COMPARISON OF A FRACTAL SMOKE OPTICS MODEL WITH LIGHT EXTINCTION MEASUREMENTS

Optical cross-sections of carbonaceous aggregates (smoke) formed by co mbustion sources have been computed based on fractal concepts. Specifi c extinction depends upon the primary particle size, the structure of the aggregate as represented by the fractal dimension, the fractal pre factor, and the real and imaginary components of the refractive index of the particle material. While the fractal dimension and primary part icle diameter are narrowly defined, the refractive index, to which the results are highly sensitive, are disputed. Specific extinction was m easured at lambda = 450, 630 and 1000 nm in a smoke-filled chamber wit h an optical path length of 1.0 m that was equipped to continuously mo nitor both particle mass and number concentration as the smoke aged du ring a 90-120 min interval. The smoke was generated by the burning of crude oil in a pool fire. Specific extinction at all three values of l ambda was found to be constant even though the aggregate number concen tration decreases by a factor of 24 owing to cluster-cluster aggregati on. The refractive indices at several wavelengths that are required to give agreement with the measured specific extinction are compared wit h literature values. The inadequacy of Mie theory for spheres in predi cting the optical properties of soot aggregates is reiterated.