Comparison of in situ measurements and Rayleigh-Debye-Gans theory for optical properties of flame generated monodispersive soot particles

摘要

Soot particles, formed as a result of incomplete fuel combustion, have been an active subject of research for decades for the scientists and researchers from all walks of life. This is primarily because of the challenge which soot's complex formation mechanism and its structure throws to the researchers thereby making the task arduous for studying its optical and physical properties. The knowledge of optical and physical properties is essential in a number of areas such as environmental applications involving visibility and radiation transfer and for optical diagnostics of aerosols and colloids. When it comes to our earth's atmosphere, carbonaceous aerosols stand out as the single most dominant particles residing in the troposphere.Quantifying the optical properties of soot particles using conventional electromagnetic theory has always been a great challenge mainly because of their complex particle morphology. Soot particles formation by combustion is a complex process, and there is a large body of literature on the subject. The combustion fuel first undergoes pyrolysis, a process of thermal degradation and cracking of complex fuel molecules, which produces lowmolecular weight radicals. PAHs are formed subsequently by a series of radical reactions with soot precursors such as acetylene radicals. Further reactions with small precursor molecules result in the growth of the PAH structures until they become large enough to serve as particle nuclei. The high temperature of the flame causes most of the hydrogens to be stripped away, leaving small spherules with a high carbon-to hydrogen ratio. These spherules, under the unavoidable influence of Brownian motion, coagulate to form aggregate particles. The individual spherules that form these aggregate particles are generally small compared to the wavelength of light so that they satisfy the Rayleigh limit of the Lorentz-Mie Scattering theory. Optical cross sections of these complex shaped aggregates cannot be treated adequately by relatively simple electromagnetic theories for spheres or cylinders. Hence, to solve for the optical cross sections of these aggregates using conventional electromagnetic theory, one has to rely on intensive computational capabilities and power, which often yields little insight in terms of one's physical understanding of the problem. In recent years however, thanks to the introduction of the concept of Fractal geometry, a simple formulation known as the Rayleigh-Debye-Gans (RDG) approximation has received much attention and recognition for providing a simple andaccurate solution to the calculation of the optical scattering and absorption by aggregates. Experimental tests of RDG scattering and absorption coefficients of aggregates have been indirect. In this paper, we provide preliminary results obtained from comparison of RDG scattering and absorption coefficients with experimental scattering and absorption coefficients. The aerosol sample used in our case was flame generated soot.