Spatially Resolved Hyperspectral Optical Extinction Measurements of Carbonaceous Particulate in a Nitrogen-Diluted Ethylene/Air Diffusion Flame and Implications for the Angstrom Exponent

Optical properties of combustion-generated carbonaceous particulate have been reported extensively for visible wavelengths however, there is limited analogous data available for the infrared. In this study, in situ laser-based extinction measurements were made in an International Sooting Flame target laboratory scale 60%/40% ethylene/nitrogen diffusion flame. Measurements were made from 500 nm to 3355 nm using two laser systems - a supercontinuum laser and an external cavity interband cascade laser - to investigate the Angstrom Exponent (AE). The resulting extinction data was tomographically reconstructed and the Extinction AE was calculated throughout the flame. No evidence was found of a polynomial wavelength dependence as has often been described for fine atmospheric particles at visible wavelengths. The contributions of scattering in this flame have been investigated previously and were found to be very low (Scattering AE 0.1, with maximum values in regions with the highest soot volume fraction), thus the EAE values closely resemble the expected Absorption AE values, particularly in young soot areas. There was a wide distribution of AE values across flame positions. In the area of the flame known for particle inception and surface growth, the wavelength dependence of the AE was greater than in areas with more mature soot. In young soot areas, data from 500-700 nm yielded AE values of 2-7, whereas when 700-3355 nm data were used in the same flame area, AE values ranged from 0.6-1.5. In mature soot areas, the AE values correspond well in magnitude ( 0.8-1.2) with black carbon particulate. Overall the wavelength dependence of young soot showed similarities with brown carbon reported in the literature while mature soot nearer to the flame front exhibited good agreement with black carbon measurements made in the atmosphere.