Aerosol light interaction

Small particles scatter and absorb light. This phenomenon has been used to investigate aerosol behavior extensively since Tyndall s work in the nineteenth century. In more recent years, instruments have been built to take advantage of light interactions to deduce particle size distributions. To appreciate how such devices work, we introduce certain basic principles of light interaction with airborne material. 

Suspended particles are the most important factor in visibility reduction. In most instances, the visual quality of air is controlled by partide scattering and is characterized by the extinction coeffident The size of particles plays a crucial role in their interaction with light. Other factors are the refractive index and shape of the particles, although their effect is harder to measure and is less well understood. If we could establish these properties, we could calculate the amount of light scattering and absorption. Alternatively, the extinction coeffident associated with an aerosol can be measured directly. 

S. Skupin, L. Berge, U. Peschel, F. Luderer, Interaction of femtosecond light filaments with obscurants in aerosols, Physical Review Letters 93, 023901 (2004)... 

Figure 5.2. Various interactions that determine the composition of a water droplet in the atmosphere (e.g., cloud, fog). Aerosol particles, which to a large extent consist of (NH4)2S04 and NH4NO3, can form the nuclei for the condensation of liquid water. Various gases can become absorbed into the aqueous phase. The atmosphere is an oxidative environment the water phase, often assisted by light, promotes oxidation reactions, for example, the oxidation of SO2 to H2SO4 and of organic matter to CO2. NH3 neutralizes mineral acids and buffers the solution phase.

Aerosols (solid or aqueous particles in the atmosphere) also interact with the radiation field. However, Rayleigh theory can no longer be applied to these particles, because the size of the aerosols is of the same order of magnitude or larger than the wavelength of the incident light. 

Figure 15.1 depicts the various processes that can occur when radiation of wavelength X0 interacts with a particle. Inelastic scattering processes include Raman scattering and fluorescence. For the interaction of solar radiation with atmospheric aerosols, elastic light scattering is the process of interest. 

Atmospheric visibility can be greatly decreased by primary particulate emissions from industrial sources and by particles generated or grown in the atmosphere in addition to naturally occurring causes. Since the visibility reduction is a function of particle size distribution, aerosol thermodynamics and interaction forces are involved in addition to light scattering. 

Aerosols can be analysed using techniques that are based on the interactions between particles and light The examination of a scattered beam of light by a detector after hitting a particle is the basis for many optical instruments. For example, the number of scattered light pulses is a measure of particle number. Furthermore, the intensity and spatial scattering pattern can also be used for determination of particle size and particle shape, respectively. Optical methods are sensitive and easy to use. These methods are classified into four categories (1) optical particle counter, (2) laser diffractometer, (3) phase Doppler system and (4) intensity deconvolution system. 

Abstract Aerosol-OT water-in-oil microemulsions in isooctane was studied. By using dynamic light scattering and viscosity we measured two coefficients relating to hydro-dynamic interparticle interaction, that is, a from DLS and Huggins coefficient Ich from viscometry. The Wq ( = [H20]/[A0T]) dependence of these coefficients in the range of Wq = 5-15 showed a critically different tendency compared to that... 

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