Extinction measurements

Extinction is determined by measuring the ratio of transmitted to incident irradiance (11.1). Many laboratories are equipped with recording spectrophotometers which can measure this quantity very quickly for liquid or solid samples. In principle this same type of instrument may be used for measuring extinction by particulate samples. The results, however, may be unreliable unless the detector is designed to reject forward-scattered light, which may be the major contributor to extinction by particles larger than the wavelength. 

Measured extinction spectra for aqueous suspensions of polystyrene spheres—the light scatterer s old friend—are shown in Fig. 11.19. Water is transparent only between about 0.2 and 1.3 jam, which limits measurements to this interval. These curves were obtained with a Cary 14R spectrophotometer, a commonly available double-beam instrument which automatically adjusts for changing light intensity during a wavelength scan and plots a continuous, high-resolution curve of optical density. To reproduce the fine structure faithfully, the curves were traced exactly as they were plotted by the instru- 

Kigure 11.19 Measured extinction by aqueous suspensions of polystyrene spheres with three different mean diameters. 

Extinction spectra could be used to size particles by matching measured features with those calculated from Mie theory provided that the size distribution is narrow and the particles are nearly spherical. 

Fxtinction is not difficult to measure—in principle. But if it is to be separated into its components an independent measurement of either scattering or 

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Fletsch and Richards [51] determined fluoride in seawater spectrophotometri-cally as the cerium alizarin complex. The cerium alizarin complex and chelate was formed in 20% aqueous acetone at pH 4.35 (sodium acetate buffer) and, after 20-60 min, the extinction measured at 625 nm (2.5 cm cell) against water. The calibration graph was rectilinear for 8-200 ig/l fluoride the mean standard deviation was 10 xg/l at a concentration of 1100 ig/l fluoride. 

In the Nile blue spectrophotometric method, 10 ml 2% aqueous hydrofluoric acid is added to a 10 ml sample contained in a polyethylene bottle. The mixture is shaken for about 2h. Aqueous ferrous sulfate 10% 10 ml and 1ml 0.1% aqueous Nile blue A are added, then extracted with o-dichlorobenzene (10 ml and 3x5 ml). The combined organic extracts are diluted to 50 ml with the solvent and the extinction measured at 647 nm. Interference from chloride ions up to 100 mg/1 can be eliminated by precipitation as silver chloride. 

Ethyl alcohol, methyl alcohol and cyclohexane (UVASOL(R)-Grade) employed as solvents shall have an extinction, measured in a 1 cm cell at 240 nm with reference to water (spectroscopic grade), not exceeding 0.10. 

One of the earliest detailed diagnostic efforts on sooting of diffusion flames was that of Wagner et al. [86-88], who made laser scattering and extinction measurements, profile determinations of velocity by LDV, and temperature measurements by thermocouples on a Wolfhard-Parker burner using ethene as the fuel. Their results show quite clearly that soot particles are generated near the reaction zone and are convected farther toward the center of the fuel stream as they travel up the flame. The particle number densities and generation rates decline with distance from the flame zone. The soot formation rate appeared to... 

Procedure To the test solution (2 ml) in a separating-funnel are added anhydrous acetic acid (10 ml) and a 5% solution of p-thiocresol in anhydrous acetic acid (0.2 ml). The phases are mixed and allowed to stand for 30 min. The complex is extracted into CHCI3 and the extinction measured. 

In more recent optical extinction measurements on specially filtered samples [60], the weak bump had disappeared. This bump may thus have been due to damaged clusters (see Sect. 3.6) or cluster aggregates which form colloidal inclusions in the sample. The main feature of the UV-visible spectrum of AU55 in solution is then a broad absorption extending across the whole visible region. 

Anderson, J., and V. K. Saxena, Temporal Changes of Mount Pinatubo Aerosol Characteristics over Northern Midlatitudes Derived from SAGE II Extinction Measurements, J. Geophys. Res., 101, 19455-19463 (1996). 

Steele, H. M., and R. P. Turco, "Separation of Aerosol and Gas Components in the Halogen Occultation Experiment and the Stratospheric Aerosol and Gas Experiment (SAGE) II Extinction Measurements Implications for SAGE II Ozone Concentrations and Trends, J. Geophys. Res., 102, 19665-19681 (1997b). 

Up to this point we have considered only extinction by a single particle. However, the vast majority of extinction measurements involve collections of very many particles. Let us now consider such a collection, which is confined to a finite volume, the scattering volume. The total Poynting vector is... 

Microwave ( = 3 cm) extinction measurements for beams incident parallel ( = 0°) and perpendicular (f = 90°) to the symmetry axis of prolate spheroids... 

In this section we compare the theory of the preceding two sections with experimental measurements of infrared extinction by small particles. Comparisons between experiment and theory for spheres of various solids, most notably alkali halides and magnesium oxide, have been published in the scientific literature many of these papers are cited in this chapter. In most of this work, however, there is an arbitrary normalization of theory and experiment, which tends to hide discrepancies. For this reason, most theoretical calculations in this section are compared with mass-normalized extinction measurements. The new measurements presented here were made in the Department of Physics at the University of Arizona. A group of solids was selected to illustrate different aspects of surface modes. Results on amorphous quartz (Si02) particles, for example, illustrate the agreement between experi-... 

The absorption curve was constructed from points obtained by the extinction measurements on a Beckman DU instrument. The solvents used were n-bexane, carbon tetrachloride, dioxan and ethanol. 

Abstract. A multi-wavelength, multi-sensor Look-Up-Table (LUT) algorithm has been developed to retrieve information about stratospheric aerosols from satellite-based observations of particulate extinction. Specifically, the LUT algorithm combines extinction measurements from SAGE n with similar measurements from the CLAES instrument, and uses the composite spectra in month-latitude-altitude bins to retrieve values and uncertainties of particle effective radius, surface area, volume and size distribution width. 

In these density profiles the latex particles, added before starting the experiment, migrate to that position in the cell where their density coincides with the density of the surrounding medium. The position of the particles can be recorded by schlieren optics or, if there is a particle density distribution, more precisely by scanning extinction measurements normally used for the characterization of proteins. Thus the density and extinction profile in the ultracentrifugation cell yield a criterion for the density distribution and hence, because of the correlation between chemical composition and particle density, a criterion for the composition distribution or heterogeneity of the latex particles. 

The local position of the particles in the cell can be recorded in all cases by scanning extinction measurements. Only in the special case of chemically uniform particles can the schlieren optical method also be used. 

The structure of the paper itself affects the behavior with respect to Beer s law which establishes a relation between extinction E and concentration C of transparent solutions, according to the expression E = kC. When extinction measured on paper is plotted against concentration, not a linear but a hyperbolic curve is obtained (CIO, Cll). The cause of the error is the sievelike structure of the paper, since only the threads are covered with stained proteins while the meshes remain completely permeable to light (Fig. 34). This light falling directly on the photosensitive layer of the cell gives, on the microscopic scale, the same error as found for uneven distribution of stained spots on a clear... 

Dinitro-o-cresol Cellulose MN Ammonia saturated butanol Cresols extracted from water with diethyl ether then TLC Bright yellow spots produced with ammonia, spots extracted from plate with 50% acetic acid-methanol and extinction measured at 370nm - [52,53]... 

The ratio of the light extinction coefficient of flame-generated smoke to the mass concentration of soot in the smoke is approximately constant and equal to os = 8.7 1.1 m2/g.64 The implication of this nearly universal value is that the soot yield, i.e., the mass of soot generated per mass unit of fuel burnt, can be estimated from light extinction measurements as follows ... 

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