COMPARISON OF A FRACTAL SMOKE OPTICS MODEL WITH LIGHT EXTINCTION MEASUREMENTS

Optical cross-sections of carbonaceous aggregates (smoke) formed by combustion sources have been computed based on fractal concepts. Specific extinction depends upon the primary particle size, the structure of the aggregate as represented by the fractal dimension, the fractal prefactor, and the real and imaginary components of the refractive index of the particle material. While the fractal dimension and primary particle diameter are narrowly defined, the refractive index, to which the results are highly sensitive, are disputed. Specific extinction was measured at lambda = 450, 630 and 1000 nm in a smoke-filled chamber with an optical path length of 1.0 m that was equipped to continuously monitor both particle mass and number concentration as the smoke aged during 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 lambda was found to be constant even though the aggregate number concentration decreases by a factor of 24 owing to cluster-cluster aggregation. The refractive indices at several wavelengths that are required to give agreement with the measured specific extinction are compared with literature values. The inadequacy of Mie theory for spheres in predicting the optical properties of soot aggregates is reiterated.