identifies the properties of complex materials that can be extracted from measurements of optical anisotropies. The presentation begins with a discussion of molecular models that are applied to polymeric materials. Using these models, optical phenomena such as birefringence, dichroism, and Rayleigh and Raman scattering are predicted. Models appropriate for particulate systems are also developed. 

The connection between the observables extracted from optical measurements, and the microstructure of polymeric and colloidal liquids is presented in Chapter 6. This is developed in terms of current models of molecular and particulate dynamics. The study of the dynamics and structure of complex liquids is interdisciplinary, involving physicists, chemists, and chemical engineers. Recognition of this wide audience is reflected in the applications that are included, where examples are drawn from each segment of the community. 

There are many pitfalls in measuring the properties of aerosols. One of the most critical is sampling of particulate matter without disturbing the aerial suspension. There are some optical devices that make measurements of an aerosol in situ without disturbance. However, most devices requires that a small sample be taken from the gas-particle suspension. Because of inertial forces acting on particles, it can be deduced readily that siphoning part of the fluid... 

The attenuation of a light beam is given in terms of the extinction coefficient, sometimes called the attenuation coefficient or turbidity, and it is a key measure of the optical behavior of particulate systems. In terms of the separate contributions for particle scattering and absorption,... 

Optical measurements of airborne combustion aerosols have been carried out for a number of years, usually with light scattering techniques. However, due to the particle size dependence of light scattering and the variable particle size distributions of smokes, it is extremely difficult to relate light scattering properties to particulate mass concentrations. The measurement of light absorption by particles can be directly related to particle mass if two conditions are met ... 

In the case of high liquid water clouds, the D(X) value can be as high as 10 as noted by Madronich [109,110] and co-workers. For example, they note that measurements of spectral ultraviolet-B irradiance under optically thick clouds show strongly enhanced attenuation by molecular and particulate absorbers and that the photon path is enhanced due to the presence of the highly scattering medium, leading to an amplification of absorption by chromophores. Using discrete ordinate and Monte Carlo model caculations, they [110] showed that photon paths (i.e., D(X)) in realistic water clouds could be enhanced by factors of 10 and more compared to cloudless sky. ... 

With diffuse-reflection optics a reference signal cannot be measured without a standard. With reference to diluted samples, the matrix particulate is best suited. In this way, contaminations of the matrix which are virtually inevitable due to the large specific surface, are compensated for. For undiluted, specially compact samples, gold covered erasive paper has proved to be an excellent ready-to-use standard (Otto, 1987). 

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