Water droplets extinction

Figure 4.6 Extinction efficiencies for water droplets in air plotting increment = 0.01 /xm...

Mie calculations with the optical constants of water given in Fig. 10.3 are shown in Fig. 11.3 extinction and absorption are plotted logarithmically, photon energy linearly. The bulk absorption coefficient of water is shown in Fig. 11.3 c. Because many of the extinction features of water and MgO, both of which are insulators, are similar, we present calculations for a single water droplet (in air) with radius 1.0 jam. Size-dependent spectral feature.4 are therefore not obscured as they are for a distribution of radii. 

Figure 11.3 Calculated extinction (a) and absorption (b) by a water droplet of radius 1.0 fim. I he absorption spectrum of water is shown in (c). 

The extinction features at energies where water is transparent are rapidly squelched in the ultraviolet as the onset of electronic transitions greatly increases bulk absorption. In the infrared, however, vibrational absorption bands in water are carried over into similar bands in extinction (dominated by absorption if a A) by a water droplet. Unlike MgO there are no appreciable spectral shifts in going from the bulk to particulate states. The reason for this lies in the strength of bulk absorption and will be discussed more thoroughly in Chapter 12. 

Figure 11.6 The effect of size dispersion on extinction of visible light by water droplets. Each curve is labeled with a, the standard deviation in the Gaussian size distribution.

To show the effect of increasing size dispersion on extinction,a series of calculations for water droplets is given in Fig. 11.6. The topmost curve reproduces the calculations of Fig. 11.5a for a single sphere the standard deviation a is increased in successively lower curves. 

Figure 11.7 High-resolution (Ax = 10 4) calculation of the ripple structure in extinction by a water droplet (m = 1.33 + 110-8). After Chylek et al. (1978a).

For each index n there is a sequence of values of x for which the mode associated with an or bn is excited. We may therefore label each (nonoverlapping) extinction peak with the type of mode [electric (an) or magnetic (/ )], the index n, and the sequential order of x (Chylek, 1976) for example, a 0, a 0, and so on, where the superscript indicates the order of x the extinction peaks between x = 50 and x = 52 for a water droplet are so labeled in Fig. 1 1.7. 

Ripple structure was observed in scattering at 90° by water droplets as they nucleated and grew in a cloud chamber (Dobbins and Eklund, 1977). We shall show in Section 11.7 that ripple structure is easily observed in extinction by... 

There are some notable differences apparent in Fig. 11.14 between the extinction curves for aluminum spheres and those for water droplets. For example, av is still constant for sufficiently small aluminum particles but the range of sizes is more restricted. The large peak is not an interference maximum aluminum is too absorbing for that. Rather it is the dominance of the magnetic dipole term bx in the series (4.62). Physically, this absorption arises from eddy current losses, which are strong when the particle size is near, but less than, the skin depth. At X = 0.1 jam the skin depth is less than the radius, so the interior of the particle is shielded from the field eddy current losses are confined to the vicinity of the surface and therefore the volume of absorbing material is reduced. 

FIGURE 15.3 Extinction efficiency QCM for a water droplet (a) wavelength of the radiation is constant at X = 0.5 pm and diameter is varied (b) diameter = 2 pm and wavelength is varied (Bohren and Huffman 1983). 

The extinction efficiency of a water droplet as a function of the size parameter a is shown in Figure 15.3. We note that Qext approaches the limiting value 2 as the size parameter increases ... 

A.M. Lentati, H.K. Chefliah Dynamics of water droplets in a counterflow field and their effect on flame extinction. Combustion and Flame, 115, 158-179 (1998). 

A determination of the particle size, which is independent from the distance of the particle to the detector, assumes that the extinction coefficients for the minimum and the maximum aperture angle do not differ significantly. Water droplets in air whose diameters are less than 1000 pm result in nearly identical extinction coefficient when the aperture is less than 0.01"". The smaller the particles, the larger the aperture angles are which lead to significantly different extinction coefficients. Water droplets with diameters less than 100 pm arise in almost identical extinction cross sections when the aperture is less than 0.1°. For other particle systems the qualitative correlation is similar. The determination of particle size that is independent of the position of the particle in the measurement volume requires an optical construction of the SE-Sensor with a correspondingly small aperture angle. 

Heskestad [21] has shown that droplet sprays can be partially scaled in fire. He developed a correlation of the water flow rate (1/ 0) needed for extinction of a pool fire, as shown in Figure 12.11. Heskesrtad determines that the water flow needed for extinction is... 

In the region where water is weakly absorbing (between about 0.5 and 5 jum-1) the extinction curve for a 1.0 jum droplet has several features (1) a series of regularly spaced broad maxima and minima called the interference structure, which oscillates approximately about the value 2 (2) irregular fine... 

Both the interference structure and the ripple structure are strongly damped when absorption becomes large, as it does in water if 1 /X is greater than about 6 pm x this is analogous to damping of interference bands in the transmission spectrum of a slab (see Fig. 2.8). If the droplet is small compared with the wavelength, then peaks in the bulk absorption spectrum are seen in the particle extinction spectrum for example, the extinction peaks in Fig. 4.6 at about 6 jam-1 for a 0.05-jum-radius droplet and at about 0.3 jum-1 for a 1.0-jam droplet are neither interference nor ripple structure but bulk absorption peaks. This illustrates the fact that absorption dominates over scattering for small a/X if there is any appreciable bulk absorption. 

The radiative transfer model in Madronich (1987) permits the proper treatment of several cloud layers, each with height-dependent liquid water contents. The extinction coefficient of cloud water is parameterized as a function of the cloud water computed by the three-dimensional model based on a parametrization given by Slingo (1989). For the Madronich scheme used in WRF/Chem, the effective radius of the cloud droplets follows Jones et al. (1994). For aerosol particles, a constant extinction profile with an optical depth of 0.2 is applied. 

Figure 4. Left side Monitoring the homogeneous freezing of supercooled H2SO4/H2O solution droplets with FTIR extinction spectroscopy. Right side Comparison of independent water measurements in the AIDA chamber. See text for details.

A. K. Lazzarini, R. H. Krauss, H. K. Chelliah, G. T. Linteris Extinction conditions of nonpremixed flames with fine droplets of water and water-NaOH solutions, 28th Symposium (International) on Combustion, pp. 2939-2945, Combustion Institute, Pittsburgh (2000). 

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