Particle extinction

The species mass extinction efficiency. Eg, can be theoretically determined from Mie s classical solution to light extinction by a sphere in an infinite medium. Computer routines are available to calculate single particle extinction efficiencies, and, hence. Eg ( ). If the mass distribution of each species is... 

Table 3 summarizes the 1979 annual average particle extinction coefficient and the mass concentrations of the fine aerosol chemical species estimated by statistical analysis of the 61 filter samples. Organics and sulfates dominated the chemically determined fine aerosol mass at China Lake in 1979. A linear least squares fit between molar concentrations of NHt and SO gave a zero intercept, a slope of 1.87 and a correlation coefficient of 0.98. It is therefore assumed that the fine sulfate aerosol was in the form of ammonium sulfate. The mass concentration of carbonaceous and sulfate aerosols were, on the average, comparable in magnitude. 

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. 

Figure 12.9d shows the dielectric function of several metals that either have been discussed in Chapter 9 or will be discussed in connection with small particle extinction in Section 12.4. The energy dependence of the dielectric function is given in the form of trajectories in the complex e plane, similar to ihe Cole-Cole plots (1941) that are commonly used for polar dielectrics the numbers indicated on the trajectories are photon energies in electron volts. 

Rayleigh scattering for.v < 1 and the large particle extinction law for.v 1 provide useful limiting relationships for the efficiency factor. Frequently the range x 1 is imponani. 

Although there is no doubt that the particle extinction method is superior to classical reflection/transmission techniques for determining the effective complex index of refraction of coal, it is more suitable for spherical particles rather than irregularly shaped particles. For coal and char particles, it is very likely that material and shape nonhomogeneity will affect the results obtained in experiments. Therefore, the particle extinction technique can be used to obtain average index of refraction data for coal particles. 

Figure 28.28 Correlation of (a) the particle extinction coefficient at 308 nm, (b) the back-scatter coefficient at 308 nm, (c) the extinction-to-back-scatter DIAL ratio, (d) the effective radius of the particle size distribution, (e) the particle surface-area and (f) the mass concentrations the data were recorded on 4 April 1992. The lidar signal profiles are smoothed with a height window of 600 m for the back-scatter coefficient and 2500 m in all other cases the error bars indicate the overall retrieval error. The optical depth of the stratospheric aerosol layer was 0.25. The dashed line indicates the tropopause. Adapted from Ansmann et al, J. Atmos. Sci., 1997, 54 2630, with permission ofthe American Meteorological Society...

The influence of photocatalyst particle size on gas solid photocatalyzed rate has recently been examined [107,108], using titanium dioxide powder entrained in a flowing gas transport reactor. Consideration of the complex number form of the particle extinction coefficient leads to calculated extinction and absorption efficiencies versus particles size that have maxima at 0.09 fim and become "constant above 10 /xm. Since particle extinction rises for particles smaller than 10 /xm, the calculated reactant conversion shows a maximum at about 10 /xm, falling to the left due to scattering and the right due to diminished photocatalyst surface area. 

To begin with, the basic concept of a single particle extinction size analysis is to be explained in more detail. A suitable configuration is demonstrated in Fig. 1. Other arrangements are feasible. 

To get an impression of the particle size distribution and its deviations the ten number density distributions and cumulative distributions of the split samples, measured by optical single particle extinction counting, are plotted in figure 1. 

Figure 8 Mean cumulative distribution function Q(x) and its statistical data of wet sieving according to figure 7, added by variation coefficients of laser diffraction and single particle extinction...

Cumulative distribution Particle extinction efficiency Particle scattering efficiency Radius... 

The aperture and lens arrangement shown in Fig. 16.1 is needed to ensure that the particles are illuminated with a parallel beam of light (i.e., to ensure that all rays are parallel) and that only the attenuated parallel light reaches the detector. Failure to include the lens and aperture at the detector allows forward-scattered light from the particles to reach the detector, in which case Eq. 16.7 does not hold. The extinction produced by a particle is a function of the particle extinction efficiency Q. ... 

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