Light absorption by particles

Table 9.8 summarizes some measurements of the contribution of light absorption by particles, b, for different types of locations from urban residential to remote. In urban areas, bap varies from 13 to 42% of 6cxl, whereas in very remote areas, the contribution of absorption is much less. This is not surprising since the combustion sources producing graphitic carbon tend to be in urban-industrial areas. However, it is noteworthy that soot has been found associated with many sulfate particles even over the remote oceans (Buseck and Posfai, 1999). 

Optical measurements of airborne combustion aerosols have been carried out for a number of years, usually with light scattering techniques. However, due to the particle size dependence of light scattering and the variable particle size distributions of smokes, it is extremely difficult to relate light scattering properties to particulate mass concentrations. The measurement of light absorption by particles can be directly related to particle mass if two conditions are met ... 

Let s consider the influence of gases and particles on the optical properties of the atmosphere. Reduction in visibility is caused by the following interactions in the atmosphere light scattering by gaseous molecules and particles, and light absorption by gases and particles (2). 

There are a number of estimates of the actinic flux at various wavelengths and solar zenith angles in the literature (e.g., see references in Madronich, 1987, 1993). Clearly, these all involve certain assumptions about the amounts and distribution of 03 and the concentration and nature (e.g., size distribution and composition) of particles which determine their light scattering and absorption properties. Historically, one of the most widely used data sets for actinic fluxes at the earth s surface is that of Peterson (1976), who recalculated these solar fluxes from 290 to 700 nm using a radiative transfer model developed by Dave (1972). Demerjian et al. (1980) then applied them to the photolysis of some important atmospheric species. In this model, molecular scattering, absorption due to 03, H20, 02, and C02, and scattering and absorption by particles are taken into account. 

Although it has been generally assumed that elemental carbon is the only component of particles that absorbs visible light, as discussed in Chapter 9, this may not be the case. Instrumentation for measuring total light absorption by all particle components based on... 

Gerber, H., and E. Hindman, 1982. Light Absorption by Aerosol Particles, Spectrum Press, Hampton, Va. 

Ki, the detector response factor, describes the signal generated when particles are present in the eluant as it transits the detector flow-through cell. Detector response arises primarily from scattering of light by the latex particles (15), although a small contribution from light absorption by the sample may occur (16). Polystyrene latex standards of known size and concentration were used to determine Ki factors for conversion of detector response into mass concentration information. 

The problem of the optical characteristics of the aerosols, soot particles included, was discussed at the First International Workshop on Light Absorption by Aerosol Particles (IWLAAP) [24]. The real part, , of the StA complex refractive index in the visible (0.3-0.7 Pm), according to V. Egan [24], varies from 1.665 to 1.775 (w c 2.0-li, for the visible). 

Figure 12.15. Simplified scheme of light absorption by a semiconductor particle and subsequent redox reaction on the surface, vb. Valence band cb, conduction band X and A, adsorbed species. Note that in this scheme, the y axis is at the same time energy and distance. Valence band holes and conduction band electrons can oxidize and/or reduce, respectively, molecules adsorbed on the surface directly ...

The final size ofthe clusters depends on the metal it decreases as the initial polymer/ ion concentration ratio increases and with PVA lies in the nanometre range (1-10 nm).The radiolytic method has been used for the synthesis ofa great number of noble and non noble metal nanocolloids in various solvents, water, alcohols, liquid ammonia, etc. Their intense colours, due to a Mie resonance in light absorption by the electron pool of the particle, depend on the surrounding medium, on the cluster size and shape, and on the metal (Fig. 4) in water, yellow for Ag spheres of a few nm 380 nm), purple for Au ( max=520nm),... 

Figure 5.10 Scheme illustrating some important events that might take place on a metal-oxide particle subsequent to light absorption by the particle. 

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