Source line

Emission spectrum from a typical line source. 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

For quantitative analysis, the resolution of the spectral analyzer must be significantly narrower than the absorption lines, which are - 0.002 nm at 400 nm for Af = 50 amu at 2500°C (eq. 4). This is unachievable with most spectrophotometers. Instead, narrow-line sources specific for each element are employed. These are usually hoUow-cathode lamps, in which a cylindrical cathode composed of (or lined with) the element of interest is bombarded with inert gas cations produced in a discharge. Atoms sputtered from the cathode are excited by coUisions in the lamp atmosphere and then decay, emitting very narrow characteristic lines. More recendy semiconductor diode arrays have been used for AAS (168) (see Semiconductors). 

Frequency-Modulation Spectroscopy. Frequency-modulation spectroscopy (tins) is a high sensitivity null-background infrared technique for measuring absorbances down to 10 with fast acquisition speeds. Fms involves frequency-modulating a laser source at COq to produce a carrier frequency having sidebands at cJq where is an integral multiple of the modulation frequency. Dye lasers and many other single-line sources can... 

Petersen, W. B., and Rumsey, E. D., "User s Guide for PAL 2.0—A Gaussian Plume Algorithm for Point, Area, and Line Sources," EPA/600/8-87/009. U.S. Environmental Protection Agency, Research Triangle Park, NC, 1987 (NTIS Accession No. PB87-168 787). 

If the receptor is within an area source, or if emission rates do not vary markedly from one area source to another over most of the simulation area, the narrow-plume hypothesis can be used to consider only the variation in emission rates from each area source in the alongwind direction. Calculations are made as if from a series of infinite crosswind line sources whose emission rate is assigned from the area source emission rate directly upwind of the receptor at the distance of the line source. The ATDL model (22) accomplishes this for ground-level area sources. The RAM model (8) does this for ground-level or elevated area sources. 

Fig. 21-7, Relative concentration in seconds per meter (s m ) exceeded in a 10-km city on 10% of all mornings (solid lines) and 10% of all afternoons (dashed lines). Source After Holzworth (2),...

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

TABLE 7.19 Characteristics of Thermal Plumes above Point and Line Sources... 

According to Bach et al. the volume flow from the vertical surfaces should be added to the volume flow calculated by the equations for point or line sources. 

For a line source, Mundt gives the following plume tise formulae ... 

Most BEOs are situated at the end of a tube, but there are also basic openings situated in walls. BEOs can be used for nearly all kinds of sources, but are usually used for point sources. Use for line or area sources usually demands flexible or movable exhausts, or a slot placed along the line source or along the sides of an area source, or a very large (circular or rectangular) opening placed close to tlie source. A high flow rate is needed to get efficient exhaust in many cases. 

The smoke generator shown in Fig. 12.3 allows the user to ad)ust the flow rate of the smoke and also to connect different types of spreaders throufdi 3 several-meter-Iong tube. This makes it possible to simulate different types of sources, such as a point source with low or high momentum, a line source, a surface source, or any other source with any geometry. Some examples are it-lusrrated in Fig. 12.4. 

An application of the smoke generation principle described above is the smoke wire." This simulates a line source and makes it possible to efteccively study the airflow patterns in a layer, which is often desirable. A thin steel wire is... 

For a new process plant, calculations can be carried out using the heat release and plume flow rate equations outlined in Table 13.16 from a paper by Bender. For the theory to he valid, the hood must he more than two source diameters (or widths for line sources) above the source, and the temperature difference must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests. - If exhaust volume requirements must he established accurately, small scale modeling can he used to augment and calibrate the analytical approach. 

A distinction must be made between continuous sources (hydrogen or deuterium lamps, incandescent tungsten lamps, high pressure xenon lamps) and spectral line sources (mercury lamps), which deliver spectrally purer light in the region of their emission lines. 

The entire development on atmospheric dispersion lias been limited to eniissioiis from a "point" (e.g., stack) source. Altliough most dispersion applications involve point sources, in some instances tlie location of the emission can be more accurately described physically and niatlicmatically by either a line source or an area source. 

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