Assessing optical properties and refractive index of combustion aerosol particles through combined experimental and modeling studies

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.