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| VSF at the small angles for natural dispersions | Prev topic | Next topic Fig. 1, Fig. 2 |
The VSF, β(θ), where θ is the scattering angle, exhibits a significant angular asymmetry in the case of natural dispersions such as atmospheric aerosols (for example, Barteneva OD 1960), or aquatic suspensions (for example, Hodara H 1973, Petzold TJ 1972). Indeed, at the scattering angle ~5° ≤ θ ≤ ~90° the VSF typically increases with the decreasing θ roughly according to a power law θ -s, where s ~2.5 for seawater (for example, Martinis M and Risović 1998, Jonasz M 1980). This rate of increase moderates to s ~ 1.5 at the scattering angles smaller than ~5° (Mobley CD et al 2002, Martinis M and Risović 1998, Morel A 1973a). Mobley et al who determined the slope parameter for an angular range of 0.6 ≤ θ ≤1.2°, based on measurements with a prototype polar nephelometer (Lee ME and Lewis 2003), used this slope to extrapolate the VSF when calculating the scattering coefficient.
Fournier GF and Forand 1994 showed that the power-law behavior of the VSF of seawater at the small angles results from the PSD of particles dispersed in seawater being approximated by a power-law (see VSF at the small angles for natural dispersions and power-laws. The difference in the power-law dependence of the VSF at the small and medium angles has been interpreted as a manifestation of a change in the fractal dimension of marine particles with the particle size (Martinis M and Risović 1998).
The variability in the VSF power-law slope can be conveniently represented with a single functional approximation of the VSF by a log-normal function (Fig. 1, see also: Agrawal YC 2005 and Volume scattering function of seawater; note that Agrawal reports the scattering angle in air rather than in water). This simple approximation does not represent well VSF data for waters dominated by quasi-monosized particle species, although a two-component log-normal approximation appears to do much better (Fig. 2). Note that the log-normal approximation has been found unsuccessful to extrapolate the VSF to the small-angles when using data for the scattering angles greater than 10° (Pegau WS et al 1995).
Note that in seawater, the turbulence may significantly contribute to (or even dominate) the VSF at angles ≤~0.1° (Bogucki DJ et al 1998) even at moderate distances (~0.2 m). Turbulence affects the scattering of light simply by varing the refractive index of water across the scattering volume. In that sense, the role of turbulence in light scattering is similar to that of relatively smooth variations of refractive index across cells in human tissue. Bogucki et al note that particles affect these irradiance fluctuations differently than turbulence and suggest that this may be used to differentiate between the two.
See also: Volume scattering function: Small-angle measurement techniques.
| CITATION: Jonasz M., Boss E. 2006. Volume scattering function at the small angles for natural dispersions (www.tpdsci.com/Tpc/VsfSmlAngNatDsp.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 07-Sep-2006 Modified: 29-Nov-2007 Peer-reviewed: 04-Dec-2007 |
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