Multiple scattering effects in disordered media

Light propagation in disordered media such as aerosols can be described for tenuous optical depths of the system by means of the Mie theory of light scattering (single scattering). Numerous experiments confirmed the validity of this approach. In many practically important situations, however, like optical measurements of atmospheric aerosols, or optical information transfer, multiple scattering can not be neglected due to substantial optical depths of the media. In such cases light propagation is basically a multiple scattering process. The knowledge of these effects in combination with the information regarding microphysical aerosol parameters is still rather limited. Currently, there is no general theoretical description available. Also the experimental data, particularly for well- defined systems is inadequate. We plan an experimental investigation of multiple scattering in well-defined aerosols in order to contribute to better understanding of these processes. Measurements will be Performed in a specially designed cloud expansion chamber providing simultaneous observation of a number of scattered and transmitted light fluxes from aerosols. The proposed method allows a quantitative determination of multiple scattering effects as a function of all significant physical aerosol parameters like size and size distribution, refractive index of particles, their concentration, as well as the geometry of the detection system. The suggested experimental arrangement provides a possibility for quantitative determination of light propagation under multiple scattering conditions for particles with various optical characteristics and for aerosols with concentrations varying over many orders of magnitude. This enables a verification of different theoretical models for description of multiple scattering and radiative transfer in disordered media. The results obtained would provide a firm data basis and an insight into the interaction between particulate matter and electromagnetic irradiation in aerosol systems with high optical depths. The experimental set-up allows also a precise, absolute measurement of very high particle concentrations, starting with the nanometer size range, hence it is suitable for calibration purposes as a standard. Comparative aerosol concentration measurements and a further instrumental development towards a portable system is also intended.

Recent years have seen increasing applications of optical frequencies as carriers of information and an opportunity for measurement and diagnostics of disordered media in which the propagation of frequencies happens. A typical example might be the atmosphere containing various aerosols (aerosol - multiphase medium consisting of liquid and / or solid particles suspended in gas / air) such as clouds, fogs, fumes, particulate pollutants, etc. The applications have been driven by the development of suitable laser light sources and the need to know the micro- physical properties of the medium in order either to optimize the information propagation, or to describe the medium to understand its composition and its possible impact on the environment. Particularly the experimental evidence in this field is still incomplete, although greatly needed to verify various theoretical models. Field experiments suffer from the inherent difficulty to control the natural aerosol properties, hence a very important role in multiple scattering studies is apportioned to well-defined laboratory investigations. Micro-physical parameters which crucially affect light propagation and multiple scattering in disordered media such as an aerosol are: aerosol particle size, particle composition and its effective complex refractive index, particle scattering characteristics and their concentration, together with physical dimensions of the system - laboratory scale or outdoor dimensions - and certainly the wavelength of the illumination source. We succeeded in developing an expansion chamber unifying the expansion process together with a simultaneous optical observation of the formed scale model cloud. The particular strength of this experimental procedure is that it offers a way for a simultaneous, but fully independent possibility to control and measure the particle concentration, size and size distribution of the droplet aerosol along with its optical properties. Moreover, we designed the experimental system in such a way that the system`s geometry can be easily varied allowing simulations of various measuring conditions. Furthermore, numerical procedures were developed to replicate various measuring situations under multiple scattering conditions, which resulted among others in improved design parameters for optical systems for measurement of transmitted radiation in disordered media. This project contributes to better understanding of light propagation and multiple scattering in disordered media and provides new and useful experimental means for determination of the impact of various aerosol parameters on radiative transfer, which is of substantial interest e.g. for remote optical sensing of atmospheric pollution, for optical information transfer in turbid medium and for the Earth radiation budget. The experimental verification of various theoretical approaches for description of light propagation in disordered media under multiple scattering conditions shows that the so-called paraxial approximation to the radiative transfer equation for narrow, well- collimated beams describes accurately multiple scattering effects in aerosol systems corresponding to frequently encountered environmental conditions. Moreover we obtained a basis for design of a portable, optical system based on the laser light scattering for measurement of concentrations of particles starting with the nanometer sized particles. Such a measuring technique is of interest for a number of metrological applications in the field of atmospheric sciences, as well as for industrial and biotechnological measurements.