Comparison of a Fractal Smoke Optics Model With Light Extinction Measurements.
Comparison of a Fractal Smoke Optics Model With Light
Dobbins, R. A.; Mulholland, G. W.; Bryner, N. P.
Atmospheric Environment, Vol. 28, No. 5, 889-897, 1994.
Sponsor:Army Research Office, Washington, DC
smoke; extinction; absorption; aging (materials);
optical density; soot aggregates; scattering cross
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.