Scattering efficiencies

Like e, t is the product of two contributions the concentration N/V of the centers responsible for the effect and the contribution per particle to the attenuation. It may help us to become oriented with the latter to think of the scattering centers as opaque spheres of radius R. These project opaque cross sections of area ttR in the light path. The actual cross section is then multiplied by the scattering efficiency factor optical cross... 

Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

FIGURE 3 17 Increase in aerosol light-scattering efficiency as a result of aerosol growth. Compare with Figures 3-24 and 3-26. Reprinted with permission from O Brien et al. 

Higher laser power is preferred since it proportionally reduces acquisition time (signal strength). The maximum sampling rate in a process application is limited by acquisition time, which depends on the molecule s Raman scattering efficiency, though adjustments to the standard data transfer hardware and software... 

Statistical Estimation of Species Mass Scattering Efficiencies... 

When the uncertainties in the values are included, it is found that the statistically determined mass scattering efficiences are not significantly different than those calculated theoretically. For example, representing the uncertainty as twice the standard error results in a statistically inferred sulfate mass scattering efficiency of 5.0 i 1.2 as compared with a theoretical value of 3.2 + 1.9. 

Another potential effect is if one species affects the concentration of others due to chemical interactions. If the concentration of a species that is hygroscopic changes, the water content and average index of refraction for the particle may change simultaneously. For example, a reduction of about 7% in the index of refraction due to increased hydration can lead to a change in the scattering efficiency of as much as 34% (Hegg et al., 1993). 

Sloane, C. S., Effect of Composition on Aerosol Light Scattering Efficiencies, Atmos. Environ., 20, 1025-1037 (1986). 

The light scattering efficiency of sulfate at a particular relative humidity (RH) chosen as a reference aRI1 this was estimated as 4.8 X 102 m2 mol-1 with an uncertainty of 1.4. 

FIGURE 14-28 Calculated mass scattering efficiency term (oR,1/un) as a function of wavelength for 0, 40, and 97% RH. Particle dry radius taken to be 0.096-yu.m (NH4)2S04 (adapted from Nemesure et al., 1995). 

Particles are known to scatter light as well as absorb it and this produces the white or pale appearance of fine powders. The even smaller nano-sized particles, however, are transparent because the scattering efficiency is reduced. This effect has led to the use of nanoparticles in sunscreens and cosmetics. These will still absorb ultraviolet light but will scatter less visible light. 

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