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.