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Abstract
The variability of optical properties of combustion particles generated from a propane diffusion flame under varying fuel-to-air (C/O) ratios was studied with a three-wavelength nephelometer, a particle soot absorption photometer, and an integrating sphere photometer. Information on particle size distribution, morphology, and elemental carbon to total carbon (EC/TC) ratios were obtained from scanning mobility particle sizer measurements, transmission electron microscopy analyses, and thermal-optical analyses. Particles generated under a low C/O ratio (0.22) showed high elemental carbon fraction (EC/TC = 0.77) and low brown carbon to equivalent black carbon (BrC/EBC) ratio (0.01), and were aggregates composed of small primary particles. Rayleigh–Debye–Gans theory reproduced experimental single-scattering albedo, ω, absorption, and scattering Ångström exponents within 56, 3, and 18%, respectively. In contrast, particles produced under a high C/O ratio (0.60) showed low elemental carbon fraction (EC/TC = 0.09) and high BrC/EBC ratio >100, and were smaller and spherical in shape. Their optical properties were better modeled with Mie theory. By minimizing the difference between calculated and measured ω and Ångström exponents, refractive indices of OC at three visible wavelengths were deduced. Contrary to the widely accepted assumption that refractive index of BC is wavelength independent, BC-rich particles exhibited absorption Ångström exponent >1.0 which implies some degree of wavelength dependence.
Copyright 2015 American Association for Aerosol Research
ACKNOWLEDGMENTS
This work is dedicated to the memory of our dear friend and distinguished colleague Heidi Bauer. The authors are grateful for the valuable support from A. Ibrahim (DLR), M. Johnson (Rolls-Royce, UK), R. Marsh, and A. Crayford (Cardiff University, UK). We also thank Professor C. M. Sorensen (Kansas State University, USA) for helpful discussions.
Funding
The study was supported by the European Aviation Safety Agency under the SAMPLE II Contract EASA.2009.OP.18. Part of the operation was financed by the European Union, European Social Fund (P-MR-09/1).
SUPPLEMENTAL MATERIAL
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