Wavelength and temperature dependences of the absorption and scattering cross sections of soot

We have inferred the wavelength and temperature dependence of the absorption and scattering cross sections of mature soot in an ethylene flame from laser-induced incandescence (LII) and transmittance measurements at 532 and 1064 nm. The LII measurements indicate that the emissivity of soot in a flame deviates from the expected 1/λ dependence. Combining the LII results with transmittance measurements yields single-scattering albedos of 0.058–0.077 at 1064 nm and 0.22–0.29 at 532 nm and values of F(m)/E(m) of 2.2–2.9 at 532 nm and 2.4–3.3 at 1064 nm. These values confirm that scattering must be taken into account when interpreting extinction data at these wavelengths. Our results also indicate increases in the absorption cross section and decreases in the scattering cross section with increasing fluence at low fluences. The increase in absorption cross section is consistent with increases in primary particle size with increasing particle temperature during laser heating. The decrease in scattering cross section could be attributable to an increase in the radius of gyration or a decrease in the fractal dimension of the aggregate with increasing temperature. Alternatively these trends might be the result of changes to the optical properties of the particles with increasing temperature.